#### Upcoming Seminars

#### Past Seminars

Autumn 2022 Seminar Speakers

Spring 2022 Seminar Speakers

Autumn 2021 Seminar Speakers

Easter 2021 Seminar Speakers

Spring 2021 Seminar Speakers

## Seminar LIX: Effective dimension of machine learning models

25 November 2023

**Author: **Amira Abbas (University of KwaZulu-Natal)

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**Abstract: **Making statements about the performance of trained models on tasks involving new data is one of the primary goals of machine learning, i.e., to understand the generalization power of a model. Various measures try to capture this ability, but usually fall short in explaining important characteristics of statistical models that we observe in practice. In this talk, I will motivate the use of a measure called the local effective dimension, and hopefully convince you that it has some interesting properties which we can extend to quantum models.

[1] Abbas, A., Sutter, D., Zoufal, C., Lucchi, A., Figalli, A., & Woerner, S. (2021). The power of quantum neural networks. Nature Computational Science, 1(6), 403-409.

[2] Abbas, A., Sutter, D., Figalli, A., & Woerner, S. (2021). Effective dimension of machine learning models. arXiv preprint arXiv:2112.04807.

## Seminar LVIII: Is longitude or latitude more important for determining your location? Intrinsic Sensitivity Limits for Multiparameter Quantum Metrology

18 November 2023

**Author: **Aaron Goldberg (National Research Council — Canada)

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**Abstract: **Most real-world measurements involve determining multiple parameters; one measurement strategy is seldom ideal for all of the parameters. The ultimate limit in precision for a set of parameters is characterized by the quantum Cramér-Rao bound, which provides a matrix inequality that is hard to uniquely optimize and even more challenging to saturate, adding to richness of multiparameter estimation. I will describe how we found a unique, optimal arrangement among any set of parameters imparted by a unitary transformation that leads to a unique method of finding optimal measurements and optimal probe states for multiparameter metrology, regardless of the chosen parametrization and the underlying parameter values. This is achieved via the metric tensor underlying the geometry of the transformation, providing another important connection between geometry and multiparameter estimation and allowing us to answer the titular question.

[1] Goldberg, A. Z., Sánchez-Soto, L. L., & Ferretti, H. (2021). Intrinsic sensitivity limits for multiparameter quantum metrology. Physical Review Letters, 127(11), 110501.

## Seminar LVII: Multi-Core Quantum Computation with Optical Interconnects

11 November 2023

**Author: **Aharon Brodutch (Entangled Networks)

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**Abstract: **Quantum computers are extremely difficult to scale up due to issues with noise, cooling, control, testing and manufacturing. These become more difficult as the number of qubits increases. The most reasonable way to scale up beyond a few thousand qubits (at best) is to use a modular architecture made up of multiple small quantum processing units (QPUs) that can exchange quantum information. Quantum interconnects are the physical devices used to establish an inter-QPU quantum communication channel by entangling pairs of qubits on different QPUs. While interconnects provide a clear path to scalability they come with their own set of challenges. First and foremost is the relatively slow rate of information exchange between QPUs. This is a result of low entanglement distribution rates and circuit operation overheads. In this talk we present new methods for dealing with the interconnect bottleneck at the logical level. Our results show that the minimal requirements for interconnects that can be used in near-term multi-QPU devices are not far from the current state-of-the-art.

## Seminar LVI: Born’s Rule Painted in a QBist Style

4 November 2023

**Author: **Christopher Fuchs (UMass Boston)

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**Abstract: **QBism is a foundational program for quantum mechanics premised on the idea that quantum probabilities must be understood as Bayesian probabilities—that is, quantified degrees of belief or gambling attitudes. Philosophers hate it. “Wah, wah, wah, your quantum states are not objective features of the world; they have to be [because my philosophy says so].” What the philosophers have never appreciated (or perhaps cared about) is that this turn in thinking has motivated a significant number of theorems and constructions in quantum information science that might not have been discovered otherwise (from the quantum no-broadcasting theorem, to the quantum de Finetti theorem, to the first criterion for certifiable continuous- variable quantum teleportation and plenty more than that). In this talk, I will outline QBism’s most ambitious technical project yet: Rewriting the quantum formalism so that it wears its Bayesian nature on its sleeve. In the process, we will see that it leads to a very deep mathematical question related to Hilbert’s (still unsolved) 12th problem and suggests a novel quantum measurement that may have any number of uses in quantum information science.

[1] John B. DeBrota, Christopher A. Fuchs, and Blake C. Stacey, ‘Symmetric informationally complete measurements identify the irreducible difference between classical and quantum systems’, Phys. Rev. Research, 2, 013074, (2020).

[2] Christopher A. Fuchs, Michael C. Hoang, Blake C. Stacey , ‘The SIC Question: History and State of Play’, Axioms, (2017).

## Seminar LV: Quantum-enhanced Markov chain Monte Carlo

28 October 2023

**Author: **David Layden (IBM)

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**Abstract: **Sampling from complicated probability distributions is a hard computational problem arising in many fields, including statistical physics, optimization, and machine learning. Quantum computers have recently been used to sample from complicated distributions that are hard to sample from classically, but which seldom arise in applications. We introduce a quantum algorithm to sample from distributions that pose a bottleneck in several applications, which we implement on a superconducting quantum processor. The algorithm performs Markov chain Monte Carlo (MCMC), a popular iterative sampling technique, to sample from the Boltzmann distribution of classical Ising models. In each step, the quantum processor explores the model in superposition to propose a random move, which is then accepted or rejected by a classical computer and returned to the quantum processor, ensuring convergence to the desired Boltzmann distribution. We find that this quantum algorithm converges in fewer iterations than common classical MCMC alternatives on relevant problem instances, both in simulations and experiments. It therefore opens a new path for quantum computers to solve useful—not merely difficult—problems in the near term.

[1] David Layden, Guglielmo Mazzola, Ryan V. Mishmash, Mario Motta, Pawel Wocjan, Jin-Sung Kim, Sarah Sheldon, “Quantum-enhanced Markov chain Monte Carlo”, arXiv PrePrint, (2022).

## Seminar LIV: Quantum computation and the additional degrees of freedom in a physical system

21 October 2023

**Author: **Jan-Åke Larsson (Linköping University)

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**Abstract: **The speed-up of Quantum Computers is the current drive of an entire scientific field with several large research programmes both in industry and academia world-wide. Many of these programmes are intended to build hardware for quantum computers. A related important goal is to understand the reason for quantum computational speed-up; to understand what resources are provided by the quantum system used in quantum computation. Some candidates for such resources include superposition and interference, entanglement, nonlocality, contextuality, and the continuity of state-space. The standard approach to these issues is to restrict quantum mechanics and characterize the resources needed to restore the advantage. Our approach is dual to that, instead extending a classical information processing systems with additional properties in the form of additional degrees of freedom, normally only present in quantum-mechanical systems. In this talk, we will have a look at these additional degrees of freedom including the effect of Pauli-group contextuality, and how quantum computers make use of them to achieve the quantum speedup.

[1] Christoffer Hindlycke, Jan-Åke Larsson, An efficient contextual ontological model of n-qubit stabilizer quantum mechanics, to appear in Phys Rev Lett (2022). https://arxiv.org/abs/2202.05081

[2] Niklas Johansson, Felix Huber, Jan-Åke Larsson, Conjugate Logic (2021). https://arxiv.org/abs/2102.06572

[3] Niklas Johansson, Jan-Åke Larsson, Quantum Simulation Logic, Oracles, and the Quantum Advantage, Entropy 21:800 (2019). https://dx.doi.org/10.3390/e21080800

## Seminar LIII: What does consciousness have to do with the collapse of the wave function?

14 October 2023

**Author:** Roger Penrose (University of Oxford)

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**Abstract: **In this talk, Prof. Roger Penrose will discuss his foundational interpretation of quantum mechanics.

## Seminar LII: A Spin Dynamics Approach to Manipulating Tissue Growth

1 April 2022

**Author:** Wendy Scott Beane (Western Michigan University)

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**Abstract: **Recent advances in quantum biology have highlighted the potential advantages of quantum approaches as future therapies. Our work has identified weak magnetic fields (WMFs) as one such approach. WMFs are theorized to affect biological systems by altering electron spin states of free radicals, with different field strengths either increasing or decreasing radical oxygen species (ROS) levels. In turn, signaling downstream of ROS is well known to control both stem cell states and tissue repair mechanisms. Using the planarian regeneration model, our group is investigating the ability of WMFs to modulate stem cell activity in vivo. Our data reveal that WMF exposure following injury, in a field strength-dependent manner, can either increase or decrease: ROS accumulation, ROS-mediated Hsp70 gene expression, adult stem cell proliferation, and blastema (new tissue) growth [1]. Furthermore, WMF exposure phenocopies molecular-genetic ROS inhibition and was predictive of subsequent results obtained with traditional methods to increase ROS signaling. Thus our results support the hypothesized action of WMFs on free radical concentrations and indicate that WMFs may be a potential non-invasive tool to modify cell proliferation and stem cell activity either positively (as in regenerative medicine) or negatively (as in cancer therapies) [2]. Together, these data suggest quantum approaches to controlling stem cells are an emerging research area.

[1] Van Huizen AV, Morton JM, Kinsey LJ, Von Kannon DG, Saad MA, Birkholz TR, Czajka JM, Cyrus J, Barnes FS, Beane WS. (2019) Weak magnetic fields alter stem cell–mediated growth. Science Advances, Jan 30;5(1):eaau7201. DOI: 10.1126/sciadv.aau7201, https://www.science.org/doi/10.1126/sciadv.aau7201

[2] Hack SJ, Kinsey LJ, Beane WS. (2021) An Open Question: Is Non-Ionizing Radiation a Tool for Controlling Apoptosis-Induced Proliferation? Int. J. Mol. Sci. 2021 Oct 16;22(20):11159. DOI: 10.3390/ijms222011159, https://www.mdpi.com/1422-0067/22/20/11159

## Seminar LI: Research at the Wikimedia Foundation: The Science of Knowledge Equity

25 March 2022

**Author:** Martin Gerlach (Wikimedia Foundation)

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**Abstract: **With roughly 20 billion monthly pageviews, 15 million monthly edits, and more than 55 million articles across 300+ languages, Wikipedia has become a canonical part of the Free Knowledge ecosystem. In this talk, I will give an overview of its success story and describe some of the big challenges it faces today. I will focus on one of the main focus areas around knowledge equity: by 2030, the Wikimedia projects (which include Wikipedia) aspire to break down the social, political, and technical barriers preventing people from accessing and contributing to free knowledge. I will present work by the Research Team at the Wikimedia Foundation in this direction to identify, measure, and bridge knowledge gaps [1,2]. In the second part of the talk, I will highlight one of the technical challenges underlying many of these problems: how to automatically identify topics from millions of articles to organize them into meaningful groups covering the wide interests of editors and readers? I will present two approaches we recently developed: i) a language-agnostic topic model to account for the more than 300 language versions in Wikipedia [3], and ii) a network approach to topic models [4,5]: establishing the formal connection between finding topics and community detection in complex networks, we show that stochastic block models can be adapted to obtain a more principled and versatile framework for topic modelling, solving the main limitations of some of the most widely-used methods such as Latent Dirichlet Allocation.

[1] Redi, M., Gerlach, M., Johnson, I., Morgan, J., & Zia, L. (2020). A Taxonomy of Knowledge Gaps for Wikimedia Projects (Second Draft). arXiv:2008.12314. http://arxiv.org/abs/2008.12314

[2] Gerlach, M., Miller, M., Ho, R., Harlan, K., & Difallah, D. (2021). Multilingual Entity Linking System for Wikipedia with a Machine-in-the-Loop Approach. Proceedings of the 30th ACM International Conference on Information & Knowledge Management, 3818–3827. https://doi.org/10.1145/3459637.3481939

[3] Johnson, I., Gerlach, M., & Sáez-Trumper, D. (2021). Language-agnostic Topic Classification for Wikipedia. In Companion Proceedings of the Web Conference 2021 (pp. 594– 601). https://doi.org/10.1145/3442442.3452347

[4] Gerlach, M., Peixoto, T. P., & Altmann, E. G. (2018). A network approach to topic models. Science Advances, 4(7), eaaq1360. https://doi.org/10.1126/sciadv.aaq1360 [5] Hyland, C. C., Tao, Y., Azizi, L., Gerlach, M., Peixoto, T. P., & Altmann, E. G. (2021). Multilayer networks for text analysis with multiple data types. EPJ Data Science, 10(1), 1–16. https://doi.org/10.1140/epjds/s13688- 021-00288-5

## Seminar L: Quantum simulation of an antiferromagnetic Heisenberg spin chain with gate-defined quantum dots

18 March 2022

**Author:** Tzu-Kan Hsiao (TU Delft)

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**Abstract: **Emergent phases of strongly-correlated fermions are of central interest in condensed matter physics. Quantum systems with engineered Hamiltonians can be used as simulators of such many-body systems to provide insights beyond the capabilities of classical computers. Magnetism naturally arises in the Mott-insulator regime of the Fermi-Hubbard model, where charges are localized and the spin degree of freedom remains. In this regime the occurrence of phenomena such as resonating valence bonds, frustrated magnetism, and spin liquids are predicted. However, to study such magnetic behaviour low-entropy, many-body spin states have to be prepared, and characterized.

In this experiment we show that gate-defined semiconductor quantum dots can be used to simulate quantum magnetism in the Mott-insulator regime. For this purpose we develop several experimental techniques including many-body spin-state preparation, singlet-triplet correlation measurements, and characterization of the quantum system with energy spectroscopy and global coherent oscillations. We use these techniques to tune and probe a homogeneously coupled Heisenberg spin chain formed in a linear array of four single-electron quantum dots, and find good agreement between experiment and numerical simulation. Our demonstrated control and techniques combined with flexibility of the quantum dot lattice geometry design opens new opportunities to simulate quantum magnetism, including spin liquid physics and quantum phase transitions.

[1] C. J. van Diepen*, T.-K. Hsiao*, U. Mukhopadhyay, C. Reichl, W. Wegscheider, and L. M. K. Vandersypen, Quantum Simulation of Antiferromagnetic Heisenberg Chain with Gate-Defined Quantum Dots, Physical Review X 11, 041025 (2021)

## Seminar XLIX: Silicon Colour Centres

11 March 2022

**Author:** Stephanie Simmons (Photonic Inc & Simon Fraser University)

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**Abstract: **The future global quantum internet will require high-performance matter-photon interfaces. The highly demanding technological requirements indicate that the matter-photon interfaces currently under study all have potentially unworkable drawbacks, and there is a global race underway to identify the best possible new alternative. For overwhelming commercial and quantum reasons, silicon is the best possible host for such an interface. Silicon is not only the most developed integrated photonics and electronics platform by far, isotopically purified silicon-28 has also set records for quantum lifetimes at both cryogenic and room temperatures [1]. Despite this, the vast majority of research into photon-spin interfaces has notably focused on visible-wavelength colour centres in other materials. In this talk I will introduce a variety of silicon colour centres and discuss their properties in isotopically purified silicon-28. Some of these centres have zero-phonon optical transitions in the telecommunications bands [2], some have long-lived spins in their ground states [3], and some, including the newly rediscovered T centre, have both [4].

[1] K. Saeedi, S. Simmons, J.Z. Salvail, et al. Science 342:830 (2013)

[2] C. Chartrand, L. Bergeron, K.J. Morse, et al. Phys. Rev. B 98:195201 (2018)

[3] K. Morse, R. Abraham, A. DeAbreu, et al. Science Advances 3:e1700930 (2017)

[4] L. Bergeron, C. Chartrand, A.T.K. Kurkjian, et al. PRXQuantum 1 020301 (2020)

## Seminar XLVIII: Building Small, Fast and Hot Si Spin Qubits

4 March 2022

**Author:** Dominik Zumbühl (University of Basel)

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**Abstract: **Quantum computers promise to execute complex tasks exponentially faster than any possible classical computer, Qubits based on hole spins in 1D Ge/Si nanowire are predicted to experience an exceptionally strong yet electrically tunable spin–orbit interaction. Here we used small gate voltage changes to tune the Rabi frequency and the driven coherence time by about a factor of 7, and its Landé g-factor by 50%. We can thus tune from a fast manipulation to an idle mode, demonstrating a spin–orbit switch. Finally, we used this control to optimize our qubit further and approach the strong driving regime, with spin-flipping times as short as ~1 ns.

One of the greatest challenges in quantum computing is achieving scalability. Classical computing previously faced a scalability issue, solved with silicon chips hosting billions of fin field-effect transistors (FinFETs). Here, we show that silicon FinFETs can host spin qubits operating above 4 K, potentially allowing in-situ integration of qubit control electronics. We achieve fast electrical control of hole spins with driving frequencies up to 150 MHz, single-qubit gate fidelities at the fault-tolerance threshold, and a Rabi oscillation quality factor greater than 87. Our devices feature both industry compatibility and quality, and are fabricated in a flexible and agile way that should accelerate further development.

## Seminar XLVII: Experimental observation of thermalisation with noncommuting charges

4 March 2022

**Author:** Aleks Lasek (NIST & University of Maryland)

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**Abstract: **Quantum simulators have recently enabled experimental observations of quantum many-body systems’ internal thermalisation. Often, the global energy and particle number are conserved, and the system is prepared with a well-defined particle number—in a microcanonical subspace. However, quantum evolution can also conserve quantities, or charges, that fail to commute with each other. Noncommuting charges have recently emerged as a subfield at the intersection of quantum thermodynamics and quantum information. Until now, this subfield has remained theoretical. We initiate the experimental testing of its predictions, with a trapped-ion simulator. We prepare 6–15spins in an approximate microcanonical subspace, a generalisation of the microcanonical subspace for accommodating noncommuting charges, which cannot necessarily have well-defined nontrivialvalues simultaneously. We simulate a Heisenberg evolution using laser-induced entangling interactions and collective spin rotations. The noncommuting charges are the three spin components. We find that small subsystems equilibrate to near a recently predicted non-Abelian thermal state. This work bridges quantum many-body simulators to the quantum thermodynamics of noncommuting charges, whose predictions can now be tested.

[1] https://arxiv.org/abs/2202.04652

## Seminar XLVI: The structure of noncontextuality, in particular for the stabilizer subtheory

25 February 2022

**Author:** Matthew Pusey (University of York)

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**Abstract: **I will give an overview of two recent papers on the generalised notion of noncontextuality due to Spekkens. In the first [1] we show that taking into account transformations imposes a surprisingly rigid structure on noncontextual models of tomographically local theories (such as quantum theory). An example of this rigidity is that the number of ontic states in the noncontextual model must equal the dimension of the operational state space. In the second paper [2] we exploit this structure to classify the no ncontextual models of the stabilizer subtheories, finding that there is a unique noncontextual model in odd dimension, and no model at all in even dimension. The unique model in odd dimensions is known as Gross’s discrete Wigner function, which could expla in why this representation plays a special role in some forms of quantum computation.

[1] https://arxiv.org/abs/2005.07161

[2] https://arxiv.org/abs/2101.06263

## Seminar XLV: Two-dimensional implementations of quantum LDPC codes

18 February 2022

**Author:** Nicolas Delfosse (Microsoft Quantum)

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**Abstract: **Quantum LDPC codes are promising to reduce the cost of fault-tolerant quantum computing (arxiv:1310.2984) but can they really be implemented with current quantum hardware which typically looks like a 2D grid of qubits equipped with local gates? In this talk we will provide two answers to this question. First we will discuss obstructions to local implementation of quantum LDPC codes which suggest that quantum LDPC are impractical with 2D local quantum hardware. Second, we propose an implementation of quantum LDPC codes based on long-range connections. With this design, we obtain a threshold of 0.28% for a family of quantum LDPC codes using 49 physical qubits per logical qubit. For a physical error rate of 10−4, this family reaches a logical error rate of 10−15 using fourteen times fewer physical qubits than the surface code.

Based on joint work with Michael Beverland and Maxime Tremblay:

[1] https://arxiv.org/abs/2109.14599

[2] https://arxiv.org/abs/2109.14609

## Seminar XLIV: Combining the radical elements of Gravity and the Quantum: Indefinite causal structure

11 February 2022

**Author:** Lucien Hardy (Perimeter Institute)

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**Abstract:** General Relativity and Quantum Theory are each conservative and radical in complementary respects. In General Relativity quantities take definite values but the theory has dynamical causal structure. Quantum Theory has fixed causal structure but it has the property of indefiniteness (quantities do not take definite values). Most likely, a theory of Quantum Gravity will combine the radical aspects – that is it will have indefinite causal structure. I will discuss a general probabilistic framework capable of admitting theories with indefinite causal structure (the causaloid framework) and also the quantum equivalence principle which offers a way to tame indefinite causal structure locally. Finally, I will discuss tentative steps on route to a theory of Quantum Gravity employing the quantum equivalence principle.

## Seminar XLIII: Quantum Error Suppression

26 November 2021

**Author:** Milad Marvian (University of New Mexico)

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**Abstract:** The theory of quantum fault-tolerance ensures that quantum computers can operate reliably in the presence of decoherence and noise. Although in principle the existence of this threshold means that scalable quantum computation is possible, in practice the value of this threshold and the required overhead are very important, as achieving them in experiments remain extremely challenging. One approach to reducing these requirements is to use active error correction in combination with passive methods that provide additional robustness against instability or noise. In this talk, we will explore various passive error reduction methods in different experimental setups, and will discuss how they can be thought as manifestations of “quantum Zeno effect”.

## Seminar XLII: Organic neuromorphic electronics and biohybrid systems

19 November 2021

**Author:** Yoeri van de Burgt (Eindhoven University of Technology)

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**Abstract:**Neuromorphic computing could address the inherent limitations of conventional silicon technology in dedicated machine learning applications. Recent work on large crossbar arrays of two-terminal memristive devices has led to the development of promising neuromorphic systems. However, delivering a compact and efficient parallel computing technology that is capable of embedding artificial neural networks in hardware remains a significant challenge.

Organic electronic materials have shown potential to overcome some of these limitations. This talk describes state-of-the-art organic neuromorphic devices and provides an overview of the current challenges in the field and attempts to address them. I demonstrate a novel concept based on an organic electrochemical transistor and show how we can use these devices in trainable biosensors and smart autonomous robotics.

Next to that, organic electronic materials have the potential to operate at the interface with biology. This can pave the way for novel architectures with bio-inspired features, offering promising solutions for the manipulation and the processing of biological signals and potential applications ranging from brain-computer-interfaces and smart robotics to bioinformatics. I will highlight our recent efforts for such hybrid biological memory devices.

[1] van de Burgt, Nature Materials, 2017, doi:10.1038/nmat4856

[2] van de Burgt, Nature Electronics, 2018, doi:10.1038/s41928-018-0103-3 [3] Keene, Nature Materials, 2020, doi:10.1038/s41563-020-0703-y

## Seminar XLI: Quantum supremacy and quantum machine learning with analog quantum simulators

12 November 2021

**Author:** Dimitris Angelakis (Singapore Centre for Quantum Technologies)

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**Abstract:**I will start with a general introduction on quantum simulation and computation with near term processors, the state of the art and the current race for building operational quantum processors. I will then review our work with Google on quantum simulations with superconducting qubits [1] and as well as our recent works in analog quantum supremacy [2] and quantum machine learning [3] with NISQ devices. If time, I will briefly summarise our work on qubit efficient quadratic optimization algorithms [4]. The first part of the talk should be accessible to non- specialists.

[1]Spectral signatures of many-body localization with interacting photons

P. Roushan, C. Neill, …D.G. Angelakis, and J. Martinis. Science, 01 Dec 2017: Vol. 358, Issue 6367, (2017)

[2] Quantum supremacy and quantum phase transitions J. Tangpanitanon, S. Thanasilp, M. A. Lemonde, N. Dangiam, D. G. Angelakis Phys. Rev. B 103, 165132 (2021)

[3]Expressibility and trainability of parameterized analog quantum systems for machine learning applications

J. Tangpanitanon, S. Thanasilp, M. A. Lemonde, N. Dangiam, D. G. Phys. Rev. Research 2, 043364 2020

[4] Qubit efficient algorithms for binary optimization problems

B. Tan, M. A. Lemonde, S. Thanasilp, J. Tangpanitanon, D. G. Angelakis Quantum 5, 454 (2021) ]

Angelakis

## Seminar XL: Negative quasiprobabilities enhance phase estimation in quantum-optics experiment

5 November 2021

**Author:** Noah Lupu-Gladstein (University of Toronto)

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**Abstract: **Hallmarks of quantum theory, such as operators’ failure to commute, impose fundamental limits on measurement precision. Foundational studies of these limits have pushed measurements and metrology to the bleeding edge. Inspired by a recent foundational result connecting metrology with quasiprobabilities [1], quantum generalizations of probabilities, we discover a filtering technique that promises an–in principle–unlimited advantage in the information rate of trials that survive the filter. We implement this filter in a proof-of-principle optical measurement of a waveplate’s birefringent phase and amplify the information per detected photon by over two orders of magnitude. We find the theoretically unlimited advantage to be bounded in practice because the filter also amplifies systematic errors. We crystallize the relationship between enhanced precision and negative quasiprobabilities by deriving an equality for pure states, confirmed by our data, between the postselected information-rate and a function of a quasiprobability distribution.

[1] D. R. M. Arvidsson-Shukur, N. Yunger Halpern, H. V. Lepage, A. A. Lasek, C. H. W. Barnes, and S. Lloyd, Quantumadvantage in postselected metrology, Nature Communications11, 3775 (2020)

## Seminar XXXIX: Measurement, information, control, decoherence

29 October 2021

**Author:** Kater Murch (Washington University)

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**Abstract: **In this talk I will discuss some basic ideas in quantum measurement from the perspective of experiments that are readily available today in the circuit quantum electrodynamics architecture. We will focus on a recent experiment where we test an entropic uncertainty relation involving weak and projective measurements. Our story spans some of the oldest ideas in quantum mechanics (uncertainty relations), involves some of the most controversial (weak values), pulls on ideas in information theory (entropies), and will be presented from the humble perspective of an experimentalist trying to make his way through the fascinating world of quantum physics.

[1] https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.126.100403 (https://arxiv.org/abs/2008.09131)

[2] http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.114.090403 (https://arxiv.org/abs/1409.0510)

[3] http://www.nature.com/nature/journal/v502/n7470/full/nature12539.html (https://arxiv.org/abs/1305.7270v1)

## Seminar XXXVIII: Silicon MOS quantum dots for spin-based quantum computation

22 October 2021

**Author:** Arne Laucht (UNSW Sydney)

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**Abstract:** Quantum computers are expected to outperform conventional computers for a range of important problems, from molecular simulation to search algorithms, once they can be scaled up to large numbers of quantum bits (qubits), typically millions. Spin qubits in silicon MOS quantum dots are one of the big contenders for a scalable, solid state-based quantum computing platform. Here, the qubits are encoded as the spin states of individual electrons confined in electrostatically-gated quantum dots. The great potential of this system has been demonstrated through various experiments over the last few years, with coherence times of up to T2=28 ms, single qubit control fidelities of 99.96%, and two-qubit control fidelities of 98%.

In my presentation, I will give an introduction to the SiMOS quantum dot spin qubits that we employ at UNSW Sydney. I will showcase some of the key experiments of the last years related to experimental challenges of scaling a silicon-CMOS based quantum processor up to the millions of qubits that will be required for fault-tolerant quantum computing. In particular, I will present our results of operating silicon spin qubits at temperatures above 1 K [1,2] that are important for the integration of conventional CMOS control electronics with the qubit system, and global control techniques that allow for the control of many qubits simultaneously.

[1] C. H. Yang, R. C. C. Leon, J. C. C. Hwang, A. Saraiva, T. Tanttu, W. Huang, J. Camirand Lemyre, K. W. Chan, K. Y. Tan, F. E. Hudson, K. M. Itoh, A. Morello, M. Pioro-Ladrière, A. Laucht, and A. S. Dzurak. Operation of a silicon quantum processor unit cell above one kelvin. Nature 580, 350 (2020).

[2] J. Y. Huang, W. H. Lim, R. C. C. Leon, C. H. Yang, F. E. Hudson, C. C. Escott, A. Saraiva, A. S. Dzurak, and A. Laucht. A High-Sensitivity Charge Sensor for Silicon Qubits above 1 K. Nano Letters 21, 6328 (2021).

[3] E. Vahapoglu, J. P. Slack-Smith, R. C. C. Leon, W. H. Lim, F. E. Hudson, T. Day, J. D. Cifuentes, T. Tanttu, C. H. Yang, A. Saraiva, M. L. W. Thewalt, A. Laucht, A. S. Dzurak, and J. J. Pla. Coherent control of electron spin qubits in silicon using a global field. arXiv:2107.14622 (2021).

## Seminar XXXVII: Training deep quantum neural networks

15 October 2021

**Author:** Kerstin Beer (Leibniz Universität Hannover)

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**Abstract:** Machine learning, particularly as applied to deep neural networks via the back-propagation algorithm, has brought enormous technological and societal change. With the advent of quantum technology it is a crucial challenge to design quantum neural networks for fully quantum learning tasks. In my talk I will present a truly quantum analogue of classical neurons and explain how to use it to form a quantum feed-forward neural networks capable of universal quantum computation. For training these networks we use the fidelity as a cost function and benchmark the proposal for the quantum task of learning an unknown unitary operation. We find remarkable generalization behavior and robustness to noisy training data. My talk will be based on a recent work of us [1]. For digging deeper in to the topic after the talk I would recommend reading about finding an optimal lower bound on the probability that such a trained network gives an incorrect output for a random input [2] and about considering graph- structured quantum data for training our quantum neural networks [3].

[1] https://www.nature.com/articles/s41467-020-14454-2

[2] https://arxiv.org/abs/2003.14103

[3] https://export.arxiv.org/abs/2103.10837

## Seminar XXXVI: Hamiltonian quantum computing

8 October 2021

**Author:** Seth Lloyd (MIT)

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**Abstract:** This talk shows how to recast quantum algorithms in a purely Hamiltonian form, substituting time-dependent Hamiltonian controls for the conventional quantum logic gate picture. Using the example of the quantum singular value transformation (“the mother of all quantum algorithms,” according to Ike Chuang), I show that Hamiltonian quantum computing can supply significant advantages in terms of time and decoherence compared with the gate based model.

[1] ‘Hamiltonian singular value transformation and inverse block encoding’, Seth Lloyd, Bobak T. Kiani, David R. M. Arvidsson-Shukur, Samuel Bosch, Giacomo De Palma, William M. Kaminsky, Zi-Wen Liu, Milad Marvian, arXiv Preprint: https://arxiv.org/abs/2104.01410

## Seminar XXXV: Quantum advantage for differential equation analysis

28 May 2021

**Author:** Giacomo de Palma (Scuola Normale Superiore di Pisa)

**Abstract:** Quantum algorithms for both differential equation solving and for machine learning potentially offer an exponential speedup over all known classical algorithms. However, there also exist obstacles to obtaining this potential speedup in useful problem instances. The essential obstacle for quantum differential equation solving is that outputting useful information may require difficult post-processing, and the essential obstacle for quantum machine learning is that inputting the training set is a difficult task just by itself. In this paper, we demonstrate, when combined, these difficulties solve one another. We show how the output of quantum differential equation solving can serve as the input for quantum machine learning, allowing dynamical analysis in terms of principal components, power spectra, and wavelet decompositions. To illustrate this, we consider continuous time Markov processes on epidemiological and social networks. These quantum algorithms provide an exponential advantage over existing classical Monte Carlo methods.

References:

1) Bobak T. Kiani, Giacomo De Palma, Dirk Englund, William Kaminsky, Milad Marvian, Seth Lloyd, arXiv preprint: https://arxiv.org/abs/2010.15776

## Seminar XXXIV: Quantum speed-ups in reinforcement learning

21 May 2021

**Author:** Valeria Saggio (University of Vienna)

**Abstract:** The field of artificial intelligence (AI) has experienced major developments over the last decade. Within AI, of particular interest is the paradigm of reinforcement learning (RL), where autonomous agents learn to accomplish a given task via feedback exchange with the world they are placed in, called an environment. Thanks to impressive advances in quantum technologies, the idea of using quantum physics to boost the performance of RL agents has been recently drawing the attention of many scientists. In my talk I will focus on the bridge between RL and quantum mechanics, and show how RL has proven amenable to quantum enhancements. I will provide an overview of the most recent results — for example, the development of agents deciding faster on their next move [1-2]— and I will then focus on how the learning time of an agent can be reduced using quantum physics. I will show that such a reduction can be achieved and quantified only if the agent and the environment can also interact quantumly, that is, if they can communicate via a quantum channel [3]. This idea has been implemented on a quantum platform that makes use of single photons as information carriers. The achieved speed-up in the agent’s learning time, compared to the fully classical picture, confirms the potential of quantum technologies for future RL applications.

References:

1) Sriarunothai, T. et al. Quantum Science and Technology 4, 015014 (2018) https://iopscience.iop.org/article/10.1088/2058-9565/aaef5e/meta

2) Paparo, G. D. et al. Physical Review X, 4(3), 031002 (2014) https://journals.aps.org/prx/abstract/10.1103/PhysRevX.4.031002

3) Saggio, V. et al. Nature 591, 229–233 (2021) https://www.nature.com/articles/s41586-021-03242-7

## Seminar XXXIII: From nanotech to living sensors: unraveling the spin physics of biosensing at the nanoscale

14 May 2021

**Author:** Clarice Aiello (University of California, Los Angeles)

**Abstract:** Substantial in vitro and physiological experimental results suggest that similar coherent spin physics might underlie phenomena as varied as the biosensing of magnetic fields in animal navigation and the magnetosensitivity of metabolic reactions related to oxidative stress in cells. If this is correct, organisms might behave, for a short time, as “living quantum sensors” and might be studied and controlled using quantum sensing techniques developed for technological sensors. I will outline our approach towards performing coherent quantum measurements and control on proteins, cells and organisms in order to understand how they interact with their environment, and how physiology is regulated by such interactions. Can coherent spin physics be established – or refuted! – to account for physiologically relevant biosensing phenomena, and be manipulated to technological and therapeutic advantage?

References:

1) Clarice D. Aiello, Masashi Hirose & Paola Cappellaro, Nature Communications, 4, 1419 (2013) https://www.nature.com/articles/ncomms2375

2) Ron Naaman, Yossi Paltiel & David H. Waldeck, Nature Reviews Chemistry, 3, (2019) https://www.nature.com/articles/s41570-019-0087-1

3) P . Hore, H. Mouritsen, Annu Rev Biophys, Jul 5;45:299-344. (2016) https://pubmed.ncbi.nlm.nih.gov/27216936/

## Seminar XXXII: Simulations of gauge field theories with quantum tools

7 May 2021

**Author:** Zohreh Davoudi (University of Maryland)

**Abstract:** Quantum field theories (QFTs) are at the core of the modern descriptions of physical phenomena. Gauge field theories, for example, are the cornerstones of the Standard Model of particle physics describing interactions of elementary particles in nature at a range of energies. Quantum simulation and quantum computation have the promise of making classically formidable QFT problems, such as computing real-time phenomena in and out of equilibrium, tractable. Significant progress has been made in theoretical and algorithmic developments for, and hardware implementation of, a number of QFTs and lattice gauge theories in recent years. In this talk, I will introduce analog, digital, and hybrid analog-digital approaches to the simulation of QFTs along with a few illustrative examples.

## Seminar XXXI: Entangled quantum cellular automata, physical complexity, and Goldilocks rules

26 March 2021

**Author:** Logan Hillberry (University of Texas – Austin)

**Abstract:** Cellular automata are interacting classical bits that display diverse emergent behaviors, from fractals to random-number generators to Turing-complete computation. In this talk, I will discuss quantum cellular automata (QCA) and show that they can exhibit complexity in the sense of the complexity science that describes biology, sociology, and economics. QCA exhibit complexity when evolving under “Goldilocks rules” that I will define by balancing activity and stasis. The Goldilocks rules generate robust dynamical features (entangled breathers), network structure and dynamics consistent with complexity, and persistent entropy fluctuations. Present-day experimental platforms — Rydberg arrays, trapped ions, and superconducting qubits — can implement our Goldilocks protocols, making testable the link between complexity science and quantum computation exposed by Goldilocks QCA.

References

1) Logan E. Hillberry, Matthew T. Jones, David L. Vargas, Patrick Rall, Nicole Yunger Halpern, Ning Bao, Simone Notarnicola, Simone Montangero, and Lincoln D. Carr, Entangled quantum cellular automata, physical complexity, and Goldilocks rules (2021), https://arxiv.org/abs/2005.01763

## Seminar XXX: Advances in Variational Quantum Algorithms

19 March 2021

**Author:** Balint Koczor (University of Oxford)

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**Abstract:** Quantum devices have recently been announced whose behaviour cannot be simulated using classical computers with practical levels of resource. In this era, quantum computers may have the potential to perform useful tasks of value despite their high levels of noise. One very promising class of approaches are generically called quantum variational algorithms in which one seeks to make use of a quantum circuit of relatively low depth by adjusting its function through varying a set of classical parameters. The emerging quantum states can be very complex, while inevitably being restricted to a small proportion of the exponentially large Hilbert space. These variational approaches promise to solve key problems, such as finding ground states of quantum systems in quantum chemistry and in materials science, but require non-trivial classical optimisation. I will give an overview of some of our recent works in the context of variational quantum algorithms. I will discuss two novel optimisation approaches: Quantum Natural Gradient and imaginary time evolution [1,2] which extract information about the geometry of quantum states and Quantum Analytic Descent [3] which offloads more work from the quantum device via an efficient analytical approximation. I will then briefly discuss applications to quantum metrology [4].

References

1) B. Koczor, S. C. Benjamin: Quantum natural gradient generalised to non-unitary circuits. arXiv preprint arXiv:1912.08660 (2019). https://arxiv.org/abs/1912.08660

2) S. McArdle, T. Jones, S. Endo, Y. Li, S. C. Benjamin, X. Yuan: Variational ansatz-based quantum simulation of imaginary time evolution. npj Quantum Information 5, no. 1 (2019): 1-6. https://www.nature.com/articles/s41534-019-0187-2

3) B. Koczor, S. C. Benjamin: Quantum Analytic Descent. arXiv preprint arXiv:2008.13774 (2020).

https://arxiv.org/abs/2008.13774

4) B. Koczor, S. Endo, T. Jones, Y. Matsuzaki, S. C. Benjamin: Variational-state quantum metrology. New Journal of Physics 22, no. 8 (2020): 083038. https://iopscience.iop.org/article/10.1088/1367-2630/ab965e

## Seminar XXIX: From quantum foundation to quantum cryptography

12 March 2021

**Author:** Xiongfeng Ma (Tsinghua University)

**Abstract: **The peculiar features of quantum mechanics, such as unpredictable randomness and nonlocal correlation, cause lots of trouble on its interpretation at the early stage of development. The unpredictable randomness is quantified by quantum coherence, while the nonlocal correlation is quantified by quantum entanglement. Today, these features are employed in information processing and become useful resources to enable tasks otherwise impossible with classical means. In modern cryptography, true randomness generation and key distribution are two challenges. These can be handled with the introduction of quantum cryptography. In this talk, I shall introduce the concepts of coherence and entanglement, which, respectively, enable true randomness generation and key distribution. Meanwhile, I shall also cover some of the recent exciting developments in this field.

References

1) ‘Quantum Coherence and Intrinsic Randomness’, X. Yuan et al., Advanced Quantum Technologies, (2019), https://onlinelibrary.wiley.com/doi/abs/10.1002/qute.201900053

2) ‘Quantum Random Number Generation’, X. Ma et al., NPJ Quantum Information, (2016), https://www.nature.com/articles/npjqi201621

## Seminar XXVIII: Optical (non)classicality, quadrature coherence and environmental decoherence of bosonic quantum field states

5 March 2021

**Author: **Stephan De Bièvre (Université de Lille – CNRS)

**Abstract: **In quantum optics, the electromagnetic field is often restricted to a finite number of field modes. A quantum state of the field is then said to be optically classical, if it is a mixture of coherent states. Given a state, the question arises how to establish if it is nonclassical and if so, how strongly. A number of witnesses, measures and monotones of optical nonclassicality have been developed for that purpose over the years. I will review some of this literature and present a new such optical nonclassicality measure recently introduced [2]: the quadrature coherence scale (QCS) of a state. The QCS links optical nonclassicality to the presence of “coherences” in the density matrix of the state far away from its diagonal. It allows to understand why strongly optically nonclassical states are extremely sensitive to environmental disturbance and therefore hard to create and maintain. And it provides an upper bound on entanglement.

References

1) Measuring the nonclassicality of bosonic field quantum states via operator ordering sensitivity, with D. B. Horoshko, G. Patera et M. I. Kolobov, Phys. Rev. Lett. 122, 080402, 2019. arXiv.org 1809.02047. https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.122.080402

2) Quadrature coherence scale driven fast decoherence of bosonic quantum field states, with A. Hertz, Phys. Rev. Lett. 2020. arXiv.org 1909.05025. https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.124.090402

3) Relating the entanglement and optical nonclassicality of multimode states of a bosonic quantum field, Anaelle Hertz, Nicolas J. Cerf, and Stephan De Bièvre. Phys. Rev. A 102, 032413, 2020. arXiv:2004.11782. https://journals.aps.org/pra/abstract/10.1103/PhysRevA.102.032413

## Seminar XXVII: Quantum Earth Mover’s Distance: A New Approach to Learning Quantum Data

26 February 2021

**Author: **Bobak Kiani (MIT)

**Abstract: **Quantifying how far the output of a learning algorithm is from its target is an essential task in machine learning. However, in quantum settings, the loss landscapes of commonly used distance metrics often produce undesirable outcomes such as poor local minima and exponentially decaying gradients. As a new approach, we consider here the quantum earth mover’s (EM) or Wasserstein-1 distance, recently proposed in [De Palma et al., arXiv:2009.04469] as a quantum analog to the classical EM distance. We show that the quantum EM distance possesses unique properties, not found in other commonly used quantum distance metrics, that make quantum learning more stable and efficient. We propose a quantum Wasserstein generative adversarial network (qWGAN) which takes advantage of the quantum EM distance and provides an efficient means of performing learning on quantum data. Our qWGAN requires resources polynomial in the number of qubits, and our numerical experiments demonstrate that it is capable of learning a diverse set of quantum data.

References

1) Bobak Toussi Kiani, Giacomo De Palma, Milad Marvian, Zi-Wen Liu, Seth Lloyd, Quantum Earth Mover’s Distance: A New Approach to Learning Quantum Data, https://arxiv.org/abs/2101.03037

## Seminar XXVI: Contextuality of quantum linear response

12 February 2021

**Author: **Matteo Lostaglio (Delft University of Technology)

**Abstract: **I present a recent work that highlights a fundamental difference between classical and quantum dynamics in the linear response regime by showing that the latter is, in general, contextual. This allows one to provide an example of a quantum engine whose favorable power output scaling *unavoidably* requires nonclassical effects in the form of contextuality. Furthermore, I describe contextual advantages for local metrology. Given the ubiquity of linear response theory, I anticipate that these tools will allow one to certify the nonclassicality of a wide array of quantum phenomena.

3) David RM Arvidsson-Shukur, Quantum advantage in postselected metrology, Nat. Comm 11, 3775 (2020)

## Seminar XXV: Electron cascade for distant spin readout

5 February 2021

**Author: **Sjaak van Diepen (Delft University of Technology)

**Abstract: **Spin-qubits based on gate-defined semiconductor quantum dots are a promising platform for quantum computation and simulation. An important advantage of quantum dots is their small footprint. The dot pitch is about 100 nm, hence 100 million dots fit on 1 mm2. A problem is that qubit readout with charge sensing based on capacitive coupling only enables to sense nearby quantum dots and placing charge sensors within the quantum dot array hosting the qubits is detrimental for connectivity of the qubits. In this work we demonstrate cascade-based readout of a spin distant from the charge sensor. The cascade consists of an initial charge transition, far away from the sensor, and subsequent charge transitions induced by Coulomb repulsion, with the final transition nearby the sensor. Combined with spin-to-charge conversion a cascade enables the readout of charge and spin occupation of quantum dots remote from the charge sensor. We experimentally demonstrate cascade-based readout with Pauli spin blockade in a quadruple dot with a sensing dot.

## Seminar XXIV: A Spin Quintet in a Silicon Double Quantum Dot: Spin Blockade and Relaxation

27 November 2020

**Author: **Theodor Lundberg (Hitachi)

**Abstract: **Spins in gate-defined silicon quantum dots are promising candidates for implementing large- scale quantum computing. To read the spin state of these qubits, the mechanism that has provided highest fidelity is spin-to-charge conversion via singlet-triplet spin blockade, which can be detected in-situ using gate-based dispersive sensing. In systems with a complex energy spectrum such as silicon quantum dots, accurately identifying when singlet-triplet blockade occurs is therefore critical for scalable qubit readout.

In this work, we present a description of spin blockade physics in a tunnel-coupled silicon double quantum dot defined in the corners of a split-gate transistor. Using gate-based magnetospectroscopy, we report successive steps of spin blockade and spin blockade lifting involving spin states with total spin angular momentum up to S = 3. Furthermore, we report the formation of a hybridized spin quintet state and show triplet-quintet and quintet-septet spin blockade. This enables investigation of the quintet relaxation dynamics from which we find a relaxation time of T1 ~ 4 μs. Finally, we develop a quantum capacitance model that is applied generally to reconstruct the energy spectrum of the double quantum dot including the spin- dependent tunnel coupling and the energy splitting between different spin manifolds. Our results open the possibility of using silicon complementary metal-oxide-semiconductor (CMOS) quantum dots as a tuneable platform for studying the interactions and dynamics of high-spin systems.

References

1) https://arxiv.org/abs/1910.10118

## Seminar XXIII: Aharonov-Bohm Phase is Locally Generated Like All Other Quantum Phases

20 November 2020

**Author: **Chiara Marletto (Oxford)

**Abstract: **In the Aharonov-Bohm (AB) effect, a superposed charge acquires a detectable phase by enclosing an infinite solenoid, in a region where the solenoid’s electric and magnetic fields are zero. Its generation seems therefore explainable only by the local action of gauge-dependent potentials, not of gauge-independent fields. This was recently challenged by Vaidman, who explained the phase by the solenoid’s current interacting with the electron’s field (at the solenoid). Still, his model has a residual nonlocality: it does not explain how the phase, generated at the solenoid, is detectable on the charge. I will explain how to solve this nonlocality by explicitly quantizing the field. In this model, the AB phase is mediated locally by the entanglement between the charge and the photons, like all electromagnetic phases. I will discuss a realistic experiment to measure this phase difference locally, by partial quantum state tomography on the charge, without closing the interference loop.

References

1) https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.125.040401

2) https://arxiv.org/abs/1906.03440

## Seminar XXII: Noise-resistant quantum control from geometric curves

13 November 2020

**Author: **Edwin Barnes (Virginia Tech)

**Abstract: **Future technologies such as quantum computing, sensing and communication demand the ability to control microscopic quantum systems with unprecedented accuracy. This task is particularly daunting due to unwanted and unavoidable interactions with noisy environments that destroy quantum information through decoherence. I will present a new theoretical framework for deriving control waveforms that dynamically combat decoherence by driving qubits in such a way that noise effects destructively interfere and cancel out. This theory exploits a rich geometrical structure hidden within the time-dependent Schrödinger equation in which quantum evolution is mapped to geometric curves. Control waveforms that suppress noise can be obtained by drawing closed curves and computing their curvatures. I will show how this can be done for single- and multi-qubit systems.

References

1) https://arxiv.org/abs/2008.01168

## Seminar XXI: Complete Resource Theory of Quantum Incompatibility as Quantum Programmability

6 November 2020

**Author: **Zhoe Wenbin (Nagoya University)

**Abstract: **Given a non-empty closed convex subset F of density matrices, we formulate conditions that guarantee the existence of an F-morphism (namely, a completely positive trace-preserving linear map that maps F into itself) between two arbitrarily chosen density matrices. While we allow errors in the transition, the corresponding map is required to be an exact F-morphism. Our findings, though purely geometrical, are formulated in a resource-theoretic language and provide a common framework that comprises various resource theories, including the resource theories of bipartite and multipartite entanglement, coherence, athermality, and asymmetric distinguishability. We show how, when specialized to some situations of physical interest, our general results are able to unify and extend previous analyses. We also study conditions for the existence of maximally resourceful states, defined here as density matrices from which any other one can be obtained by means of a suitable F-morphism. Moreover, we quantitatively characterize the paradigmatic tasks of optimal resource dilution and distillation, as special transitions in which one of the two endpoints is maximally resourceful.

References

1) https://arxiv.org/abs/2005.09188

## Seminar XX: Nonlinear Bell inequality for macroscopic measurements

30 October 2020

**Author: **Adam Bene Watts (MIT)

**Abstract: **How precise do measurements need to be to detect quantum mechanical effects? In this talk I will describe a device-independent protocol which tests for entanglement in a macroscopic system using only measurements with macroscopic precision. The test involves measurements of the covariance between macroscopic measurements and assumes independence between the microscopic subsystems making up each macroscopic system. As such, it cannot be used to disprove local hidden variable theories, but can be used to certify nonclassical correlations under reasonable experimental assumptions. Time permitting, I will also describe some possible experimental implementations. This talk is based off of joint work with Aram Harrow and Nicole Yunger Halpern.

References

1) https://arxiv.org/abs/1911.09122

## Seminar XIX: The usefulness of negativity: Quantum advantage in post-selected metrology

23 October 2020

**Author: **David Arvidsson-Shukur (University of Cambridge)

**Abstract: **In this talk, I will show that post-selection offers a non-classical advantage in metrology. In every parameter-estimation experiment, the final measurement or the post-processing incurs some cost. Post-selection can improve the rate of Fisher information (the average information learned about an unknown parameter from an experimental trial) to cost. This improvement, we will see, stems from the negativity of the Kirkwood-Dirac (KD) quasi-probability distribution, a quantum extension of a probability distribution. In a classical theory, in which all observables commute, the KD distribution can be expressed as real and non-negative. In a quantum-mechanical theory, however, I will show that non-commutation forces the KD distribution to include negative or non-real quasi-probabilities. The distribution’s non- classically negative values enable post-selected experiments to outperform even post-selection- free experiments whose input states and final measurements are optimised: Post-selected quantum experiments can yield anomalously large information-cost rates. Finally, I will outline a preparation-and-post-selection procedure that can result in an arbitrarily large Fisher information. In collaboration with Aephraim Steinberg’s quantum-optics group, we are currently conducting an experiment to demonstrate this result.

References

1) D. Arvidsson-Shukur et al., Nature Comms., 11, 3775, (2020)

2) https://arxiv.org/abs/1903.02563

3) https://www.cam.ac.uk/research/news/quantum-negativity-can-power-ultra-precise-measurements

## Seminar XVIII: Conditions tighter than non-commutation needed for non-classicality

16 October 2020

**Author: **Jacob Chevalier Drori (University of Cambridge)

**Abstract: **Kirkwood discovered in 1933, and Dirac discovered in 1945, a representation of quantum states that has undergone a renaissance recently. The Kirkwood-Dirac (KD) distribution has been employed to study nonclassicality across quantum physics, from metrology to chaos to the foundations of quantum theory. The KD distribution is a quasiprobability distribution, a quantum generalization of a probability distribution, which can behave nonclassically by having negative or nonreal elements. Negative KD elements signify quantum information scrambling and potential metrological quantum advantages. Nonreal elements encode measurement disturbance and thermodynamic nonclassicality. KD distributions’ nonclassicality has been believed to follow necessarily from noncommutation of operators. We show that noncommutation does not suffice. We prove sufficient conditions for the KD distribution to be nonclassical (equivalently, necessary conditions for it to be classical). We also quantify the KD nonclassicality achievable under various conditions. This work resolves long-standing questions about nonclassicality and may be used to engineer quantum advantages.

References

1) https://arxiv.org/abs/2009.04468

## Seminar XVII: Suppression of phonon-assisted processes in InGaAs

9 October 2020

**Author: **Stuart Holmes (UCL)

**Abstract: **I discuss the metal-to-insulator transition in disordered InGaAs two-dimensional electron gas devices that are thermally isolated from a temperature reservoir. At high temperatures the devices are diffusion dominated, whilst at low temperatures they show insulating behaviour below conductance (s) ~ e2/h. The device characteristics for a localization behaviour cross- over from ‘Mott’-variable range hopping to interaction-dominated, ‘Efros-Shklovskii’ conditions where a Coulomb gap can be observed are studied. The suppression of phonon- assisted processes in this region of transport can lead to the condition s(T) = 0 at finite temperature T with a breakdown in thermalization. Progress towards this transport regime needed to study Many Body Localization effects are discussed with the importance of this for future technology.

## Seminar XVI: Quantum Computing with Graphene Plasmons

2 October 2020

**Author: **Irati Alonso Calafell (University of Vienna)

**Abstract: **Among the various approaches to quantum computing, all-optical architectures are especially promising due to the robustness and mobility of single photons. However, the creation of the two- photon quantum logic gates required for universal quantum computing remains a challenge. Here we propose a universal two-qubit quantum logic gate, where qubits are encoded in surface plasmons in graphene nanostructures, that exploits graphene’s strong third- order nonlinearity and long plasmon lifetimes to enable single-photon-level interactions. In particular, we utilize strong two-plasmon absorption in graphene nanoribbons, which can greatly exceed single-plasmon absorption to create a “square-root-of-swap” that is protected by the quantum Zeno effect against evolution into undesired failure modes. Our gate does not require any cryogenic or vacuum technology, has a footprint of a few hundred nanometers, and reaches fidelities and success rates well above the fault-tolerance threshold, suggesting that graphene plasmonics offers a route towards scalable quantum technologies.

References

1) Alonso Calafell, et al. npj Quantum Information 5, 37 (2019)

## Seminar XV: Large dispersive interaction between a CMOS double quantum dot and microwave photons

25 September 2020

**Author: **David Ibberson (Hitachi)

**Abstract: **We report a large coupling rate, g/2π = 183 MHz, between the charge state of a double quantum dot in a CMOS split-gate silicon nanowire transistor and microwave photons in a lumped-element resonator, which is formed by wire-bonding to a superconducting inductor fabricated on a separate chip. We enhance the coupling by exploiting the large interdot lever arm of an asymmetric split-gate device, α =0.72, and by inductively coupling to the resonator to increase its impedance, Z = 560 Ω. In the dispersive regime, the large coupling strength at the DQD hybridisation point produces a frequency shift comparable to the resonator linewidth, the optimal setting for maximum state visibility. We exploit this regime to demonstrate rapid gate-based readout of the charge degree of freedom, with an SNR of 3.3 in 50 ns. In the resonant regime, the fast charge decoherence rate precludes reaching the strong coupling regime, but we show a clear route to spin-photon circuit quantum electrodynamics using hybrid CMOS systems.

References

1) https://arxiv.org/abs/2004.00334

## Seminar XIV: Noncommuting conserved quantities in quantum many-body thermalization

18 September 2020

**Author: **Nicole Yunger Halpern (ITAMP Harvard)

**Abstract: **In statistical mechanics, a small system exchanges conserved quantities—heat, particles, electric charge, etc.—with a bath. The small system may thermalize to the canonical ensemble, the grand canonical ensemble, etc. The conserved quantities are represented by operators usually assumed to commute with each other. But noncommutation distinguishes quantum physics from classical. What if the operators fail to commute? I will argue, using quantum- information-theoretic thermodynamics, that the small system thermalizes to near a “non- Abelian thermal state.” I will present a protocol for realizing this state experimentally, supported with numerical simulations of a spin chain. The protocol is suited to ultracold atoms, trapped ions, quantum dots, and more. This work introduces a nonclassical phenomenon— noncommutation of conserved quantities—into a decades-old thermodynamics problem.

References

1) NYH, Beverland, and Kalev, Phys. Rev. E 101, 042117 (2020).

2) NYH, Faist, Oppenheim, and Winter, Nat. Comms. 7, 12051 (2016).

3) NYH, J. Phys. A 51, 094001 (2018).

## Seminar XIII: March Meeting Rehearsal

28 February 2020

**Author:** Aleks Lasek

**Title:** GPU-accelerated simulations of realistic quantum systems with a focus on Quantum Information and Computation.

**Abstract: **In this talk we present our method of numerically simulating realistic quantum systems. With recent increases in GPU processing power, we are able to simulate two-particle 2D systems on thousands of spatial sites in a reasonable time. Even though today’s quantum computers can manipulate multiple qubits, the long-term fidelity of many-qubit operations is still poor. Numerical simulations of quantum computers can help guide experiments towards better fidelities. We focus on simulating physical qubits, which leads to a better understanding of where the errors that are observed in quantum computers and experiments come from. We describe the techniques used to make our calculations possible, as well as present simulation results of realistic quantum experiments, including performing an entangling operation on two electron-spin qubits, controlling a charge qubit, and sound driven single electron transfer between quantum rails.

**Author:** Yordan Yordanov

**Title:** Implementing single-qubit POVMs on a circuit-based quantum computer

**Abstract:** We present a deterministic protocol to implement general single-qubit POVMs on near-term circuit-based quantum computers. The protocol has a modular structure, such that an n-element POVM is implemented as a sequence of (n-1) circuit modules. Each module performs a 2-element POVM. Two variations of the protocol are suggested, one optimal in terms of number of ancilla qubits, the other optimal in terms of number of qubit gate operations and quantum circuit depth. We use the protocol to implement 2- and 3-element POVMs on two publicly available quantum computing devices. The results we obtain are positive, and suggest that implementing non-trivial POVMs could be within the reach of current noisy intermediate scale quantum computing devices.

## Seminar XII: Playing with donors and electrons in silicon devices

7 February 2020

**Author:** Thierry Ferrus

**Abstract: **In the history of semiconductors, defects and impurities have often been perceived negatively by the scientific communities and industries, mostly for their detrimental effects on the mobility or the stability of devices. This problem became more acute with downscaling and the development of nanometre scale transistors. This perception remains mostly unchanged until Bruce Kane proposed the use of single ions as the basis for quantum computation 20 years ago. Since then, defects such as the NV centres in diamonds or single implanted donors in silicon have been subjected to intensive attractiveness and thoroughful experimental and theoretical studies with the hope of developing quantum information platforms. However, the major obstacle still remains the ability to individually control them either spatially, optically, electrically or even magnetically. Through the description of some key fabrication techniques, technology advances and well know electronic properties, I will show how one can realise and manipulate single donors, isolated structures or arrays of donors to be used as quantum bits or future memory applications.

## Seminar XI: Can Three-Body Recombination Purify a Quantum Gas?

31 January 2020

**Author:** Lena Dogra

**Abstract: **While dissipation in quantum systems usually limits the observation of coherent phenomena, there are also cases when it can have interesting and beneficial consequences. I will talk about an elegant example of this, where purely quantum-statistical effects cause a notorious dissipative process in a many-body quantum system to act like a less pedantic cousin of Maxwell’s daemon. Specifically, losses from three-body recombination in an ultracold Bose gas could purify the sample, that is, reduce the entropy per particle and increase the Bose-Einstein condensed fraction. Based on: PRL 123, 020405 (2019)

https://arxiv.org/abs/1905.09811

## Seminar X: Entanglement-breaking properties of repeated interaction systems

24 January 2020

**Author:** Yordan Yordanov

**Abstract: **In this week’s seminar I will talk about quantum computational chemistry. More particularly we will look at how the ground energy of a molecule can be found by solving the electronic structure problem using the variational quantum eigensolver (VQE) algorithm. I will explain how the VQE can be implemented on a quantum computer by illustrating how to map a fermionic hamiltonian to a qubit hamiltonian, and how to construct a suitable trial ansatz state.

## Seminar IX: Entanglement-breaking properties of repeated interaction systems

17 January 2020

**Author:** Eric Hanson

**Abstract:** We consider a simple but useful model in which a system of interest interacts with a sequence of identical probes, one after the other, and study what types of induced dynamics eventually break any prior entanglement the system of interest may have with other systems. In this work, all systems are modeled as finite-dimension quantum objects, and the talk will be elementary and accessible. Based on https://arxiv.org/abs/1902.08173.

## Seminar VIII: Hybrid superconducting-semiconducting quantum integrated circuits on two-dimensional electron gas platforms

29 November 2019

**Author:** Kaveh Delfanazari

**Abstract: **In this talk, I will first discuss a device—based on the proximitized superconducting two-dimensional (2D) electron gases in InGaAs heterostructures—design, fabrication, characterization and low-temperature measurements in sub-Kelvin temperature ranges. I will then review our activity and progress, such as the observation of coherent quantum transport, quantum interference, and magnetoconductance oscillations in 2D Josephson junctions. I will summarise by discussing the promising approaches for the realization of the scalable quantum processors based on hybrid super-semi devices.

## Seminar VII: Introduction to Machine Learning and AI

22 November 2019

**Author:** Bono Xu

**Abstract:** I will present an overview of the history of machine learning, from simple logistic regression to neural networks. Some basic concepts in machine learning will be introduced, such as supervised/unsupervised learning and loss-bias compromise. I will also use examples of my current work, which is electric-energy disaggregation in domestic homes, as well as from my previous PhD project, which was on identifying double Higgs production from electron-positron collisions, to illustrate how machine learning can be applied to solve problems in physics and in the real world. Lastly, I will share some more recent developments in neural networks, with examples from state-of-art models and performances.

## Seminar VI: Using a 32-bit magnetic barcode to store binary codes

15 November 2019

**Author:** Peter Newton

**Abstract:** In a recent publication, we introduced a magnetic barcode comprised of 32 composite element bits. We used the magno-optic Kerr effect to characterise the magnetic hysteresis of each bit independently and from this extrapolated 12 bits that could be independently coded as a 1 or a 0. We then used magnetic force microscopy measurements to verify the successful writing of a simple code after application of an appropriate alternating in-plane magnetic field . In this talk I will introduce the design of the magnetic elements, our experimental methods and results, and initiate discussion on the problems inherent with writing to a single bit at a time.

## Seminar V: Quantum Entanglement and Separability in Multipartite States

8 November 2019

**Author:** Niall Devlin

**Abstract: **Quantum Entanglement was identified nearly 90 years ago, yet remains an active topic of research. Fundamental questions regarding the quantification of the entanglement in a quantum state, and even if a state is entangled at all, remain unanswered. In this presentation I will provide an introduction to the concept of bi- and multi-partite entanglement. The latter being the presence of entanglement between more than two subsystems of a Hilbert Space. I will discuss the separability of a quantum state and present an algorithm I developed to determine the separability of a state. I will also review the concept of entanglement in the framework of a Quantum Resource Theory.

## Seminar IV: Sound-driven single-electron transfer in a circuit of coupled quantum rails

1 November 2019

**Author:** Hugo Lepage

**Abstract: **In a recent Nature Communications publication, we presented the experimental demonstration of a single electron directional coupler. Electrons carried by surface acoustic waves (SAWs) enter a tunnel-coupled region and electric voltages on metallic surface gates determine the quantum rail in which the electrons are measured. This transfer mechanism makes SAW technologies a promising candidate to convey quantum information through a circuit of quantum logic gates. High performace computing simulations provide a detailed picture of the behaviour of the electrons as they are carried across the semiconductor device. These simulations show the shortcomings of the current experimental implementations and promising avenues for novel designs that maximize qubit coherence and fidelity.

https://doi.org/10.1038/s41467-019-12514-w

## Seminar III: Introduction to Majorana zero modes and their use in topological-quantum computing

25 October 2019

**Author:** Aleksander Lasek

**Abstract:** I will present an introduction to Majorana zero modes (MZM), which are non Abelian quasi-particles (anyons). I will give an overview of the theory and explain how they can be used as qubits to implement a topological quantum computer, which uses braiding operations on anyons as logic gates. This has the advantage of more noise resistance compared to a quantum computer using energy eigenstates, as small perturbations will not change topological properties. I plan to simulate quantum MZMs devices that are studied at the Cavendish by Dr. K. Delfanazari and others.

## Seminar II: Implementing generalised measurements (POVMs) on quantum processors

18 October 2019

**Author:** Yordan Yordanov

**Abstract: **I will present a protocol for efficient implementation of general single-qubit POVMs on circuit-based quantum computers. I will give a brief theoretical introduction, and outline the method I developed. Then I will describe explicitly some simple examples, and present results from implementing these examples on the IBM and Rigetti quantum computers.

## Seminar I: Quantum Metrology

11 October 2019

**Author:** David Arvidsson Shukur

**Abstract: **I will give an introductory seminar on quantum metrology and Fisher information. Metrology concerns the measurement or estimation of physical parameters. Quantum metrology relies on quantum phenomena to improve estimations beyond classical bounds. The talk will be general and hopefully relevant to experimentalists as well as theorists.