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Wednesday, March 8, 2023 12:00 pm - 1:00 pm EST (GMT -05:00)

IQC Student Seminar featuring Ramy Tannous

So you want to build a satellite?

Are you curious about the Quantum Encryption and Science Satellite mission, also known as QEYSSat? Are you wondering "Why put quantum in space"? Or perhaps you are curious to know what it takes to put quantum hardware in space? In this talk, we will discuss why it is advantageous to have quantum in space. We will also explore the various design challenges that need to be considered for space hardware. Finally, we will discuss the history of quantum space activities at the Institute for Quantum Computing, particularly QEYSSat, which is a joint project between the Canadian Space Agency (CSA) and the University of Waterloo.

Wednesday, March 15, 2023 11:00 am - 12:00 pm EDT (GMT -04:00)

Quantum Matters Seminar Series: Language Models for Quantum Simulation

Roger Melko: Language models for quantum simulation

Abstract: As the frontiers of artificial intelligence advance more rapidly than ever before, generative language models like ChatGPT are poised to unleash vast economic and social transformation. In addition to their remarkable performance on typical language tasks (such as writing undergraduate research papers), language models are being rapidly adopted as powerful ansatze states for quantum many-body systems.  In this talk, I will discuss the use of language models for learning quantum states realized in experimental Rydberg atom arrays. By combining variational optimization with data-driven learning using qubit projective measurements, I will show how language models are poised to become one of the most powerful computational tools in our arsenal for the design and characterization of quantum simulators and computers.

Tuesday, March 21, 2023 12:00 pm - 1:00 pm EDT (GMT -04:00)

IQC Student Seminar featuring Aditya Jain

Improved diagnostics and implementation for quantum error correction

Abstract: Fault-tolerant quantum computing will require accurate estimates of the resource overhead, but standard metrics such as gate fidelity and diamond distance have been shown to be poor predictors of logical performance. We present a scalable experimental approach based on Pauli error reconstruction to predict the performance of concatenated codes. Numerical evidence demonstrates that our method significantly outperforms predictions based on standard error metrics for various error models, even with limited data. We illustrate how this method assists in the selection of error correction schemes.
Tuesday, March 28, 2023 12:00 pm - 1:00 pm EDT (GMT -04:00)

IQC Student Seminar featuring Junan Lin

Characterizing states and measurements: principles and approaches

Abstract: The problem of separately characterizing state preparation and measurement (SPAM) processes has not been frequently discussed in the literature. In this talk, I will first review the theoretical challenge behind SPAM characterization due to a gauge freedom, and then describe two different principles that can be applied to get around it. The first one can be understood as an effective propagation of state preparation noise from the target system to an ancillary qubit, whereas the second one utilizes measurements and post-selection to reduce the state preparation noise and can be interpreted as a form of algorithmic cooling. For the first method, I will present experimental and simulation data obtained from real quantum processors. For the second method, I will analyze its overhead through an upper bound on the expected number of runs to achieve a given error-reduction ratio.
Wednesday, April 5, 2023 11:00 am - 12:00 pm EDT (GMT -04:00)

Quantum Matters Seminar Series: Strange metals from not-so-strange quasiparticles

Brad Ramshaw

Abstract: Strange metals have linear-in-temperature (T-linear) down to low temperature. Strange metals are found in many families of correlated electron materials, leading to the conjecture that a universal bound - the "Planckian" bound - limits the scattering rate of electrons to a value set by fundamental constants. If the Planckian bound exists, it would provide a natural explanation for why a host of seemingly disparate systems, including high-temperature superconductors and twisted bilayer graphene, all have T-linear resistivity. Perhaps more dramatically, T-linear resistivity suggests that electron-electron interactions are so strong that conventional concepts such as quasiparticles and Boltzmann transport do not apply in strange metals. I will present our work on the cuprate Nd-LSCO and the 5-layer superconducting nickelate that shows that conventional quasiparticle transport is alive and well, even in the strange metal regime where the Planckian bound is saturated. This suggests that we may not need to abandon the quasiparticle picture entirely, but that we need to better understand the source of scattering in these materials. 

Wednesday, April 5, 2023 12:00 pm - 1:00 pm EDT (GMT -04:00)

IQC Student Seminar Featuring Xi Dai

Landau-Zener tunneling: from weak to strong environment coupling

Abstract:

Landau-Zener tunneling, which describes the transitions in a two-level system during the passage through an anti-crossing, is a model applicable to a wide range of physical phenomena. Dissipation due to coupling between the system and the environment is an important factor in determining the transition rates. Using a superconducting tunable capacitively shunted flux qubit, we observe the crossover from weak to strong coupling to the environment. The weak coupling limit corresponds to small system-environment coupling and leads to environment-induced thermalization. In the strong coupling limit, environmental polarizations dress the system and transitions occur between the dressed states. Our results confirm previous theoretical studies of dissipative Landau-Zener tunneling in the weak and strong coupling limits, and motivate further work on understanding the intermediate regime. This work is relevant for understanding the role of open system effects in quantum annealing, where Landau-Zener transitions at small gaps, occurring in large scale systems, are important to understand for improving the success probability.

 

This talk is based on the preprint arXiv:2207.02017.

Debanjan Chowdhury: The good, the bad and the strange: Unconventional metallic behaviour in the vicinity of Mott insulators

Abstract: In recent years, we have witnessed remarkable experimental breakthroughs in uncovering the intriguing properties of correlated metals in the vicinity of Mott transitions. Describing these phenomena theoretically remains an open challenge. This talk will focus on three recent examples of puzzling electronic behavior near Mott insulating phases and address the various conundrums. In the first part of the talk, I will discuss the microscopic origin of an unconventional T-linear resistivity with Planckian scattering in a quasi-two-dimensional “good” metal with long mean-free path, consisting of highly conducting metallic and Mott insulating layers, respectively. In the second part, I will address the origin of a low-temperature “bad” metallic behavior in the vicinity of a continuous bandwidth-tuned metal-insulator transition in a moiré semiconductor. I will end by presenting some new theoretical insights into the experimental observation of an anomalous particle-hole continuum and overdamped plasmon in the density response of cuprate “strange” metals. 

Wednesday, April 19, 2023 12:00 pm - 1:00 pm EDT (GMT -04:00)

IQC Student Seminar featuring Andrew Jena

AEQuO: A Comprehensive Measurement Allocation Protocol

Abstract: The variational quantum eigensolver (VQE) is a hybrid quantum-classical algorithm for solving the electronic structure problem, a problem foundational to the field of computational chemistry. In the VQE, a classical optimizer directs the state preparation protocol while a quantum device is used to measure the expectation value of the Hamiltonian with the prepared state. We developed the adaptive estimation of quantum observables (AEQuO) protocol in an attempt to use all of information from the measurement outcomes to minimize the number of measurements needed. I will give an overview of the VQE and the techniques we employ in our protocol, and I will discuss where our approach fits in with the previous measurement allocation techniques.
Tuesday, April 25, 2023 12:00 pm - 1:00 pm EDT (GMT -04:00)

IQC Student Seminar featuring Emma Bergeron

Development of InSb Surface Quantum Wells for hybrid superconducting device applications. 

Abstract: Surface quantum well (QW) heterostructures in III-V semiconductors are compatible with proximitized superconductivity and offer a scalable planar platform for superconductor-semiconductor systems, such as those suggested for topological quantum computation and those suitable for topological phase transitions involving Majorana zero modes. Amongst III-V binary semiconductors, Indium Antimonide (InSb) has the smallest electron effective mass, highest spin orbit coupling and largest Land´e g-factor. Such material properties makes the pursuit of InSb QWs desirable for a number of quantum device applications, including quantum sensing, quantum metrology, and quantum computing.

Unfortunately, high quality two-dimensional electron gases (2DEGs) in InSb QWs have so far been difficult to realize. InSb QWs have generally relied on the use of modulation doping for 2DEG formation, but these structures have frequently reported issues with parasitic parallel conduction and unstable carrier densities. We report on the transport characteristics of field effect 2DEGs in surface InSb quantum wells which overcome these challenges and are suitable for future hybrid superconducting device applications.

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Wednesday, April 26, 2023 11:00 am - 12:00 pm EDT (GMT -04:00)

Quantum Matters Seminar Series: No, you have not discovered a Majorana Fermion

No, you have not discovered a Majorana Fermion

Abstract: Is what I tell myself. There was a time when I thought I may have discovered it, others did too. Around 2012 several groups including ours found evidence of these quantum excitations in electrical circuits containing nanowires of semiconductor covered by a superconductor. The dramatic signatures were peaks in conductance that appeared under conditions expected from theory for Majorana modes, which are their own anti-modes and may possess non-Abelian properties. But a few years later, similar features in the data were identified due to an interesting, but a more mundane effect - which we call trivial states such as Andreev bound states. Over time more and more data pointed at the trivial and not at the exotic explanation. But because Majorana claims kept coming, this led to some digging and even retractions. What we learned after 10 years is that we have a much better handle on what effects show up in these nanowires, which positions us well for the ultimate Majorana discovery which we should be able to tell apart from all the non-Majorana things we saw. The second lesson we learned is that materials quality of device constituents, superconductors and semiconductors, as well as how samples are fabricated - are the make-or-break factors for making this happen. So while  I cannot report an exciting physics discovery, I can walk you through the scientific process that took place, a 10-year event of independent value which taught me how to do science better.