Current graduate students

CS/Math Seminar - Yu Tong, Caltech

Quantum-Nano Centre, 200 University Ave West, Room QNC 1201 + ZOOM Waterloo, ON CA N2L 3G1

In the last few years a number of works have proposed and improved provably efficient algorithms for learning the Hamiltonian from real-time dynamics. In this talk, I will first provide an overview of these developments, and then discuss how the Heisenberg limit, the fundamental precision limit imposed by quantum mechanics, can be reached for this task. I will demonstrate how the Heisenberg limit requires techniques that are fundamentally different from previous ones, and the important roles played by quantum control and thermalization. I will also discuss open problems that are crucial to making these algorithms implementable on current devices.

Variational methods for quantum sensing

Quantum-Nano Centre, 200 University Ave West, Room QNC 1201 Waterloo, ON CA N2L 3G1

The precise estimation of unknown physical quantities is foundational across science and technology. Excitingly, by harnessing carefully-prepared quantum correlations, we can design and implement sensing protocols that surpass the intrinsic precision limits imposed on classical approaches. Applications of quantum sensing are myriad, including gravitational wave detection, imaging and microscopy, geoscience, and atomic clocks, among others.

However, current and near-term quantum devices have limitations that make it challenging to capture this quantum advantage for sensing technologies, including noise processes, hardware constraints, and finite sampling rates. Further, these non-idealities can propagate and accumulate through a sensing protocol, degrading the overall performance and requiring one to study protocols in their entirety.

In recent work [1], we develop an end-to-end variational framework for quantum sensing protocols. Using parameterized quantum circuits and neural networks as adaptive ansätze of the sensing dynamics and classical estimation, respectively, we study and design variational sensing protocols under realistic and hardware-relevant constraints. This seminar will review the fundamentals of quantum metrology, cover common sensing applications and protocols, introduce and benchmark our end-to-end variational approach, and conclude with perspectives on future research.

[1] https://arxiv.org/abs/2403.02394

Random state generation for quantum key distribution using weak coherent pulse source

Quantum-Nano Centre, 200 University Ave West, Room QNC 2101
Waterloo, ON CA N2L 3G1

Supervisor: Thomas Jennewein

IQC Seminar - Jameson O'Reilly, Duke University

Quantum-Nano Centre, 200 University Ave West, Room QNC 0101  Waterloo, ON CA N2L 3G1

Trapped atomic ions are a leading candidate platform for quantum simulation and computing but system sizes are limited by motional mode crowding and transport overhead. Multiple reasonably-sized, well-controlled modules can be connected into one universal system using photonic interconnects, in which photons entangled with ions in each trap are collected into and detected in a Bell-state analyzer. The speed of these interconnects has heretofore been limited by the use of 0.6 NA objectives and the need to periodically pause entanglement attempts for recooling. In this work, we use a system with two in-vacuo 0.8 NA lenses on either side of an ion trap to collect 493 nm photons from barium ions and demonstrate the most efficient free-space ion trap photonic interconnect to date. In addition, we introduce an ytterbium ion as a sympathetic coolant during the entangling attempts cycle to remove the need for recooling, enabling a record photon-mediated entanglement rate between two trapped ions. The major remaining error source is imperfections in the photon polarization encoding, so we also develop a new protocol for remotely entangling two ions using time-bin encoded photons and present preliminary results of an experimental implementation. Finally, we prepare the first remote entangled state involving two barium ions in separate vacuum chambers.

Overparameterization and Expressivity of Realistic Quantum Systems

Quantum-Nano Centre, 200 University Ave West, Room QNC 1201 Waterloo, ON CA N2L 3G1

Quantum computing devices require exceptional control of their experimental parameters to prepare quantum states and simulate other quantum systems, in particular while subject to noise. Of interest here are notions of trainability, how difficult is it to classically optimize parameterized, realistic quantum systems to represent target states or operators of interest, and expressivity, how much of a desired set of these targets is our parameterized ansatze even capable of representing? We observe that overparameterization phenomena, where systems are adequately parameterized, are resilient in noisy settings at short times and optimization can converge exponentially with circuit depth. However fidelities decay to zero past a critical depth due to accumulation of either quantum or classical noise. To help explain these noise-induced phenomena, we introduce the notion of expressivity of non-unitary, trace preserving operations, and highlight differences in average behaviours of unitary versus non-unitary ensembles. We rigorously prove that highly-expressive noisy quantum circuits will suffer from barren plateaus, thus generalizing reasons behind noise-induced phenomena. Our results demonstrate that appropriately parameterized ansatze can mitigate entropic effects from their environment, and care must be taken when selecting ansatze of channels.

IQC Special Colloquium - Ivana Dimitrova, Harvard University

Quantum-Nano Centre, 200 University Ave West, Room QNC 0101 Waterloo, ON CA N2L 3G1

Scaling up the number of qubits available in experimental systems is one of the most significant challenges in quantum computation. A promising path forward is to modularize the quantum processors and then connect many processors using quantum channels, realized using photons and optical fibers. For Rydberg atom arrays, one of the leading platforms for quantum information processing, this could be done by developing an interface for photons, such as an optical cavity. In addition, an optical cavity can be used for fast mid-circuit readout for error detection. In this talk, I will discuss recent progress with two types of cavities and their feasibility as a photonic link. First, we show coherent control of Rydberg qubits and two-atom entanglement as close as 130um away from a nanophotonic cavity. Second, we show fast high-fidelity qubit state readout at a fiber Fabry Perot cavity. In addition, a fiber cavity also allows for cavity-mediated atom-atom gates, which could enable novel quantum networking capabilities. 

CS/Math Seminar - Lauritz van Luijk, Leibniz Universität Hannover

Quantum-Nano Centre, 200 University Ave West, Room QNC 1201 + ZOOM Waterloo, ON CA N2L 3G1

Embezzlement refers to the counterintuitive possibility of extracting entangled quantum states from a reference state of an auxiliary system (the "embezzler") via local quantum operations while hardly perturbing the reference. I will explain a deep connection between this operational task and the mathematical classification of von Neumann algebras.

This result implies that relativistic quantum fields are universal embezzlers: Any entangled state of any dimension can be embezzled from them with arbitrary precision. In particular, this provides an operational characterization of the infinite amount of entanglement present in the vacuum state of relativistic quantum field theories and explains the classic result that the vacuum maximally violates Bell's inequalities: Alice and Bob can simply embezzle a maximally entangled qubit pair and perform a Bell measurement.

The talk is based on joined work with A Stottmeister, RF Werner, and H Wilming (see arXiv:2401.07292, arXiv:2401.07299).

IQC Seminar - Zohreh Davoudi, University of Maryland

Quantum-Nano Centre, 200 University Ave West, Room QNC 1201 Waterloo, ON CA N2L 3G1

Quantum computing gauge theories of relevance to Nature requires a range of theoretical and algorithmic developments to make simulations amenable in the near and far terms. With a focus on the SU(2) lattice gauge theory with matter, I will motivate the need for efficient theoretical formulations, introduce general quantum algorithms that can simulate them efficiently, and discuss strategies for analyzing the required quantum resources accurately. These considerations will be of relevance to simulating other gauge theories of increasing complexity, including quantum chromodynamics.

IQC Special Colloquium Leo Zhou, California Institute of Technology

Quantum-Nano Centre, 200 University Ave West, Room QNC 1201 Waterloo, ON CA N2L 3G1

Finding the minimum of the energy of a many-body system is a fundamental problem in many fields. Although we hope a quantum computer can help us solve this problem faster than classical computers, we have a very limited understanding of where a quantum advantage may be found. In this talk, I will present some recent theoretical advances that shed light on quantum advantages in this domain. First, I describe rigorous analyses of the Quantum Approximate Optimization Algorithm applied to minimizing energies of classical spin glasses. For certain families of spin glasses, we find the QAOA has a quantum advantage over the best known classical algorithms. Second, we study the problem of finding a local minimum of the energy of quantum systems. While local minima are much easier to find than ground states, we show that finding a local minimum under thermal perturbations is computationally hard for classical computers, but easy for quantum computers. These results highlight exciting new directions in leveraging physics-inspired algorithms to achieve quantum advantages in broadly useful problems.

IQC Seminar - Ashutosh Marwah, University of Montreal

Quantum-Nano Centre, 200 University Ave West, Room QNC 1201
Waterloo, ON CA N2L 3G1

For a state $\rho_{A_1^n B}$, we call a sequence of states $(\sigma_{A_1^k B}^{(k)})_{k=1}^n$ an approximation chain if for every $1 \leq k \leq n$, $\rho_{A_1^k B} \approx_\epsilon \sigma_{A_1^k B}^{(k)}$. In general, it is not possible to lower bound the smooth min-entropy of such a $\rho_{A_1^n B}$, in terms of the entropies of $\sigma_{A_1^k B}^{(k)}$ without incurring very large penalty factors. In this paper, we study such approximation chains under additional assumptions. We begin by proving a simple entropic triangle inequality, which allows us to bound the smooth min-entropy of a state in terms of the R\'enyi entropy of an arbitrary auxiliary state while taking into account the smooth max-relative entropy between the two. Using this triangle inequality, we create lower bounds for the smooth min-entropy of a state in terms of the entropies of its approximation chain in various scenarios. In particular, utilising this approach, we prove approximate versions of the asymptotic equipartition property and entropy accumulation. In a companion paper, we show that the techniques developed in this paper can be used to prove the security of quantum key distribution in the presence of source correlations.