Events

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 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.

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

Rajibul Islam
Faculty, Institute for Quantum Computing
Associate Professor, Department of Physics and Astronomy, University of Waterloo
Co-founder, Open Quantum Design

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

Quantum computing promises to advance our computational abilities significantly in many high-impact research areas. In this period of rapid development, the experimental capabilities needed to build quantum computing devices and prototypes are highly specialized and often difficult to access. In this public talk, we'll discuss how to build quantum computing devices one atom a time using the ion-trap approach. We'll show how we build quantum bits out of individually isolated atoms, explore how we use them to simulate other complex systems, and showcase how we're building open-access hardware to advance research in this exciting field.

Design and Implementation of an Experimental Setup for Entanglement Harvesting

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

Supervisor: Adrian Lupascu

Developing a robust quantum simulator with trapped ions

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

Supervisor: Rajibul Islam

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

CS/Math Seminar - Avantika Agarwal IQC

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

The Polynomial-Time Hierarchy (PH) is a staple of classical complexity theory, with applications spanning randomized computation to circuit lower bounds to ''quantum advantage'' analyses for near-term quantum computers. Quantumly, however, even though at least four definitions of quantum PH exist, it has been challenging to prove analogues for these or even basic facts from PH. This work studies three quantum-verifier based generalizations of PH, two of which are from [Gharibian, Santha, Sikora, Sundaram, Yirka, 2022] and use classical strings (QCPH) and quantum mixed states (QPH) as proofs, and one of which is new to this work, utilizing quantum pure states (pureQPH) as proofs. We first talk about solutions to open problems from GSSSY22 which include a collapse theorem for QCPH and a quantum-classical Karp-Lipton. We then talk about our results for pureQPH, including lower bounds relating QCPH to pureQPH, and finally discuss some interesting open problems related to QCPH. This talk is based on https://arxiv.org/abs/2401.01633, a joint work with Sevag Gharibian, Venkata Koppula and Dorian Rudolph.

CS/MATH Seminar - Kieran Mastel from IQC ZOOM + IN PERSON

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

The recent MIP*=RE theorem of Ji, Natarajan, Vidick, Wright, and Yuen shows that the complexity class MIP* of multiprover proof systems with entangled provers contains all recursively enumerable languages. In prior work Grilo, Slofstra, and Yuen showed (via a technique called simulatable codes) that every language in MIP* has a perfect zero knowledge (PZK) MIP* protocol.  The MIP*=RE theorem uses two-prover one-round proof systems, and hence such systems are complete for MIP*. However, the construction in Grilo, Slofstra, and Yuen uses six provers, and there is no obvious way to get perfect zero knowledge with two provers via simulatable codes. This leads to a natural question: are there two-prover PZK-MIP* protocols for all of MIP*?

In this talk we answer the question in the affirmative. For the proof, we use a new method based on a key consequence of the MIP*=RE theorem, which is that every MIP* protocol can be turned into a family of boolean constraint system (BCS) nonlocal games. This makes it possible to work with MIP* protocols as boolean constraint systems, and in particular allows us to use a variant of a construction due to Dwork, Feige, Kilian, Naor, and Safra which gives a classical MIP protocol for 3SAT with perfect zero knowledge. To show quantum soundness of this classical construction, we develop a toolkit for analyzing quantum soundness of reductions between BCS games, which we expect to be useful more broadly. This talk is based on joint work with William Slofstra