Current graduate students

Improving the Fidelity of CNOT Circuits on NISQ Hardware

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

We introduce an improved CNOT synthesis algorithm that considers nearest-neighbour interactions and CNOT gate error rates in noisy intermediate-scale quantum (NISQ) hardware. Our contribution is twofold. First, we define a \Cost function by approximating the average gate fidelity Favg. According to the simulation results, \Cost fits the error probability of a noisy CNOT circuit, Prob = 1 - Favg, much tighter than the commonly used cost functions. On IBM's fake Nairobi backend, it fits Prob with an error at most 10^(-3). On other backends, it fits Prob with an error at most 10^(-1). \Cost accounts for the machine calibration data, and thus accurately quantifies the dynamic error characteristics of a NISQ-executable CNOT circuit. Moreover, it circumvents the computation complexity of calculating Favg and shows remarkable scalability. 

Second, we propose an architecture-aware CNOT synthesis algorithm, NAPermRowCol, by adapting the leading Steiner-tree-based synthesis algorithms. A weighted edge is used to encode a CNOT gate error rate and \Cost-instructed heuristics are applied to each reduction step. Compared to IBM's Qiskit compiler, it reduces \Cost by a factor of 2 on average (and up to a factor of 8.8). It lowers the synthesized CNOT count by a factor of 13 on average (up to a factor of 162). Compared with algorithms that are noise-agnostic, it is effective and scalable to improve the fidelity of CNOT circuits. Depending on the benchmark circuit and the IBM backend selected, it lowers the synthesized CNOT count up to 56.95% compared to ROWCOL and up to 21.62% compared to PermRowCol. It reduces the synthesis \Cost up to 25.71% compared to ROWCOL and up to 9.12% compared to PermRowCol. NAPermRowCol improves the fidelity and execution time of a synthesized CNOT circuit across varied NISQ hardware. It does not use ancillary qubits and is not restricted to certain initial qubit maps. It could be generalized to route a more complicated quantum circuit, and eventually boost the overall efficiency and accuracy of quantum computing on NISQ devices. 

Joint-work with: Dohun Kim, Minyoung Kim, and Michele Mosca

IQC Special Colloquium - Aziza Suleymanzade, Harvard University

200 University Ave W. Waterloo ON - ZOOM only

The experimental development of quantum networks marks a significant scientific milestone, poised to enable secure quantum communication, distributed quantum computing, and entanglement-enhanced nonlocal sensing. In this talk, I will discuss the recent advancements in the field along with the outstanding challenges through my work on two different platforms: Silicon Vacancy defects in diamond nanophotonic cavities and Rydberg atoms coupled to hybrid cavities. First, I will present our recent results on distributing entanglement across a two-node network with on-chip solid-state defects in cavities which we built at Harvard. We demonstrated high-fidelity entanglement between communication and memory qubits and showed long-distance entanglement over the 35 km of deployed fiber in the Cambridge/Boston area. Second, I will describe our work at the University of Chicago on using Rydberg atoms as transducers of quantum information between optical and microwave photons, with the goal of integrating Rydberg platforms with superconducting circuits and paving the way for advanced quantum network architectures. The talk will conclude with a perspective on the potential of this hybrid platform approach in constructing quantum networks, highlighting the uncharted scientific and technological opportunities it could unlock.

CS/Math Seminar - Kunal Marwaha - University of Chicago

200 University Ave. Waterloo ON. QNC 1201 + ZOOM

We study a variant of QMA where quantum proofs have non-negative amplitudes in both completeness and soundness. This class was introduced by Jeronimo and Wu [STOC '23] to understand QMA(2). We show that this variant is very powerful even without considering multiple unentangled quantum provers. In fact, QMA+ with some constant gap is equal to NEXP, even though QMA+ with some other constant gap is equal to QMA.

The Clifford theory of the n-qubit Clifford group

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

The n-qubit Pauli group and its normalizer the n-qubit Clifford group have applications in quantum error correction and device characterization. Recent applications have made use of the representation theory of the Clifford group. We apply the tools of (the coincidentally named) Clifford theory to examine the representation theory of the Clifford group using the much simpler representation theory of the Pauli group. We find an unexpected correspondence between irreducible characters of the n-qubit Clifford group and those of the (n + 1)-qubit Clifford group. This talk will rely on the explanation of Clifford theory given last week.

IQC Seminar - Zahra Khanian, Technical University of Munich

200 University Ave W. Waterloo On Can QNC 1201

In the seminal 1948 paper "a mathematical theory of communication", Shannon introduced the concept of a classical source as a random variable and established its optimal compression rate, given by Shannon entropy. Nearly five decades later, Schumacher rigorously defined the notion of a quantum source and its compressibility. Schumacher's definition involved a quantum system and correlations with a purifying reference system. In our work, we build upon Schumacher's quantum source model, extending it to the most general form allowed by quantum mechanics. This extension involves considering the source and the reference in a mixed state, along with the presence of additional systems treated as side information. We address and solve various problems posed by these modifications, determining the optimal compression rates. While our work contributes significant progress in quantum source compression, we point out remaining open questions that require further exploration.

A quick introduction to Clifford theory

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

Clifford theory studies the connection between representations of a group and those of its normal subgroups. In recent work, I examined the Clifford theory of the Clifford group to determine parts of its character table for future applications. The goal of this talk is to introduce the representation theory and Clifford theory of finite groups sufficiently to understand next week's talk when I will explain the Clifford theory of the n-qubit Clifford group. Note that these are two distinct Cliffords. I may also briefly discuss the applications of Clifford theory in quantum error correction, time permitting.

Research Advancement Centre, 475 Wes Graham Way, Room RAC 2009, Waterloo, ON, CA N2L 6R2

Graph-Theoretic Techniques for Optimizing NISQ Algorithms

Supervisor: Dr. Michele Mosca. Thesis available from mgo@uwaterloo.ca. Oral defence Friday, February 2, 9:00 a.m., online.

Unruh phenomena and thermalization for qudit detectors

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

The Unruh effect is the flat space analogue to Hawking radiation, describing how an observer in flat spacetime perceives the quantum vacuum state to be in a thermal state when moving along a constantly accelerated trajectory. This effect is often described operationally using the qubit-based Unruh-DeWitt detector.

We study Unruh phenomena for more general qudit detectors coupled to a quantized scalar field, noting the limitations to the utility of the detailed balance condition as an indicator for Unruh thermality of higher-dimensional qudit detector models. We illustrate these limitations using two types of qutrit detector models based on the spin-1 representations of SU(2) and the non-Hermitian generalization of the Pauli observables (the Heisenberg-Weyl operators).

[2309.04598] Unruh phenomena and thermalization for qudit detectors (arxiv.org)

Development of III-V semiconductor surface quantum wells for hybrid superconducting device applications

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

Supervisor: Jonathan Baugh