Events

Tight bounds for Pauli channel learning with and without entanglement

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

Quantum entanglement is a crucial resource for learning properties from nature, but a precise characterization of its advantage can be challenging. In this work, we consider learning algorithms without entanglement as those that only utilize separable states, measurements, and operations between the main system of interest and an ancillary system. Interestingly, these algorithms are equivalent to those that apply quantum circuits on the main system interleaved with mid-circuit measurements and classical feedforward. Within this setting, we prove a tight lower bound for Pauli channel learning without entanglement that closes the gap between the best-known upper bound. In particular, we show that Θ(n^2/ε^2) rounds of measurements are required to estimate each eigenvalue of an n-qubit Pauli channel to ε error with high probability when learning without entanglement. In contrast, a learning algorithm with entanglement only needs Θ(1/ε^2) copies of the Pauli channel. Our results strengthen the foundation for an entanglement-enabled advantage for Pauli noise characterization. We will talk about ongoing experimental progress in this direction.

Reference: Mainly based on [arXiv: 2309.13461]

Optimization of the Optical Infrastructure and Trapping Potential for a Quantum Processor Based on Trapped Barium Ions

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

Supervisor: Kazi Rajibul Islam

Creating and probing laser-cooled atomic ensembles inside a hollow-core optical fibre

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

Supervisor: Michal Bajcsy

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

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)

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.

IQC Colloquium - Sisi Zhou, The Perimeter Institute

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

 Quantum metrology studies estimation of unknown parameters in quantum systems. The Heisenberg limit of estimation precision 1/N, with N being the number of probes, is the ultimate sensing limit allowed by quantum mechanics that quadratically outperforms the classically-achievable standard quantum limit 1/√N. The Heisenberg limit is attainable using multi-probe entanglement in the ideal, noiseless case. However, in presence of noise, many quantum systems only allow a constant factor of improvement over the standard quantum limit. To elucidate the noise effect in quantum metrology, we prove a necessary and sufficient condition for achieving the Heisenberg limit using quantum controls. We show that when the condition is satisfied, there exist quantum error correction protocols to achieve the Heisenberg limit; when the condition is violated, no quantum controls can break the standard quantum limit (although quantum error correction can be used to maximize the constant-factor improvement). We will also discuss the modified sensing limits when only restricted types of quantum controls can be applied. 

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.