Seminar

Impromptu Whiteboard Poster Session

Quantum Nano Centre (QNC) Room 1201, 200 University Avenue West, Waterloo, ON

This week’s student seminar will take place in the form of an impromptu whiteboard poster session, where attendees will be divided into groups and will discuss each other's current work using the whiteboard. This is to encourage students to talk about their work in progress, and practice communication skills by talking to non-experts (quantum is a big field!). As always, pizza will be provided for attendees after the seminar.

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Using Symmetries to Improve Quantum de Finetti Reductions

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

Kim de Laat, University of Waterloo

Quantum Nano Centre (QNC) Room 0101, 200 University Avenue West, Waterloo, ON

The field of quantum computing has a unique opportunity to pre-empt many of the inequities that have riddled AI and computer science. But radical technologies require new, radical solutions. In this talk, I take issue with the leaky pipeline metaphor as a way of structuring policy interventions concerning inequality in STEM fields. I outline three reasons why overreliance on the leaky pipeline metaphor is problematic: (1) it does not accurately represent the phenomenon it is meant to describe; (2) it is incomplete; and (3) it does not capture the full heterogeneity of experiences with inequality in STEM disciplines. I conclude the talk by sharing feedback from the quantum technology community concerning potential pitfalls in the pursuit of equity in quantum, and what we can do about it.

Programmable Individual Optical Addressing for Trapped-ion Quantum Information Processors

Trapped ions are among the most advanced platforms for quantum computation and simulation. Programmable, arbitrary, and precise control—usually through laser-induced light-matter interaction—is required to tune ion-ion interactions. These interactions translate into diverse parameters of the system under study. Current technologies grapple with scalability issues in large ion chains and with "crosstalk" due to micron-level inter-ion separation.

In this talk, we present our development of two optical addressing systems optimized for non-coherent and coherent quantum controls, respectively.

The first addressing system employs a reprogrammable hologram to modulate the wavefront of the addressing beam, thereby engineering the amplitude and phase profile of light across the ion chain. Our implementation compensates for optical aberrations in the system down to λ/20 RMS and exhibits less than 10⁻⁴ intensity cross-talk error. This results in more than 99.9% fidelity when resetting the state or 99.66% when reading out the state of an individual ion without influencing adjacent ions. This scheme can be readily extended to over a hundred ions and adapted to other platforms, such as neutral atom arrays.

Additionally, we introduce another addressing design, tailored for coherent quantum operations through Raman transitions. This design uses a mirrored acoustic-optical deflector (AOD) setup to optimize optical power scaling and sidestep the undesired site-dependent frequency shift commonly observed in AOD-based setups.

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Critical Phase and Spin Sharpening in SU(2)-Symmetric Monitored Quantum Circuits

Monitored quantum circuits exhibit entanglement transitions at certain measurement rates. Such a transition separates phases characterized by how much information an observer can learn from the measurement outcomes. We study SU(2)-symmetric monitored quantum circuits, using exact numerics and a mapping onto an effective statistical-mechanics model. Due to the symmetry's non-Abelian nature, measuring qubit pairs allows for nontrivial entanglement scaling even in the measurement-only limit. We find a transition between a volume-law entangled phase and a critical phase whose diffusive purification dynamics emerge from the non-Abelian symmetry. Additionally, we identify a “spin-sharpening transition.” Across the transition, the rate at which measurements reveal information about the total spin quantum number changes parametrically with system size.

Reference https://journals.aps.org/prb/abstract/10.1103/PhysRevB.108.054307

IQC Seminar - Sahel Ashhab, National Institute of Information and Communications, Japan

Superconducting qubits are based on nonlinear electric circuits that support multiple quantum states. Although only two states are used to realize a qubit, it is possible to utilize more states and realize qudits (d-level quantum systems). We have proposed and experimentally demonstrated optimized implementations of qutrit gates. We have also investigated the time-optimal implementation of two-qubit gates in weakly anharmonic superconducting circuits.

Positivity and Sum-of-Squares in Quantum Information

A multivariate polynomial is said to be positive if it takes only non-negative values over reals. Hilbert's 17th problem concerns whether every positive polynomial can be expressed as a sum of squares of other polynomials. In general, we say a noncommutative polynomial is positive (resp. matrix positive) if plugging operators (resp. matrices) always yields a positive operator. Many problems in math and computer science are closely connected to deciding whether a given polynomial is positive and finding certificates (e.g., sum-of-squares) of positivity.

In the study of nonlocal games in quantum information, we are interested in tensor product of free algebras. Such an algebra models a physical system with two spatially separated subsystems, where in each subsystem we can make different quantum measurements. The recent and remarkable MIP*=RE result shows that it is undecidable to determine whether a polynomial in a tensor product of free algebras is matrix positive. In this talk, I'll present joint work with Arthur Mehta and William Slofstra, in which we show that it is undecidable to determine positivity in tensor product of free algebras. As a consequence, there is no sum-of-square certificate for positivity in such algebras.

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CS/Math Seminar - Rajesh Pereira, the University of Guelph

We explore two convex subsets of the sets of quantum channels. The set of mixed unitary channels and the set of entanglement breaking channels. We study the geometry of these sets, and show how certain geometric and spectral properties of these sets can be studied using positive definite functions, stochastic matrices and other mathematical tools.

Some Learning Bounds and Guarantees for Testing (Quantum) Hypotheses

Machine learning is a powerful tool, yet we often do not know how well a learning algorithm might perform on any given task. One standard approach to bound the accuracy of a learning algorithm is to reduce the learning task to hypothesis testing. Fano's inequality then states that a large amount of mutual information between the learner's observations and the set of unknown parameters is a necessary condition for success.

In this talk, I will describe how such a condition is also sufficient for succeeding at some learning task, thereby providing a purely information-theoretic guarantee for learning. Noting that this guarantee has an immediate extension to quantum information theory, I will then introduce the task of "testing quantum hypotheses", in which the unknown parameters of the learning task are prepared in a quantum register in superposition (rather than being sampled stochastically) and the learner's success at this task is measured by their ability to establish quantum correlations with that register. I will discuss ongoing attempts to characterize this scenario.

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