Seminar

Improved Synthesis of Restricted Clifford+T Circuits

In quantum information theory, the decomposition of unitary operators into gates from some fixed universal set is of great research interest. Since 2013, researchers have discovered a correspondence between certain quantum circuits and matrices over rings of algebraic integers. For example, there is a correspondence between a family of restricted Clifford+T circuits and the group On(Z[1/2]). Therefore, in order to study quantum circuits, we can study the corresponding matrix groups and try to solve the constructive membership problem (CMP): given a set of generators and an element of the group, how to factor this element as a product of generators? Since a good solution to CMP yields a smaller decomposition of an arbitrary group element, it helps us implement quantum circuits using fewer resources. 

Noncommuting charges: Bridging theory to experiment

Noncommuting conserved quantities have recently launched a subfield of quantum thermodynamics. In conventional thermodynamics, a system of interest and an environment exchange quantities—energy, particles, electric charge, etc.—that are globally conserved and are represented by Hermitian operators. These operators were implicitly assumed to commute with each other, until a few years ago. Freeing the operators to fail to commute has enabled many theoretical discoveries—about reference frames, entropy production, resource-theory models, etc. Little work has bridged these results from abstract theory to experimental reality. This work provides a methodology for building this bridge systematically: we present a prescription for constructing Hamiltonians that conserve noncommuting quantities globally while transporting the quantities locally. The Hamiltonians can couple arbitrarily many subsystems together and can be integrable or nonintegrable. Our Hamiltonians may be realized physically with superconducting qudits, with ultracold atoms, and with trapped ions.

Quantum measurements are inherently probabilistic. Further defying our classical intuition, quantum theory often forbids us to precisely determine the outcomes of simultaneous measurements. This phenomenon is captured and quantified through uncertainty relations. Although studied since the inception of quantum theory, this problem of determining the possible expectation values of a collection of quantum measurements remains, in general, unsolved. In this talk, we will go over some basic notions of graph theory that will allow us to derive uncertainty relations valid for any set of dichotomic quantum observables. We will then specify the many cases for which these relations are tight, depending on properties of some graphs, and discuss a conjecture for the untight cases. Finally, we will show some direct applications to several problems in quantum information, namely, in constructing entropic uncertainty relations, separability criteria and entanglement witnesses.

The optimal rates for compression of mixed states was found by Koashi and Imoto in 2001 for the blind case and by Horodecki and independently by Hayashi for the visible case respectively in 2000 and 2006. However, it was not known so far whether the strong converse property holds for these compression problems. In this work, we show that the strong converse holds for the blind compression scheme. For the visible scheme, the strong converse holds up to the continuity of the regularized Renyi entanglement of purification.

Dissipative landau Zener transition in the weak and strong coupling limits

Landau Zener (LZ) transition is a paradigm to describe a wide range of physical phenomenon. Dissipation is inevitable in realistic devices and can affect the LZ transition probabilities. I will describe how we can model the effect of the environment depending on whether it is weakly or strongly coupled to the system. I will also present our experimental results where we found evidence of crossover from weak to strong coupling limit.

TC Fraser - Perimeter Institute - Zoom event

The quantum marginals problem (QMP) aims to understand how the various marginals of a joint quantum state are related to one another by deciding whether or not a given collection of marginals is compatible with some joint quantum state. Although existing techniques for the QMP are well developed for the special case of disjoint marginals, the same is not true for the generic case of overlapping marginals. The leading technique for the generic QMP, published by Yu et. al. (2021), resorts to evaluating a hierarchy of semidefinite programs.

Generation and detection of spin-orbit coupled neutron beams

Structured waves and spin-orbit coupled beams have become an indispensable probe in both light and matter-wave optics [1-2], for neutron specifically, showing distinct scattering dynamics for some samples [3-4]. We present a method of generating neutron orbital angular momentum (OAM) states utilizing 3He neutron spin filters along with four specifically oriented triangular coils and magnetic field shielding. These states are verified via their spin-dependent intensity profiles [5]. The period and OAM number of these spin-orbit states can be altered dynamically via the magnetic field strength within the coils and the total number of coils to tailor the neutron beam towards a particular application or specific material [6].

The Office of Research at the University of Waterloo will be hosting a webinar to provide practical information and answer questions related to the new NSERC Alliance funding options that support Canada’s National Quantum Strategy.

Julien LAURAT (ENS Paris)

Quantum interconnects are central to the scale up of quantum information architectures. In this endeavour, I will first report on the implementation of a highly-efficient quantum memory based on a large ensemble of cold atoms. I will discuss how we implemented such efficient memories over the recent years, from typical efficiency values of about 30% in 2015 to 70% in 2018 and close to 90% in this recent work. Moreover, we used this platform to demonstrate highly efficient and reversible entanglement transfer into and out of two quantum memories. I will also present the realization of a faithful quantum-bit-encoding converter that relies on a teleportation process based on hybrid entanglement of light between CV and DV optical qubits.

Developments in device-independent cryptography

Device-independent cryptography connects the foundational topic of Bell inequalities to the operational task of achieving secure cryptography. With significant progress being made in Bell test experiments, various avenues for further developing device-independent cryptography have been opened. I will give an overview of some background and recent developments in the field, as well as some research questions that should be of interest going forward.