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Wednesday, May 1, 2024 12:00 pm - 1:00 pm EDT (GMT -04:00)

IQC Student Seminar Featuring Alexander Frei

Fermionic encodings: BK Superfast, ternary trees, and even fermionic encodings

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

We give an introduction to fermionic encoding schemes applicable in the context of quantum simulation of fermionic systems in condensed matter physics, lattice gauge theories, and in quantum chemistry.
 
For this we will focus on the circuit depth overhead for a variety of constructions of fermionic encodings, more precisely in terms of their weight given by the choice of encoding within the Pauli group, and as such also in terms of their circuit depth due to multi-qubit rotation gates.
 
In particular we will introduce the Fenwick tree encoding due to Bravyi and Kitaev, as well as an optimal all-to-all encoding scheme in terms of ternary trees due to Jiang et al, and put those in perspective with the well-known fermionic encoding given by the Jordan-Wigner transformation. Such encoding schemes of fermionic systems with all-to-all connectivity become relevant especially in the context of molecular simulation in quantum chemistry.
 
We then further discuss the encoding of the algebra of even fermionic operators, which becomes particularly handy in the estimation of ground state energies for complex materials and their phase transitions in condensed matter physics.
 
In particular, we will introduce here the so-called Bravyi--Kitaev superfast encoding for the algebra of even fermionic operators, as well as the compact encoding due to Klassen and Derby as a particular variant thereof. These encoding schemes require the further use of stabilizer subspaces and so of fault-tolerant encoding schemes for their practical implementation for the purpose of quantum simulation. We then finish with a further improvement, the so-called supercompact encoding, due to Chen and Xu. In particular, we will focus here on its code parameters (more precisely its encoding rate and code distance) and put those in perspective with the previous compact encoding due to Klassen and Derby.
 
This talk is meant as an expository talk on available encoding schemes for fermionic systems, together with their best practices for the purpose of quantum simulations.

Wednesday, May 8, 2024 12:00 pm - 1:00 pm EDT (GMT -04:00)

Student Seminar Featuring Sam Winnick

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

Clifford gates are ubiquitous in quantum computing. We consider the multiqudit analog for arbitrary d>1, which for example, includes the qudit Fourier transform. In this talk, we discuss the structure of the multiqudit projective Clifford group and give a high-level overview of a Clifford-based functional programming language whose underlying type system incorporates the resulting encoding scheme for projective Cliffords. This is joint work with Jennifer Paykin.

Wednesday, May 22, 2024 8:30 am - 9:30 am EDT (GMT -04:00)

Paul Oh PhD Thesis Defense

Entangled photon source for a long-distance quantum key distribution

Remote

Satellite-based Quantum Key Distribution (QKD) leverages quantum principles to offer unparalleled security and scalability for global quantum networks, making it a promising solution for next-generation secure communication systems. However, many technical challenges need to be overcome. This thesis focuses on theoretical modeling and experimental validation for long-distance QKD, as well as the development and testing of the quantum source necessary for its implementation, to take strides towards realization. While various approaches exist for demonstrating long-distance QKD, here we focus on discussing the approach of sending entangled photon pairs from an optical quantum ground station (OQGS), one through free-space on one end (uplink), and the other one through ground on the other end. This is also because our research team at the Quantum Photonics Laboratory (QPL), collaborating with the Canadian Space Agency (CSA), is planning to demonstrate Canada's first ground-to-space QKD in the near future. The mission is called Quantum Encryption and Science Satellite (QEYSSat) mission, which is planned to deploy a Low-Earth Orbit (LEO) satellite for the purpose for demonstrating QKD.

In the thesis, we first discuss the considerations relevant to establishing a long-distance quantum link. Since a substantial amount of research has already been conducted on optical fiber communication through ground-based methods, our focus is specifically directed towards ground-to-space (i.e., free space) quantum links. One of the most concerning aspects in free- space quantum communication is signal attenuation caused by environmental factors. We particularly examine pointing errors that arise from satellite tracking systems. To investigate this further, we designed a tracking system employing a specific tracking algorithm and conducted tracking tests to validate its accuracy, using the International Space Station (ISS) as a test subject. Our findings illustrate the potentially significant impact of inaccurate ground station-to- satellite alignment on link attenuation, according to our theoretical model. Given that photons serve as the signals for the QKD, we also investigate the background light noise resulting from light pollution, which is another concerning aspect, as it could worsen the link attenuation. Conducting light pollution measurements around our Optical Quantum Ground Station (OQGS), we estimate the minimum photon pair rate required for successful QKD, taking into account both the obtained values from these measurements and the expected level of link loss.

Having determined the minimum photon pair rate and other requirements for the long-distance QKD, we proceed to fully elaborate on the development process of the Entangled Photon Source (EPS), which is one of the crucial devices for demonstrating entanglement-based QKD. We use a nonlinear crystal for generating photon pairs, and experimentally obtain the photon pair rate produced from it. Here, the thesis also includes a detailed explanation of the customization process for the crystal oven. Next, we implement a beam displacer scheme along with the Sagnac loop scheme to create a robust interferometer, responsible for creating quantum entanglement. In addition, we demonstrate a novel approach to effectively compensate for the major weaknesses of the interferometer, namely spatial and temporal walk-offs. Finally, we conduct the entanglement test and demonstrate its suitability for long-distance QKD. As a side project, we

investigate the performance degradation of nonlinear crystals in response to proton radiation, exploring the potential of deploying the EPS in space for downlink QKD in the future. This thesis provides a comprehensive analysis and testing of elements required for long-distance QKD, contributing to the advancement of future global quantum networks.

Supervisor: Thomas Jennewein