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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

Monday, May 27, 2024 2:30 pm - 3:30 pm EDT (GMT -04:00)

Semiconductor spin qubits for quantum networking

IQC Colloquium - Akira Oiwa, Osaka University

Quantum-Nano Centre, 200 University Ave West, Room QNC 1501 Waterloo, ON CA N2L 3G1

Semiconductor spin qubits are well recognized as a promising platform for scalable fault-tolerant quantum computers (FTQCs) because of relatively long spin coherence time in solid state devices and high-electrical tuneability of the quantum states [1]. In addition, semiconductors have a great potential for applications in quantum communications because of their abilities in optical devices. Therefore, especially in quantum repeater applications, the semiconductor spin qubits provide a route to efficiently connect qubit modules or quantum computers via optical fibers and construct global quantum networks, contributing to realize secure quantum communications and distributed quantum computing [2]. In this talk, we present the physical process enabling the quantum state conversion from single photon polarization states to single electron spin states in gate-defined quantum dots (QDs) and its experimental demonstration [3]. As recent significant achievements, we discuss that the enhancement of the conversion efficiency from a single photon to a single spin in a quantum dot using photonic nanostructures [4]. Finally, we present a perspective of high conversion efficiency quantum repeater operating directly at a telecom wavelength based on semiconductor spin qubits.

[1] G. Burkard et al., Rev. Mod. Phys. 95, 025003 (2023). [2] A. Oiwa et al., J. Phys. Soc. Jpn. 86, 011008 (2017); L. Gaudreau et al., Semicond. Sci. Technol. 32, 093001 (2017). [3] T. Fujita et al., Nature commun. 10, 2991 (2019); K. Kuroyama et al., Phys. Rev. B 10, 2991 (2019). [4] R. Fukai et al., Appl. Phys. Express 14, 125001 (2021); S. Ji et al., Jpn. J. Appl. Phys. 62, SC1018 (2023).

Wednesday, June 12, 2024 2:30 pm - 3:30 pm EDT (GMT -04:00)

Algebraic Methods in Quantum Compiling

IQC Seminar - Sarah Meng Li - University of Amsterdam, Centrum Wiskunde & Informatica (CWI)

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

: Quantum compiling translates a quantum algorithm into a sequence of elementary operations. There exists a correspondence between certain quantum circuits and matrices over some number rings. This number-theoretic perspective reveals important properties of gate sets and leads to improved quantum compiling protocols. Here, we demonstrate several algebraic methods in quantum circuit characterization and optimization, based on my master’s research at IQC.

First, we design two improved synthesis algorithms for Toffoli-Hadamard circuits, achieving an exponential reduction in circuit size. Second, we define a unique normal form for qutrit Clifford operators. This allows us to find a set of relations that suffice to rewrite any qutrit Clifford circuit to its normal form, adding to the family of number-theoretic characterization of quantum operators.

Wednesday, June 19, 2024 12:00 pm - 1:00 pm EDT (GMT -04:00)

IQC Student Seminar Featuring Bruno De Souza Leao Torres

Optimal coupling for local entanglement extraction from a quantum field

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

The entanglement structure of quantum fields is of central importance in various aspects of the connection between spacetime geometry and quantum field theory.  However, it is challenging to quantify entanglement between complementary regions of a quantum field theory due to the formally infinite amount of entanglement present at short distances. We present an operationally motivated way of analyzing entanglement in a QFT by considering the entanglement which can be transferred to a set of local probes coupled to the field. In particular, using a lattice approximation to the field theory, we show how to optimize the coupling of the local probes with the field in a given region to most accurately capture the original entanglement present between that region and its complement. This coupling prescription establishes a bound on the entanglement between complementary regions that can be extracted to probes with finitely many degrees of freedom.

Based on: J. High Energ. Phys. 2023, 58 (2023), arXiv:2301.08775

Wednesday, July 3, 2024 12:00 pm - 1:00 pm EDT (GMT -04:00)

IQC Student Seminar Featuring Lars Kamin

Weight estimation for optical detection setups

QNC building, 200 University Ave. Room 1201, Waterloo 

Realistic models of optical detection setups are crucial for numerous quantum information tasks. For instance, squashing maps allow for more realistic descriptions of the detection setups by accounting for multiphoton detections. To apply squashing maps, one requires a population estimation of multiphoton subspaces of the input to the detection setup. So far, there has been no universal method for those subspace estimations for arbitrary detection setups.

We introduce a generic subspace estimation technique applicable to any passive linear optical setup, accounting for losses and dark counts. The resulting bounds are relevant for adversarial tasks such as QKD or entanglement verification. Additionally, this method enables a generic passive detection setup characterization, providing the necessary measurement POVM for e.g. QKD security proofs.

Thursday, July 4, 2024 10:00 am - 12:00 pm EDT (GMT -04:00)

Quantum Optomechanics Tutorial

Professor Brad Hauer, Institute for Quantum Computing

QNC building, 200 University Ave. Room 0101, Waterloo 

Join new IQC faculty member Professor Brad Hauer for a tutorial on quantum optomechanics and a preview of new research directions at IQC. This tutorial is designed for the USEQIP program to be accessible to advanced undergraduates, and all IQC members are welcome (no registration required).

Cavity optomechanics, which studies the interplay between confined electromagnetic fields and mechanical motion, has seen a flurry of activity over the past two decades. In particular, optomechanical devices have had great success in preparing, manipulating, and observing quantum states of motion in nanoscale mechanical resonators. With applications in quantum information and quantum sensing on the horizon, cavity optomechanical devices remain an exciting prospect for real-world quantum technologies, as well as probes of important physical quantities on both microscopic and cosmological scales.

In my tutorial, I will provide a brief overview of cavity optomechanics, describing both the theoretical fundamentals and physical implementations. Following this introduction, I will detail a number of recent experiments realizing quantum effects in mesoscale mechanical resonators, including ground state cooling and entanglement of their motion. I will also discuss how cavity optomechanics is being used to further our understanding of the universe through next-generation dark matter and gravity wave detectors. Finally, I will briefly discuss my own research studying newly developed mm-wave optomechanical circuits and how I plan to use these devices to continue advancing the field.

Monday, July 8, 2024 10:30 am - 11:30 am EDT (GMT -04:00)

Quantum computational advantage in simulating many-body spin dynamics

IQC Colloquium - Dr. Chae-Yeun Park, Xanadu

QNC building, 200 University Ave. Room QNC 1201 Waterloo 

Understanding the dynamics of quantum many-body systems is one of the fundamental objectives of physics. The existence of an efficient quantum algorithm for simulating these dynamics with reasonable resource requirements suggests that this problem might be among the first practically relevant tasks quantum computers can tackle. Although an efficient classical algorithm for simulating such dynamics is not generally expected, the classical hardness of many-body dynamics has been rigorously proven only for certain commuting Hamiltonians. In this talk, I will show that computing the output distribution of quantum many-body dynamics is classically difficult, classified as #P-hard, also for a large class of non-commuting many-body spin Hamiltonians. Our proof leverages the robust polynomial estimation technique and the #P-hardness of computing the permanent of a matrix. By combining this with the anticoncentration conjecture of the output distribution, I will argue that sampling from the output distribution generated by the dynamics of a large class of spin Hamiltonians is classically infeasible. Our findings can significantly reduce the number of qubits required to demonstrate quantum advantage using analog quantum simulators.