Events

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).

Blackbox Optimization for Free-space Quantum Key Distribution

MC, 200 University Ave West, Room MC 5417
Waterloo, ON CA N2L 3G1

Supervisors: Thomas Jennewein and Norbert Lütkenhaus

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.

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

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.

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.

Aspects of Quantum Information in Quantum Field Theory: Particle detector models, entanglement, and complexity

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

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.

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

QNC building, 200 University Ave. Room 1201, Waterloo 

We use numerical optimal-control-theory methods to determine the minimum number of two-qubit CNOT gates needed to perform quantum state preparation and unitary operator synthesis for few-qubit systems. In the first set of calculations, we consider all possible gate configurations for a given number of qubits and a given number of CNOT gates, and we determine the maximum achievable fidelity for the specified parameters. This information allows us to identify the minimum number of gates needed to perform a specific target operation. It also allows us to enumerate the different gate configurations that can be used for a perfect implementation of the target operation. We find that there are a large number of configurations that all produce the desired result, even at the minimum number of gates. This last result motivates the second set of calculations, in which we consider only a small fraction of the super-exponentially large number of possible gate configurations for an increasing number of qubits. We find that the fraction of gate configurations that allow us to achieve the desired target operation increases rapidly as soon as the number of gates exceeds the theoretical lower bound for the required number of gates. As a result, a random search can be a highly efficient approach for quantum circuit synthesis. Our results demonstrate the important role that numerical optimal control theory can play in the development of quantum compilers.

QNC building, 200 University Ave. Room 0101, Waterloo 

This workshop focuses on encoding quantum information in more than two states.

The main theme is to go beyond binary encodings: from quBits to quDits, where D > 2.

Now is a very interesting time, as we see a lot of experimental progress and new possibilities in this area. This workshop brings together researchers  – both experimentalists and theorists – to explore quDit-based applications in all areas of quantum technology.