Join us for Quantum Connections May 1-2, 2024. This year we’re highlighting Quantum Perspectives: the impacts and outlooks driving our future.
Quantum-Nano Centre, 200 University Ave West, Room QNC 1201 + ZOOM Waterloo, ON CA N2L 3G1
In the last few years a number of works have proposed and improved provably efficient algorithms for learning the Hamiltonian from real-time dynamics. In this talk, I will first provide an overview of these developments, and then discuss how the Heisenberg limit, the fundamental precision limit imposed by quantum mechanics, can be reached for this task. I will demonstrate how the Heisenberg limit requires techniques that are fundamentally different from previous ones, and the important roles played by quantum control and thermalization. I will also discuss open problems that are crucial to making these algorithms implementable on current devices.
Quantum-Nano Centre, 200 University Ave West, Room QNC 1201 Waterloo, ON CA N2L 3G1
The precise estimation of unknown physical quantities is foundational across science and technology. Excitingly, by harnessing carefully-prepared quantum correlations, we can design and implement sensing protocols that surpass the intrinsic precision limits imposed on classical approaches. Applications of quantum sensing are myriad, including gravitational wave detection, imaging and microscopy, geoscience, and atomic clocks, among others.
However, current and near-term quantum devices have limitations that make it challenging to capture this quantum advantage for sensing technologies, including noise processes, hardware constraints, and finite sampling rates. Further, these non-idealities can propagate and accumulate through a sensing protocol, degrading the overall performance and requiring one to study protocols in their entirety.
In recent work [1], we develop an end-to-end variational framework for quantum sensing protocols. Using parameterized quantum circuits and neural networks as adaptive ansätze of the sensing dynamics and classical estimation, respectively, we study and design variational sensing protocols under realistic and hardware-relevant constraints. This seminar will review the fundamentals of quantum metrology, cover common sensing applications and protocols, introduce and benchmark our end-to-end variational approach, and conclude with perspectives on future research.
[1] https://arxiv.org/abs/2403.02394
Quantum-Nano Centre, 200 University Ave West, Room QNC 2101
Waterloo, ON CA N2L 3G1
Supervisor: Thomas Jennewein
A commonly researched method of quantum cryptography is quantum key distribution (QKD). In this method, quantum states are used to generate secret keys which can then be used for secure communication between two users. Due to the fundamental principles of quantum mechanics, the QKD protocols produce keys that can be guaranteed as secure from eavesdroppers, thus also ensuring the security of the subsequent communication using the secret keys.
Quantum-Nano Centre, 200 University Ave West, Room QNC 0101 Waterloo, ON CA N2L 3G1
Trapped atomic ions are a leading candidate platform for quantum simulation and computing but system sizes are limited by motional mode crowding and transport overhead. Multiple reasonably-sized, well-controlled modules can be connected into one universal system using photonic interconnects, in which photons entangled with ions in each trap are collected into and detected in a Bell-state analyzer. The speed of these interconnects has heretofore been limited by the use of 0.6 NA objectives and the need to periodically pause entanglement attempts for recooling. In this work, we use a system with two in-vacuo 0.8 NA lenses on either side of an ion trap to collect 493 nm photons from barium ions and demonstrate the most efficient free-space ion trap photonic interconnect to date. In addition, we introduce an ytterbium ion as a sympathetic coolant during the entangling attempts cycle to remove the need for recooling, enabling a record photon-mediated entanglement rate between two trapped ions. The major remaining error source is imperfections in the photon polarization encoding, so we also develop a new protocol for remotely entangling two ions using time-bin encoded photons and present preliminary results of an experimental implementation. Finally, we prepare the first remote entangled state involving two barium ions in separate vacuum chambers.
Quantum-Nano Centre, 200 University Ave West, Room QNC 1201 Waterloo, ON CA N2L 3G1
Quantum computing devices require exceptional control of their experimental parameters to prepare quantum states and simulate other quantum systems, in particular while subject to noise. Of interest here are notions of trainability, how difficult is it to classically optimize parameterized, realistic quantum systems to represent target states or operators of interest, and expressivity, how much of a desired set of these targets is our parameterized ansatze even capable of representing? We observe that overparameterization phenomena, where systems are adequately parameterized, are resilient in noisy settings at short times and optimization can converge exponentially with circuit depth. However fidelities decay to zero past a critical depth due to accumulation of either quantum or classical noise. To help explain these noise-induced phenomena, we introduce the notion of expressivity of non-unitary, trace preserving operations, and highlight differences in average behaviours of unitary versus non-unitary ensembles. We rigorously prove that highly-expressive noisy quantum circuits will suffer from barren plateaus, thus generalizing reasons behind noise-induced phenomena. Our results demonstrate that appropriately parameterized ansatze can mitigate entropic effects from their environment, and care must be taken when selecting ansatze of channels.
Quantum-Nano Centre, 200 University Ave West, Room QNC 0101 Waterloo, ON CA N2L 3G1
Scaling up the number of qubits available in experimental systems is one of the most significant challenges in quantum computation. A promising path forward is to modularize the quantum processors and then connect many processors using quantum channels, realized using photons and optical fibers. For Rydberg atom arrays, one of the leading platforms for quantum information processing, this could be done by developing an interface for photons, such as an optical cavity. In addition, an optical cavity can be used for fast mid-circuit readout for error detection. In this talk, I will discuss recent progress with two types of cavities and their feasibility as a photonic link. First, we show coherent control of Rydberg qubits and two-atom entanglement as close as 130um away from a nanophotonic cavity. Second, we show fast high-fidelity qubit state readout at a fiber Fabry Perot cavity. In addition, a fiber cavity also allows for cavity-mediated atom-atom gates, which could enable novel quantum networking capabilities.
Quantum-Nano Centre, 200 University Ave West, Room QNC 1201 + ZOOM Waterloo, ON CA N2L 3G1
Embezzlement refers to the counterintuitive possibility of extracting entangled quantum states from a reference state of an auxiliary system (the "embezzler") via local quantum operations while hardly perturbing the reference. I will explain a deep connection between this operational task and the mathematical classification of von Neumann algebras.
This result implies that relativistic quantum fields are universal embezzlers: Any entangled state of any dimension can be embezzled from them with arbitrary precision. In particular, this provides an operational characterization of the infinite amount of entanglement present in the vacuum state of relativistic quantum field theories and explains the classic result that the vacuum maximally violates Bell's inequalities: Alice and Bob can simply embezzle a maximally entangled qubit pair and perform a Bell measurement.
The talk is based on joined work with A Stottmeister, RF Werner, and H Wilming (see arXiv:2401.07292, arXiv:2401.07299).
Quantum-Nano Centre, 200 University Ave West, Room QNC 1201 Waterloo, ON CA N2L 3G1
Quantum computing gauge theories of relevance to Nature requires a range of theoretical and algorithmic developments to make simulations amenable in the near and far terms. With a focus on the SU(2) lattice gauge theory with matter, I will motivate the need for efficient theoretical formulations, introduce general quantum algorithms that can simulate them efficiently, and discuss strategies for analyzing the required quantum resources accurately. These considerations will be of relevance to simulating other gauge theories of increasing complexity, including quantum chromodynamics.