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).

Tuesday, April 9, 2024 1:30 pm - 2:30 pm EDT (GMT -04:00)

Photonic Links for Rydberg Atom Arrays

IQC Special Colloquium - Ivana Dimitrova, Harvard University

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. 

Tuesday, April 2, 2024 2:30 pm - 3:30 pm EDT (GMT -04:00)

Quantum Computational Advantages in Energy Minimization

IQC Special Colloquium Leo Zhou, California Institute of Technology

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

Finding the minimum of the energy of a many-body system is a fundamental problem in many fields. Although we hope a quantum computer can help us solve this problem faster than classical computers, we have a very limited understanding of where a quantum advantage may be found. In this talk, I will present some recent theoretical advances that shed light on quantum advantages in this domain. First, I describe rigorous analyses of the Quantum Approximate Optimization Algorithm applied to minimizing energies of classical spin glasses. For certain families of spin glasses, we find the QAOA has a quantum advantage over the best known classical algorithms. Second, we study the problem of finding a local minimum of the energy of quantum systems. While local minima are much easier to find than ground states, we show that finding a local minimum under thermal perturbations is computationally hard for classical computers, but easy for quantum computers. These results highlight exciting new directions in leveraging physics-inspired algorithms to achieve quantum advantages in broadly useful problems.

Thursday, April 11, 2024 1:30 pm - 2:30 pm EDT (GMT -04:00)

Breaking ergodicity: quantum scars, quantum many-body scars and regular eigenstates

IQC Special Colloquium - Ceren B. Dag Harvard University

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

Quantum many-body scars (QMBS) consist of a few low-entropy eigenstates in an otherwise chaotic many-body spectrum and can weakly break ergodicity resulting in robust oscillatory dynamics. The notion of QMBS follows the original single-particle scars introduced within the context of quantum billiards, where scarring manifests in the form of a quantum eigenstate concentrating around an underlying classical unstable periodic orbit. A direct connection between these notions remains an outstanding question. Here, I will first show that a spinor condensate, owing to its collective interactions, is amenable to the diagnostics of scars. We characterize this system's rich dynamics, spectrum, and phase space, consisting of both regular and chaotic states. The former are low in entropy, violate the Eigenstate Thermalization Hypothesis, and can be traced back to integrable effective Hamiltonians, whereas most of the latter are scarred by the underlying classical unstable periodic orbits, while satisfying Eigenstate Thermalization Hypothesis. I will exhibit evidence on how the existing QMBS in the literature are akin to the regular states, rather than the quantum scars. Then I will move on to introduce a spatially many-body model with a mean-field limit by decreasing the range of the interactions. Remarkably, we find that unstable periodic orbits affect the early-time many-body dynamics giving rise to a new type of QMBS. I will classify the QMBS in two main classes, discuss their distinct properties, and show how both QMBS states show up in our model in different parameter regimes. This talk aims (i) to clarify the connection of QMBS to quantum scars and regular eigenstates, and (ii) illustrate the fundamental principle of classical-quantum correspondence in a many-body system, and its current limitations.

The (Quantum) Signal and the Noise: towards the intermediate term of quantum computation

University of Waterloo, 200 University Ave West QNC 0101 + ZOOM

Can we compute on small quantum processors? In this talk, I explore the extent to which noise presents a barrier to this goal by quickly drowning out the information in a quantum computation. Noise is a tough adversary: we show that a large class of error mitigation algorithms -- proposals to "undo" the effects of quantum noise through mostly classical post-processing – can never scale up. Switching gears, we next explore the effects of non-unital noise, a physically natural (yet analytically difficult) class of noise that includes amplitude-damping and photon loss. We show that it creates effectively shallow circuits, in the process displaying the strongest known bound on average convergence of quantum states under such noise. Concluding with the computational complexity of learning the outputs of small quantum processors, I will set out a program for wrapping these lower bounds into new directions to look for near-term quantum computational advantage. 

IQC Colloquium/IEEE-SSCS Distinguished Lecture - René-Jean Essiambre, Nokia/Bell Labs

University of Waterloo, 200 University Ave W. Waterloo, QNC 0101

The first part of this presentation will provide a brief overview of optical technologies that enabled high-capacity fiber-optic communication systems, from single-mode fibers to fibers supporting multiple spatial modes. A perspective on the evolution of high-capacity systems will be discussed. The second part of the talk will focus on power-e ciency optical detection systems. More specifically, we will describe an experimental demonstration of a system operating at 12.5 bits/photon with optical clock transmission and recovery on free-running transmitters and receivers.

About René-Jean Essiambre Dr. Essiambre worked in the areas of fiber lasers, nonlinear fiber optics, advanced modulation formats, space-division multiplexing, information theory, and high-photon-e ciency systems. He participated in the design of commercial fiber-optic communication systems where several of his inventions were implemented. He has given over 150 invited talks and helped prepare and delivered the 2018 Physics Nobel Prize Lecture on behalf of Arthur Ashkin. He served on or chaired many conference committees, including OFC, ECOC, CLEO, and IPC. He received the 2005 Engineering Excellence Award from OPTICA and is a fellow of the IEEE, OPTICA, IAS-TUM, and Bell Labs. He was President of the IEEE Photonics Society (2022-2023) and is currently the Past-President (2024-2025).

Monday, March 25, 2024 2:30 pm - 3:30 pm EDT (GMT -04:00)

Fundamental physics at the quantum limits of measurement

IQC Colloquium - Daniel Carney, Berkeley Labs

200 University Ave. W. Waterloo Ontario, QNC 0101

The search for new fundamental physics -- particles, fields, new objects in the sky, etc -- requires a relentless supply of more and more sensitive detection modalities. Experiments looking for new physics are starting to regularly encounter noise sources generated by the quantum mechanics of measurement itself. This noise now needs to be engineered away. The search for gravitational waves with LIGO, and their recent use of squeezed light, provides perhaps the most famous example. More broadly, searches for various dark matter candidates, precision nuclear physics, and even tests of the quantization of gravity are all now working within this quantum-limited regime of measurement. In this talk, I will give an overview of this set of ideas, focusing on activity going on now and what can plausibly be achieved within the next decade or so.

Monday, March 11, 2024 2:30 pm - 3:30 pm EDT (GMT -04:00)

Quantum error-correcting codes are far from classical: a quantitative examination

Special Colloquium - Zhi Li, Perimeter Institute

University of Waterloo 200 University Ave. W Waterloo QNC 0101

Quantum error-correcting codes play a pivotal role in enabling fault-tolerant quantum computation. These codes protect quantum information through intricately designed redundancies that encode the information in a global manner. Unlike classical objects, in a quantum error-correcting code, the knowledge of individual subregions, even when combined, reveals nothing about the overall state.

In this talk, we explore the quantification of how far quantum error-correcting code are from classical states. We examine this question from three different perspectives: circuit complexity (the mimimal number of circuit depth needed to prepare a quantum state), expansion number (the minimal number of terms needed to expand the wavefunction), and a quantity we termed product overlap, which characterizes the maximal overlap between a given state and any product state. We will demonstrate why any quantum error-correcting code states must exhibit exponentially small product overlap, and how it implies lower bounds for the circuit complexity and the expansion number.

IQC Special Colloquium - Aziza Suleymanzade, Harvard University

200 University Ave W. Waterloo ON - ZOOM only

The experimental development of quantum networks marks a significant scientific milestone, poised to enable secure quantum communication, distributed quantum computing, and entanglement-enhanced nonlocal sensing. In this talk, I will discuss the recent advancements in the field along with the outstanding challenges through my work on two different platforms: Silicon Vacancy defects in diamond nanophotonic cavities and Rydberg atoms coupled to hybrid cavities. First, I will present our recent results on distributing entanglement across a two-node network with on-chip solid-state defects in cavities which we built at Harvard. We demonstrated high-fidelity entanglement between communication and memory qubits and showed long-distance entanglement over the 35 km of deployed fiber in the Cambridge/Boston area. Second, I will describe our work at the University of Chicago on using Rydberg atoms as transducers of quantum information between optical and microwave photons, with the goal of integrating Rydberg platforms with superconducting circuits and paving the way for advanced quantum network architectures. The talk will conclude with a perspective on the potential of this hybrid platform approach in constructing quantum networks, highlighting the uncharted scientific and technological opportunities it could unlock.

Monday, February 5, 2024 2:30 pm - 3:30 pm EST (GMT -05:00)

Achieving quantum sensing limits in noisy environment

IQC Colloquium - Sisi Zhou, The Perimeter Institute

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

 Quantum metrology studies estimation of unknown parameters in quantum systems. The Heisenberg limit of estimation precision 1/N, with N being the number of probes, is the ultimate sensing limit allowed by quantum mechanics that quadratically outperforms the classically-achievable standard quantum limit 1/√N. The Heisenberg limit is attainable using multi-probe entanglement in the ideal, noiseless case. However, in presence of noise, many quantum systems only allow a constant factor of improvement over the standard quantum limit. To elucidate the noise effect in quantum metrology, we prove a necessary and sufficient condition for achieving the Heisenberg limit using quantum controls. We show that when the condition is satisfied, there exist quantum error correction protocols to achieve the Heisenberg limit; when the condition is violated, no quantum controls can break the standard quantum limit (although quantum error correction can be used to maximize the constant-factor improvement). We will also discuss the modified sensing limits when only restricted types of quantum controls can be applied.