Events

Math CS Seminar - Jinge Bao, National University of Singapore

200 University Ave W. Waterloo - ZOOM

We initiate a systematic study of the time complexity of quantum divide and conquer algorithms for classical problems. We establish generic conditions under which search and minimization problems with classical divide and conquer algorithms are amenable to quantum speedup and apply these theorems to an array of problems involving strings, integers, and geometric objects. They include LONGEST DISTINCT SUBSTRING, KLEE'S COVERAGE, several optimization problems on stock transactions, and k-INCREASING SUBSEQUENCE. For most of these results, our quantum time upper bound matches the quantum query lower bound for the problem, up to polylogarithmic factors.

https://arxiv.org/abs/2311.16401

Improving the Fidelity of CNOT Circuits on NISQ Hardware

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

We introduce an improved CNOT synthesis algorithm that considers nearest-neighbour interactions and CNOT gate error rates in noisy intermediate-scale quantum (NISQ) hardware. Our contribution is twofold. First, we define a \Cost function by approximating the average gate fidelity Favg. According to the simulation results, \Cost fits the error probability of a noisy CNOT circuit, Prob = 1 - Favg, much tighter than the commonly used cost functions. On IBM's fake Nairobi backend, it fits Prob with an error at most 10^(-3). On other backends, it fits Prob with an error at most 10^(-1). \Cost accounts for the machine calibration data, and thus accurately quantifies the dynamic error characteristics of a NISQ-executable CNOT circuit. Moreover, it circumvents the computation complexity of calculating Favg and shows remarkable scalability. 

Second, we propose an architecture-aware CNOT synthesis algorithm, NAPermRowCol, by adapting the leading Steiner-tree-based synthesis algorithms. A weighted edge is used to encode a CNOT gate error rate and \Cost-instructed heuristics are applied to each reduction step. Compared to IBM's Qiskit compiler, it reduces \Cost by a factor of 2 on average (and up to a factor of 8.8). It lowers the synthesized CNOT count by a factor of 13 on average (up to a factor of 162). Compared with algorithms that are noise-agnostic, it is effective and scalable to improve the fidelity of CNOT circuits. Depending on the benchmark circuit and the IBM backend selected, it lowers the synthesized CNOT count up to 56.95% compared to ROWCOL and up to 21.62% compared to PermRowCol. It reduces the synthesis \Cost up to 25.71% compared to ROWCOL and up to 9.12% compared to PermRowCol. NAPermRowCol improves the fidelity and execution time of a synthesized CNOT circuit across varied NISQ hardware. It does not use ancillary qubits and is not restricted to certain initial qubit maps. It could be generalized to route a more complicated quantum circuit, and eventually boost the overall efficiency and accuracy of quantum computing on NISQ devices. 

Joint-work with: Dohun Kim, Minyoung Kim, and Michele Mosca

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.

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 Seminar - Ashutosh Marwah, University of Montreal

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

For a state $\rho_{A_1^n B}$, we call a sequence of states $(\sigma_{A_1^k B}^{(k)})_{k=1}^n$ an approximation chain if for every $1 \leq k \leq n$, $\rho_{A_1^k B} \approx_\epsilon \sigma_{A_1^k B}^{(k)}$. In general, it is not possible to lower bound the smooth min-entropy of such a $\rho_{A_1^n B}$, in terms of the entropies of $\sigma_{A_1^k B}^{(k)}$ without incurring very large penalty factors. In this paper, we study such approximation chains under additional assumptions. We begin by proving a simple entropic triangle inequality, which allows us to bound the smooth min-entropy of a state in terms of the R\'enyi entropy of an arbitrary auxiliary state while taking into account the smooth max-relative entropy between the two. Using this triangle inequality, we create lower bounds for the smooth min-entropy of a state in terms of the entropies of its approximation chain in various scenarios. In particular, utilising this approach, we prove approximate versions of the asymptotic equipartition property and entropy accumulation. In a companion paper, we show that the techniques developed in this paper can be used to prove the security of quantum key distribution in the presence of source correlations.

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).

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.

CS/Math Seminar - Lauritz van Luijk, Leibniz Universität Hannover

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).

IQC Seminar - Jameson O'Reilly, Duke University

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.

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.