MC, 200 University Ave West, Room MC 5417
Waterloo, ON CA N2L 3G1
Supervisors: Thomas Jennewein and Norbert Lütkenhaus
Quantum-Nano Centre, 200 University Ave West, Waterloo, ON CA N2L 3G1 ZOOM ONLY
Quantum data access and quantum processing can make certain classically intractable learning tasks feasible. However, quantum capabilities will only be available to a select few in the near future. Thus, reliable schemes that allow classical clients to delegate learning to untrusted quantum servers are required to facilitate widespread access to quantum learning advantages. Building on a recently introduced framework of interactive proof systems for classical machine learning, we develop a framework for classical verification of quantum learning. We exhibit learning problems that a classical learner cannot efficiently solve on their own, but that they can efficiently and reliably solve when interacting with an untrusted quantum prover. Concretely, we consider the problems of agnostic learning parities and Fourier-sparse functions with respect to distributions with uniform input marginal. We propose a new quantum data access model that we call "mixture-of-superpositions" quantum examples, based on which we give efficient quantum learning algorithms for these tasks. Moreover, we prove that agnostic quantum parity and Fourier-sparse learning can be efficiently verified by a classical verifier with only random example or statistical query access. Finally, we showcase two general scenarios in learning and verification in which quantum mixture-of-superpositions examples do not lead to sample complexity improvements over classical data. Our results demonstrate that the potential power of quantum data for learning tasks, while not unlimited, can be utilized by classical agents through interaction with untrusted quantum entities.
Quantum-Nano Centre, 200 University Ave West, Room QNC 1201 Waterloo, ON CA N2L 3G1
In this talk, I will describe the use of entropic uncertainty relations in QKD security proofs. I will show how this proof method requires a bound on the classical statistics of the underlying quantum state, and thus ultimately reduces to a sampling problem. I will then describe how the sampling problem is addressed in the literature under certain unphysical assumptions on the QKD hardware. Finally, I will describe how these assumptions can be removed, thereby rendering this proof technique applicable to practical scenarios.
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.
Quantum-Nano Centre, 200 University Ave West, Room QNC 1201 + ZOOM Waterloo, ON CA N2L 3G1
We define a map from an arbitrary quantum circuit to a local Hamiltonian whose ground state encodes the quantum computation. All previous maps relied on the Feynman-Kitaev construction, which introduces an ancillary ‘clock register’ to track the computational steps. Our construction, on the other hand, relies on injective tensor networks with associated parent Hamiltonians, avoiding the introduction of a clock register. This comes at the cost of the ground state containing only a noisy version of the quantum computation, with independent stochastic noise. We can remedy this - making our construction robust - by using quantum fault tolerance. In addition to the stochastic noise, we show that any state with energy density exponentially small in the circuit depth encodes a noisy version of the quantum computation with adversarial noise. We also show that any ‘combinatorial state’ with energy density polynomially small in depth encodes the quantum computation with adversarial noise. This serves as evidence that any state with energy density polynomially small in depth has a similar property. As an application, we give a new proof of the QMA-completeness of the local Hamiltonian problem (with logarithmic locality) and show that contracting injective tensor networks to additive error is BQP- hard. We also discuss the implication of our construction to the quantum PCP conjecture, combining with an observation that QMA verification can be done in logarithmic depth.
Based on joint work with Anurag Anshu and Nikolas P. Breuckmann. (https://arxiv.org/abs/2309.16475)
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
Research Advancement Center, 485 Wes Graham Way, Room 2009 Waterloo, ON N2L 6R2
Single-photon sources (SPSs) are an elementary building block for quantum technologies. An ideal SPS is deterministic, on-demand and produces exactly one photon per pulse. Additionally, desirable features include a high repetition rate, an all-electrical driving mechanism and compatibility with semiconductor manufacturing techniques. Despite great advances in the field of single photon emitters, an SPS with all the features outlined above remains elusive. In this talk, we will present our proposed SPS, consisting of a single-electron pump integrated in proximity to a lateral PN-junction, which would allow our SPS to meet all the criteria listed above. We discuss progress towards our goal, and also discuss an unconventional electroluminescence mechanism observed during recent experiments.
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.
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.
Quantum-Nano Centre, 200 University Ave West, Room QNC 2101
Waterloo, ON CA N2L 3G1
Supervisors: Dr. Eduardo Martin-Martinez, Dr. Beni Yoshida