Seminar

Designing a precision gravitational experiment and budgeting uncertainties

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

Neutrons have a long history at the forefront of precision metrology. Following in the footsteps of the first experiment that measured the effect of gravity on a quantum particle (the C.O.W. experiment), we aim to generate structured neutron momentum profiles and apply these states to measure the gravitational constant, big-G. The significant discrepancy between modern big-G experimental results underscores the need for new experiments whose systematic uncertainties can be decoupled from existing techniques. Previously, perfect-crystal neutron interferometers were used to measure local gravitational acceleration, little-g, unfortunately, the low neutron flux (a few neutrons per second) of these devices makes them impractical for precision measurements of big-G. The recently demonstrated Phase-Grating Moiré Interferometer (PGMI) offers an increase in neutron flux of several orders of magnitude while preserving the large interferometer area, and thus the sensitivity, of a perfect-crystal interferometer. This device possesses a set of systematic uncertainties that are independent from those in existing techniques that measure big-G. In this talk, I will discuss the feasibility of measuring big-G using a neutron PGMI apparatus with a test mass on the order of 1 tonne. Further, I will address how we can optimize this setup to maximize the phase shift from a 1-tonne mass and quantify the various sources of uncertainty in the proposed experiment.

CS/Math Seminar - Amir Arqand, IQC 

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

The entropy accumulation theorem, and its subsequent generalized version, is a powerful tool in the security analysis of many device-dependent and device-independent cryptography protocols. However, it has the drawback that the finite-size bounds it yields are not necessarily optimal, and furthermore, it relies on the construction of an affine min-tradeoff function, which can often be challenging to construct optimally in practice. In this talk, we address both of these challenges simultaneously by deriving a new entropy accumulation bound. Our bound yields significantly better finite-size performance, and can be computed as an intuitively interpretable convex optimization, without any specification of affine min-tradeoff functions. Furthermore, it can be applied directly at the level of R´enyi entropies if desired, yielding fully-R´enyi security proofs. Our proof techniques are based on elaborating on a connection between entropy accumulation and the frameworks of quantum probability estimation or f-weighted R´enyi entropies, and in the process we obtain some new results with respect to those frameworks as well.

IQC CS/Math Seminar - Amolak Ratan Kalra, University of Waterloo

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

In this talk I will discuss qutrit circuit synthesis over various families of universal gate sets. I will describe a method which relates the question of exact synthesis for both single qubits and single qutrits to the problem of solving a system of polynomial equations mod p.
 

Stable and Localized Emission from Ambipolar Dopant-Free Lateral p-n Junctions

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

Combining the architectures of a dopant-free lateral p-n junction and a single-electron pump in a GaAs/AlGaAs heterostructure material system could yield high-rate, electrically-driven quantum emitters with performances surpassing the competition in quantum sensing, communication and cryptography. Observed drawbacks of the dopant-free p-n junctions are a rapid decay in electroluminescence during operation, as well as delocalized emission that lowers the measured quantum efficiency. This talk details novel measurement protocols and gate architectures implemented by us to overcome these challenges.

IQC Seminar - Universty of Maryland

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

 In the realm of quantum computing,non-equilibrium quasiparticle tunneling may be a significant loss mechanism in transmon qubits. Understanding the behavior of these quasiparticles across junctions may lead to improved qubit devices . One approach involves the fabrication of asymmetric transmons through gap-engineering techniques aimed at mitigating quasiparticle tunneling and subsequent loss. In our research, we have conducted repeated measurements of the relaxation time (T1) in Al/AlOx/Al transmons featuring electrodes with varying superconducting gap values. Specifically, one device utilized a first-layer electrode formed via thermal evaporation of nominally pure Al, while the counter-electrode incorporated oxygen-doped Al. This device exhibited notable fluctuations in T1, ranging from approximately 100 μs to slightly over 300 μs at 20 mK. Additionally, we explored different configurations of junction layouts in an effort to enhance device performance.

IQC Special Seminar, Jerome Wiesemann, Fraunhofer Heinrich Hertz Institute HHI

Quantum key distribution (QKD) is on the verge of becoming a robust security solution, backed by security proofs that closely model practical implementations.  As QKD matures, a crucial requirement for its widespread adoption is establishing standards for evaluating and certifying practical implementations, particularly against side-channel attacks resulting from device imperfections that can undermine security claims. Today, QKD is at a stage where the development of such standards is increasingly prioritized. This works aims to address some of the challenges associated with this task by focusing on the process of preparing an in-house QKD system for evaluation. We first present a consolidated and accessible baseline security proof for the one-decoy state BB84 protocol with finite-keys, expressed in a unified language. Building upon this security proof, we identify and tackle some of the most critical side-channel attacks by characterizing and implementing countermeasures both in the QKD system and within the security proof. In this process, we iteratively evaluate the risk of the individual attacks and re-assess the security of the system. Evaluating the security of QKD systems additionally involves performing attacks to potentially identify new loopholes. Thus, we also aim to perform the first real-time Trojan horse attack on a decoy state BB84 system, further highlighting the need for robust countermeasures. By providing a critical evaluation of our QKD system and incorporating robust countermeasures against side-channel attacks, our research contributes to advancing the practical implementation and evaluation of QKD as a trusted security solution.

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

Clifford gates are ubiquitous in quantum computing. We consider the multiqudit analog for arbitrary d>1, which for example, includes the qudit Fourier transform. In this talk, we discuss the structure of the multiqudit projective Clifford group and give a high-level overview of a Clifford-based functional programming language whose underlying type system incorporates the resulting encoding scheme for projective Cliffords. This is joint work with Jennifer Paykin.

Fermionic encodings: BK Superfast, ternary trees, and even fermionic encodings

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

We give an introduction to fermionic encoding schemes applicable in the context of quantum simulation of fermionic systems in condensed matter physics, lattice gauge theories, and in quantum chemistry.
 
For this we will focus on the circuit depth overhead for a variety of constructions of fermionic encodings, more precisely in terms of their weight given by the choice of encoding within the Pauli group, and as such also in terms of their circuit depth due to multi-qubit rotation gates.
 
In particular we will introduce the Fenwick tree encoding due to Bravyi and Kitaev, as well as an optimal all-to-all encoding scheme in terms of ternary trees due to Jiang et al, and put those in perspective with the well-known fermionic encoding given by the Jordan-Wigner transformation. Such encoding schemes of fermionic systems with all-to-all connectivity become relevant especially in the context of molecular simulation in quantum chemistry.
 
We then further discuss the encoding of the algebra of even fermionic operators, which becomes particularly handy in the estimation of ground state energies for complex materials and their phase transitions in condensed matter physics.
 
In particular, we will introduce here the so-called Bravyi--Kitaev superfast encoding for the algebra of even fermionic operators, as well as the compact encoding due to Klassen and Derby as a particular variant thereof. These encoding schemes require the further use of stabilizer subspaces and so of fault-tolerant encoding schemes for their practical implementation for the purpose of quantum simulation. We then finish with a further improvement, the so-called supercompact encoding, due to Chen and Xu. In particular, we will focus here on its code parameters (more precisely its encoding rate and code distance) and put those in perspective with the previous compact encoding due to Klassen and Derby.
 
This talk is meant as an expository talk on available encoding schemes for fermionic systems, together with their best practices for the purpose of quantum simulations.

CS/MATH Seminar - Kieran Mastel from IQC ZOOM + IN PERSON

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

The recent MIP*=RE theorem of Ji, Natarajan, Vidick, Wright, and Yuen shows that the complexity class MIP* of multiprover proof systems with entangled provers contains all recursively enumerable languages. In prior work Grilo, Slofstra, and Yuen showed (via a technique called simulatable codes) that every language in MIP* has a perfect zero knowledge (PZK) MIP* protocol.  The MIP*=RE theorem uses two-prover one-round proof systems, and hence such systems are complete for MIP*. However, the construction in Grilo, Slofstra, and Yuen uses six provers, and there is no obvious way to get perfect zero knowledge with two provers via simulatable codes. This leads to a natural question: are there two-prover PZK-MIP* protocols for all of MIP*?

In this talk we answer the question in the affirmative. For the proof, we use a new method based on a key consequence of the MIP*=RE theorem, which is that every MIP* protocol can be turned into a family of boolean constraint system (BCS) nonlocal games. This makes it possible to work with MIP* protocols as boolean constraint systems, and in particular allows us to use a variant of a construction due to Dwork, Feige, Kilian, Naor, and Safra which gives a classical MIP protocol for 3SAT with perfect zero knowledge. To show quantum soundness of this classical construction, we develop a toolkit for analyzing quantum soundness of reductions between BCS games, which we expect to be useful more broadly. This talk is based on joint work with William Slofstra

IQC Seminar - Alexander George-Kennedy, Georgia Tech

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

Protecting quantum information against noise is a widespread goal in quantum computation. In addition to implementing quantum error correcting codes, classical pre-processing steps of circuit optimization and qubit routing can greatly increase the fidelity of the result of a quantum computation. Prior work has shown that neural networks and/or reinforcement learning can be used to discover quantum error correcting codes, perform qubit routing optimized for circuit depth, and find optimal points to insert dynamical decoupling pulse sequences in a quantum circuit. We extend prior work by creating a deep reinforcement learning directed transpiler. We treat the problem of qubit routing and circuit optimization together, and can regard it as a single-player “game,” where the objective is minimizing the output circuit's estimated noise, subject to the connectivity constraints of the architecture. The “moves” in this game available to the transpiler are selecting the qubit layout, introducing SWAP gates subject to architecture constraints, and rewriting the circuit according to equivalency rules (such as introducing dynamical decoupling sequences, or simply optimizing away repeated self-adjoint gates). We train a transpiler for a specific quantum device, in our experiments, each of the available 5-qubit IBM devices, crucially including the reported error rates per gate per qubit per device as part of the transpiler training data. Running the transpilers on a series of random circuits across different devices, we compare the transpiler output circuits with IBM's transpiler outputs. We find an average improvement of 17% reduction in output error rate compared to the IBM transpiler. This is an improvement on prior work that also uses a neural network as a noise-indicating objective function, but with no explicit loading of device error rates, a different vectorization of circuits, and a greedy circuit rewrite policy. Our work is ongoing, as we intend to extend the transpiler's capability in the vein of prior work to construct error correcting codes during optimization.