Workshop on Quantum Simulation Research

The aim of this workshop is to provide opportunities for discussion and collaboration on quantum simulation. 

Invited speakers

• Neill Lambert (RIKEN)

• Wataru Mizukami (QIQB, Osaka University)

• Enrico Rinaldi (Quantinuum and RIKEN)

• Kazuhiro Seki (RIKEN)

Onsite venue:  Room 410, Advanced Research Laboratory, Komaba Campus, University of Tokyo.

Online: the Zoom URL will be emailed to registered participants.

Please fill out the registration form, by April 5 for onsite participation and by April 10 for online participation.

Program

11:00 Onsite check-in

11:15-12:00 Talk by Lambert

14:00-14:45 Talk by Mizukami

15:15-16:00 Talk by Rinaldi

16:30-17:15 Talk by Seki

Titles and abstracts

Lambert: 

Modelling open quantum systems: from weak to strong coupling

I will give a brief introduction to open quantum systems, via our software library QuTiP (the quantum toolbox in python).  I will briefly show how QuTiP can be used to model a range of open physical systems, including quantum circuits, taking into account noise and realistic pulse shapes.  I will then explain how one can model strong non-Markovian coupling between quantum systems and their environment with a new pseudomode method.  Finally I will briefly discuss the prospects for quantum simulation of quantum environments.

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Mizukami:

Current status of quantum algorithms for chemistry and beyond

Quantum chemistry is considered to be a promising application for quantum computers.  With this expectation in mind, the development of quantum chemistry algorithms using quantum computers has flourished over the last five years.  As a result, various chemical calculations have become possible on quantum circuit emulators.  On the other hand, significant challenges have emerged that hinder the practical application of quantum chemistry calculations on real quantum devices.  In this talk, I will present the background and current status of quantum chemistry calculations on quantum computers, including our research.  I will then discuss the challenges in this field, such as expectation value measurements, and the attempts to overcome them.

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Rinaldi: 

Classical sampling algorithms for estimating quantum computing resources for bosonic systems

To simulate bosons on a qubit- or qudit-based quantum computer, one has to regularize the theory by truncating infinite-dimensional local Hilbert spaces to finite dimensions.  In the search for practical quantum applications, it is important to know how big the truncation errors can be. In general, it is not easy to estimate errors unless we have a good quantum computer.  In this paper we show that traditional sampling methods on classical devices, specifically Markov Chain Monte Carlo, can address this issue with a reasonable amount of computational resources available today.  As a demonstration, we apply this idea to the scalar field theory on a two-dimensional lattice, with a size that goes beyond what is achievable using exact diagonalization methods.  This method can be used to estimate the resources needed for realistic quantum simulations of bosonic theories, and also, to check the validity of the results of the corresponding quantum simulations.

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Seki:  

Quantum-classical hybrid method for microcanonical ensembles   

We propose a method to calculate finite-temperature properties of many-body systems for microcanonical ensembles, which may find a potential application of near-term quantum computers [1].  In our formalism, a microcanonical ensemble is specified with a target energy and a width of the energy window, by expressing the density of states as a sum of Gaussians centered at the target energy with its spread associated with the width of the energy window.  Using the Fourier representation of the Gaussian, we then show that thermodynamic quantities such as entropy and temperature can be calculated by evaluating the trace of the time-evolution operator, and the trace of the time-evolution operator multiplied by the Hamiltonian of the system.  We also describe how these traces can be evaluated using random diagonal-unitary circuits suitable for quantum computation.  We demonstrate the proposed method by numerically calculating thermodynamic quantities of the one-dimensional spin-1/2 Heisenberg model on small clusters and show that the proposed method is most effective for the target energy around which a larger number of energy eigenstates exist.

[1] K. Seki and S. Yunoki, Phys. Rev. B 106, 155111 (2022).

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Organizers: Masanori Hanada (Surrey), Takuya Okuda (Komaba), Masahito Yamazaki (Kavli IPMU)

Supported by Grant-in-Aid for Transformative Research Areas (A) “Extreme Universe” No. 21H05190.

Contact: quantum.simulation.komaba@gmail.com