# Tomi Harry JohnsonNational University of Singapore | NUS · Centre f

Tomi Harry Johnson

Physics

## About

19

Publications

3,609

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864

Citations

Introduction

I am a theoretical physicist working on several research areas within cold atom, condensed matter, computational and non-equilibrium stochastic physics.
Aside from research, I enjoy spending time teaching undergraduates. At the moment, I teach the mathematical methods first year undergraduate physics course at Keble College.
To see an up to date list of my publications and teaching experience, go to http://www.tomijohnson.co.uk

Additional affiliations

October 2013 - April 2014

October 2013 - present

April 2013 - September 2013

**Institute for Scientific Interchange**

Position

- PostDoctoral Researhcer

## Publications

Publications (19)

We study an impurity atom trapped by an anharmonic potential, immersed within a cold atomic Fermi gas with attractive interactions that realizes the crossover from a Bardeen-Cooper-Schrieffer (BCS) superfluid to a Bose-Einstein condensate (BEC). Considering the qubit comprising the lowest two vibrational energy eigenstates of the impurity, we demon...

Estimating the temperature of a cold quantum system is difficult. Usually one measures a well-understood thermal state and uses that prior knowledge to infer its temperature. In contrast, we introduce a method of thermometry that assumes minimal knowledge of the state of a system and is potentially nondestructive. Our method uses a universal temper...

We investigate cold bosonic impurity atoms trapped in a vortex lattice formed
by condensed bosons of another species. We describe the dynamics of the
impurities by a bosonic Hubbard model containing occupation-dependent
parameters to capture the effects of strong impurity-impurity interactions.
These include both a repulsive direct interaction and...

Estimating the temperature of a cold quantum system is difficult. Usually,
one measures a system with a well-understood thermal state and uses that prior
knowledge to infer its temperature. In contrast, we introduce a method of
thermometry that assumes no knowledge of a system's thermal state and is
potentially non-destructive. Our method uses a un...

We introduce a detector that selectively probes the phononic excitations of a
cold Bose gas. The detector is composed of a single impurity atom confined by a
double-well potential, where the two lowest eigenstates of the impurity form an
effective probe qubit that is coupled to the phonons via density-density
interactions with the bosons. The syste...

Estimating the expected value of an observable appearing in a non-equilibrium
stochastic process usually involves sampling. If the observable's variance is
high, many samples are required. In contrast, we show that performing the same
task without sampling, using tensor network compression, efficiently captures
high variances in systems of various...

Wigner separated the possible types of symmetries in quantum theory into
those symmetries that are unitary and those that are antiunitary. Unitary
symmetries have been well studied whereas antiunitary symmetries and the
physical implications associated with time-reversal symmetry breaking have had
little influence on quantum information science. He...

Quantum simulators are devices that actively use quantum effects to answer
questions about model systems and, through them, real systems. Here we expand
on this definition by answering several fundamental questions about the nature
and use of quantum simulators. Our answers address two important areas. First,
the difference between an operation ter...

Quantum simulators are devices that actively use quantum effects to answer questions about model systems and, through them, real systems. Here we expand on this definition by answering several fundamental questions about the nature and use of quantum simulators. Our answers address two important areas. First, the difference between an operation ter...

Determining community structure in interacting systems, ranging from
technological to social, from biological to chemical, is a topic of central
importance in the study of networks. Extending this concept to apply to quantum
systems represents an open challenge and a crucial missing component towards a
theory of complex networks based on quantum me...

In this theoretical study, we analyze quantum walks on complex networks,
which model network-based processes ranging from quantum computing to biology
and even sociology. Specifically, we analytically relate the average long time
probability distribution for the location of a unitary quantum walker to that
of a corresponding classical walker. The d...

We derive ab initio local Hubbard models for several optical lattice
potentials of current interest, including the honeycomb and Kagom\'{e}
lattices, verifying their accuracy on each occasion by comparing the
interpolated band structures against the originals. To achieve this, we
calculate the maximally-localized generalized Wannier basis by implem...

Simulating quantum circuits using classical computers lets us analyse the inner workings of quantum algorithms. The most complete type of simulation, strong simulation, is believed to be generally inefficient. Nevertheless, several efficient strong simulation techniques are known for restricted families of quantum circuits and we develop an additio...

We study non-Markovianity and information flow for qubits experiencing local
dephasing with an Ohmic class spectrum. We demonstrate the existence of a
temperature-dependent critical value of the Ohmicity parameter s for the onset
of non-Markovianity and give a physical interpretation of this phenomenon by
linking it to the form of the reservoir spe...

Motivated by a recent experiment (Catani J.
et al., Phys. Rev. A, 85 (2012) 023623) we study breathing oscillations in the width of a harmonically trapped impurity interacting with a separately trapped Bose gas. We provide an intuitive physical picture of such dynamics at zero temperature, using a time-dependent variational approach. The amplitudes...

Using near-exact numerical simulations we study the propagation of an
impurity through a one-dimensional Bose lattice gas for varying bosonic
interaction strengths and filling factors at zero temperature. The impurity is
coupled to the Bose gas and confined to a separate tilted lattice. The precise
nature of the transport of the impurity is specifi...

We present an analysis of Bose-Fermi mixtures in optical lattices for the case where the lattice potential of the fermions is tilted and the bosons (in the superfluid phase) are described by Bogoliubov phonons. It is shown that the Bogoliubov phonons enable hopping transitions between fermionic Wannier-Stark states; these transitions are accompanie...

We adapt the time-evolving block decimation (TEBD) algorithm, originally devised to simulate the dynamics of one-dimensional quantum systems, to simulate the time evolution of nonequilibrium stochastic systems. We describe this method in detail; a system's probability distribution is represented by a matrix product state (MPS) of finite dimension a...