My office in the Cambridge Centre for Quantum Information and Foundations is on the ground floor of Pavilion F of the Centre for Mathematical Sciences.

Professor of Quantum Physics,
DAMTP, University of Cambridge

Distinguished Visiting Research Chair at Perimeter Institute
for Theoretical Physics in Waterloo, Ontario

Fellow of Wolfson
College, Cambridge

Director of Studies in Mathematics at
Darwin College, Cambridge

Affiliate at the Institute for Quantum
Computing, University of Waterloo, Ontario

Visiting Scholar at Wolfson
College, Oxford

Visiting Scholar at Massey
College, Toronto

These are my main current research interests. Most of my papers on these subjects are on the physics arxiv. Some of the research topics I've worked on, and relevant papers and talks, are described below. (These descriptions and lists are incomplete: work in progress. Note that some papers belong in more than one list.)

My talk on relativistic quantum cryptography at QCRYPT 2012 is
here.
Our collaborative
paper

on an experimental implementation of bit commitment
using quantum information and relativistic signalling constraints
was presented at QCRYPT 2013.

Device-independent relativistic quantum bit commitment

Deterministic relativistic quantum bit commitment

Experimental bit commitment based on quantum communication and special
relativity

Security Details for Bit Commitment by Transmitting Measurement Outcomes

Secure and Robust Transmission and Verification of Unknown Quantum
States in Minkowski Space

Fundamental quantum optics experiments conceivable with satellites -- reaching relativistic distances and velocities

Quantum Tasks in Minkowski Space

Unconditionally Secure Bit Commitment by Transmitting Measurement Outcomes

Location-Oblivious Data Transfer with Flying Entangled Qudits

Unconditionally Secure Bit Commitment with Flying Qudits

A No-summoning theorem in Relativistic Quantum Theory

Quantum Tagging for Tags Containing Secret Classical Data

Quantum Tagging: Authenticating Location via Quantum Information and Relativistic Signalling Constraints

Variable Bias Coin Tossing

Why Classical Certification is Impossible in a Quantum World

Unconditionally Secure Commitment of a Certified Classical Bit is Impossible

Secure Classical Bit Commitment using Fixed Capacity Communication Channels

Unconditionally Secure Bit Commitment

Coin Tossing is Strictly
Weaker Than Bit Commitment

The paper
Experimental bit commitment based on quantum communication and special
relativity

describes a collaboration between experimentalists and theorists
based in Geneva, Cambridge and Singapore to implement (a variation
of) one of my
protocols for bit commitment using quantum information and
relativistic signalling constraints and to analyse its security
under realistic practical conditions.

Another group also recently experimentally implemented my protocol on a
smaller scale; their work is described
here .

Cambridge University's news article on the work is here.

Unconditionally secure device-independent quantum key distribution
with only two devices

Maximally Non-Local and Monogamous Quantum Correlations

No Signalling and Quantum Key Distribution

Our results on unconditionally secure device-independent quantum key distribution with only two devices were presented at QIP 2013. Here are the slides of Roger Colbeck's talk.

Unconditionally secure device-independent quantum key distribution
with only two devices

Prisoners of their own device: Trojan attacks on device-independent quantum cryptography

Maximally Non-Local and Monogamous Quantum Correlations

No Signalling and Quantum Key Distribution

Roger Colbeck's talk at QCRYPT 2012 on our joint work with Jonathan Barrett on memory attacks on device-independent quantum cryptography is here.

Our joint work on this is here:

Private Randomness Expansion With Untrusted Devices

Quantum Tagging for Tags Containing Secret Classical Data

Quantum Tagging: Authenticating Location via Quantum Information and
Relativistic Signalling Constraints

A piece by Gilles Brassard in Nature on this topic is here. (Link requires subscription.)

Erika Andersson and collaborators have invented some very
interesting schemes that use quantum information -- or
data generated from quantum information -- to securely
sign messages. Their schemes have significant security
and efficiency advantages over earlier quantum digital
signature schemes, and are well adapted for practical
implementation. I was happy to be invited to join them
in developing this research further:

Quantum digital signatures with quantum key distribution components

The following papers are on "ordinary" quantum cryptography, i.e.
forms of security obtainable from the properties of quantum
information alone, without relying on relativistic constraints.
The security proofs require the validity of quantum theory,
not just the no-signalling principle.

Large N Quantum Cryptography

Quantum Bit String Commitment

A proposal for founding mistrustful quantum cryptography on coin
tossing

Cheat Sensitive Quantum Bit Commitment

Entangled Mixed States and Local Purification

A Comparison of Quantum Oracles

More information can be found on the book's
amazon page.

Unlike the other editors, I'm sceptical about whether
many-worlds quantum theory can actually be made into a well-defined
and scientifically useful theory, and one of my contributions
to the book is the question mark in the title.

Another is my chapter
One World Versus Many, which includes
an extended critique of recent
attempts to make sense of Everett's many-worlds ideas.

What would it mean to apply quantum theory, without restriction and without involving any notion of measurement and state reduction, to the whole universe? What would realism about the quantum state then imply?

This book brings together an illustrious team of philosophers and physicists to debate these questions. The contributors broadly agree on the need, or aspiration, for a realist theory that unites micro- and macro-worlds. But they disagree on what this implies. Some argue that if unitary quantum evolution has unrestricted application, and if the quantum state is taken to be something physically real, then this universe emerges from the quantum state as one of countless others, constantly branching in time, all of which are real. The result, they argue, is many worlds quantum theory, also known as the Everett interpretation of quantum mechanics. No other realist interpretation of unitary quantum theory has ever been found.

Others argue in reply that this picture of many worlds is in no sense inherent to quantum theory, or fails to make physical sense, or is scientifically inadequate. The stuff of these worlds, what they are made of, is never adequately explained, nor are the worlds precisely defined; ordinary ideas about time and identity over time are compromised; no satisfactory role or substitute for probability can be found in many worlds theories; they can't explain experimental data; anyway, there are attractive realist alternatives to many worlds.

Twenty original essays, accompanied by commentaries and discussions, examine these claims and counterclaims in depth. They consider questions of ontology - the existence of worlds; probability - whether and how probability can be related to the branching structure of the quantum state; alternatives to many worlds - whether there are one-world realist interpretations of quantum theory that leave quantum dynamics unchanged; and open questions even given many worlds, including the multiverse concept as it has arisen elsewhere in modern cosmology. A comprehensive introduction lays out the main arguments of the book, which provides a state-of-the-art guide to many worlds quantum theory and its problems.

Here are reviews by Jeremy Butterfield, Amit Hagar and Peter Lewis. (These links may require subscription.) A more recent and detailed review by Guido Bacciagaluppi appeared in Metascience .

Does it Make Sense to Speak of Self-Locating Uncertainty in the Universal Wave Function? Remarks on Sebens and Carroll

An earlier critique I wrote of earlier many-worlds ideas is

Against Many-Worlds Interpretations.

My review of Peter Byrne's interesting
biography of Everett,
* The Many Worlds of Hugh Everett III: Multiple Universes, Mutual Assured Destruction, and the Meltdown of a Nuclear Family*,
is
here.

The conference details are here.

The talks are archived
here.

Quantum Non-local Correlations are not Dominated

Path Integrals and Reality

Might quantum-induced deviations from the Einstein equations
detectably affect gravitational wave propagation?

Fundamental quantum optics experiments conceivable with satellites -- reaching relativistic distances and velocities

Beable-Guided Quantum Theories: Generalising Quantum Probability Laws

Beyond Boundary Conditions: General Cosmological Theories

A Proposed Test of the Local Causality of Spacetime

Nonlinearity without Superluminality

Causal Quantum Theory and the Collapse Locality Loophole

The Geneva group carried out a beautiful experiment
aiming to close the collapse locality loophole in Bell experiments,
described in the last paper above. In their experiment
the outcomes of Bell measurements were, for the first time,
macroscopically recorded in space-like
separated regions by fast-moving piezocrystals.
"Macroscopically" here means that matter distributions were
altered in such a way that the gravitational fields
corresponding to distinct outcomes are (according to guesstimates
due to Penrose and Diosi) distinguishable.
The experiment is described
here .

With Joseph Emerson, Wayne Myrvold and Rafael Sorkin, I organized
a meeting,
The Quantum Landscape: Generalizations of Quantum Theory
and Experimental Tests, at Perimeter Institute in May 2013.
The talks and panel discussions are all video archived
here.

Quantum Non-local Correlations are not Dominated

Sphere colourings and Bell inequalities

Maximally Non-Local and Monogamous Quantum Correlations

A Proposed Test of the Local Causality of Spacetime

Quantum nonlocality, Bell inequalities and the memory loophole

Causal Quantum Theory and the Collapse Locality Loophole

Locality and reality revisited

Non-local Correlations are Generic in Infinite-Dimensional Bipartite Systems

Simulating Quantum Mechanics by Non-Contextual Hidden Variables

Non-Contextual Hidden Variables and Physical Measurements

Consistent Sets and Contrary Inferences: Reply to Griffiths and Hartle

Causality in Time-Neutral Cosmologies

Comment on "Spacetime Information"

Quantum Prediction Algorithms

Quantum Histories and Their Implications

Consistent Sets Yield Contrary Inferences in Quantum Theory

Quasiclassical Dynamics in a Closed Quantum System

Remarks on Consistent Histories and Bohmian Mechanics

On the Consistent Histories Approach to Quantum Mechanics

Lorentzian Quantum Reality: Postulates and Toy Models

A Solution to the Lorentzian Quantum Reality Problem

Path Integrals and Reality

Beable-Guided Quantum Theories: Generalising Quantum Probability Laws

Real World Interpretations of Quantum Theory

I wrote a popular account of the problems in reconciling quantum theory with a
scientific account of reality, and hence with the rest of science, for
Aeon magazine (published in January 2014):

Our Quantum Reality Problem

I am a member of the advisory panel for the
Cambridge Centre for
the Study of Existential Risk.

A critical look at risk assessments for global catastrophes

The paper is discussed by Martin Rees in his book
Our Final Century and by Richard Posner in his book
Catastrophe, Risk and Response.

An article by Dennis Overbye in the New York Times is
here.

American Mensa has a
collection of references on global risk reduction and comments
here.

The MIT Technology Review blog's report.

My earlier papers on these subjects include a classification of the unitary highest weight representations of the Virasoro, Ramond and Neveu-Schwarz algebras, which uses the so-called GKO construction (also known as the coset construction), which relates highest weight representations of these algebras to those of affine Kac-Moody algebras.

These results are central to understanding two-dimensional conformal
field theories, which describe the scaling behaviour of a large class
of two-dimensional systems at criticality. At the critical point,
lattice models, and the physical systems they represent, have a
fractal-like structure and become scale invariant.
Here is an example of an Ising model critical state at various scales:

Because the physics is local, the models actually display local
scale invariance or *conformal invariance*, which in two dimensions
is a very rich symmetry, represented in field theory
by the action of an infinite dimensional Lie algebra, the
Virasoro algebra.
The
unitary classification of Virasoro algebra highest weight representations
explains
the previously puzzling appearance of particular
simple rational numbers as critical exponents for the Ising
model, tricritical Ising model, 3-state Potts model, tricritical
3-state Potts model, and an infinite series of two dimensional
lattice models, several of which describe the critical behaviour
of naturally occurring two dimensional systems.
The unitary classification of Ramond and Neveu-Schwarz algebra
highest weight representations highlights the naturally occurring
supersymmetry occurring in two dimensional systems described
by the tricritical Ising model and a further infinite series of
models.

Some results on the representation theory of N=2 superconformal
algebras, which also describe naturally occurring two dimensional
systems (and have applications in string theory)
are
here.

An early paper on the ADHM construction in 4k dimensions is
here.

My other work on the representation theory of the Virasoro
algebra includes descriptions of its
singular vectors
(see also
here)
and a recursion formula for the
signature characters
of its highest weight representations.
The technique for calculating signature characters gives an
alternative way of characterising unitary
highest weight representations of Lie algebras: some calculations
for simple Lie algebras are
here.

Jim McElwaine, Professor of Advanced Computational Modelling of Geohazards, Durham University.

Jonathan Barrett, Lecturer in Computer Science (Foundations), University of Oxford.

Roger Colbeck, Lecturer in Mathematics, Quantum Information and Foundations group, Department of Mathematics, University of York.

Damian Pitalua-Garcia recently (Jan 2014) began a postdoc in the Laboratoire d'information quantique at the Université Libre de Bruxelles.

Tagging Systems,

A. Kent, R. Beausoleil, W. Munro and T. Spiller,

US patent 7075438 (2006).

Quantum Information Processing using Electromagnetically Induced Transparency,

R. Beausoleil, A. Kent, P. Kuekes, W. Munro,
T. Spiller and R. Williams,

US patent 7560726 (2009).

Security systems and monitoring methods using quantum states,

A Kent, WJ Munro, TP Spiller, RG Beausoleil,

US Patent 7483142 (2009).

Quantum cryptography,

A. Kent, R. Beausoleil, W. Munro and T. Spiller,

US Patent 7983422 (2011).

I often go along to the
Cambridge Science and
Literature Reading Group.
A while ago I founded the
Wolfson Contemporary Reading Group.

Everyone working on quantum theory who reads has to write at least one review of Michael Frayn's play "Copenhagen". Mine, published a while ago in Alternatives Théâtrales, is here.

*Email:*

A.P.A.Kent at damtp.cam.ac.uk

*Mail:*

Centre for Mathematical Sciences

Wilberforce Road

Cambridge CB3 0WA

United Kingdom

Last updated April 2015.