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
Charter Honorary Fellow of the
John Bell Institute for the Foundations of Physics
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
A Quantum Paradox of Choice: More Freedom Makes Summoning a Quantum
State Harder
A Quantum Paradox of Choice: More Freedom Makes Summoning a Quantum
State Harder
Unconditionally secure device-independent quantum key distribution
with only two devices
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
Roger Colbeck's talk at QCRYPT 2012 on our joint work with Jonathan Barrett
on memory attacks on device-independent quantum cryptography is
here.
Quantum Tagging for Tags Containing Secret Classical Data
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 join them
in developing some of this research further:
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.
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.
Research: Quantum foundations, quantum information theory and quantum
cryptography
Relativistic Quantum Cryptography
Talks
on an experimental implementation of bit commitment
using quantum information and relativistic signalling constraints
was presented at QCRYPT 2013.
Papers
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
Related Experimental Work
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 .
Commentaries
Our Physical Review Letter reporting the experimental implementation
of relativistic quantum bit commitment
was highlighted as an Editor's Suggestion.
The accompanying American Physical Society Focus article is
here.
Cambridge University's news article on the work is
here.
Relativistic Quantum Information Theory
A No-summoning theorem in Relativistic Quantum Theory
Quantum Tasks in Minkowski Space
Commentaries
Our quantum paradox of choice was reported in
Science News. (Full article requires subscription.)
Quantum Key Distribution with security based (only) on
No-Signalling
Papers
Maximally Non-Local and Monogamous Quantum Correlations
No Signalling and Quantum Key Distribution
Talks
Device-Independent Quantum Cryptography
Papers
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
Talks
Commentaries
The MIT Technology Review blog's
report on the above work.
Note our author correction letter published below the
original article. (The originally posted article
wrongly suggested our attacks apply
to all types of quantum cryptography rather than specifically to
device-independent implementations by parties who cannot rely on
any property of the quantum devices they are using).
Quantum Randomness Expansion
Roger Colbeck and I invented the idea of quantum randomness expansion,
first described and discussed in
Roger's PhD thesis.
Our joint work on this is here:
Private Randomness Expansion With Untrusted Devices
Quantum Tagging (Quantum Position Authentication)
Papers
Quantum Tagging: Authenticating Location via Quantum Information and
Relativistic Signalling Constraints
Commentaries
Quantum Digital Signature Schemes
Quantum digital signatures with quantum key distribution components
Secure Quantum Signatures Using Insecure Quantum Channels
Other Work on Quantum Cryptography
Large N Quantum Cryptography
Quantum Bit String Commitment
A proposal for founding mistrustful quantum cryptography on coin
tossing
Cheat Sensitive Quantum Bit Commitment
Quantum Channels and Quantum Communication
Optimal Entanglement Enhancement for Mixed States
Entangled Mixed States and Local Purification
Quantum Computing and Quantum Gates
Inferring superposition and entanglement from measurements in a single basis
A Comparison of Quantum Oracles
Many Worlds Quantum Theory and its problems
Our book
Many Worlds? Everett, Quantum Theory, and Reality was published
by Oxford University Press in June 2010.
Synopsis
Reviews
A review by Robert Wald, published in
Classical and Quantum Gravity,
is
here.
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 .
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.
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
This preprint is among those mentioned in a BBC News article surveying recent
ideas speculating on possible links between quantum theory and
consciousness:
The Strange Link Between the Human Mind and Quantum Physics
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.
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:
Department of Applied Mathematics & Theoretical Physics