Adrian Kent

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.

Academic Affiliations

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

Research: Quantum foundations, quantum information theory and quantum cryptography

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.)

Relativistic Quantum Cryptography


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.


Bit commitment
using quantum information and relativistic signalling constraints

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

Experimental implementation of one of my bit commitment protocols
using quantum information and relativistic signalling constraints

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 .


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.

Quantum Key Distribution with security based (only) on No-Signalling


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.

Device-Independent Quantum Cryptography


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.


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)


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.)

Quantum Digital Signature Schemes

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 trying to help develop this research further:
Quantum digital signatures with quantum key distribution components

Other Work on Quantum Cryptography

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

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? Many Worlds? Everett, Quantum Theory, and Reality was published by Oxford University Press in June 2010.

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.


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 .

Popular Account

A popularized discussion of some of the arguments considered in the book was published in Plus Magazine here.

Other articles on many worlds quantum theory

Taking up earlier ideas of Vaidman, Sebens and Carroll have recently (2014) argued that probability in many worlds quantum theory can be understood as referring to the uncertainty that observers should purportedly feel about which branch they are in, after a measurement has taken place but before they have observed the result. Some of the (very many!) problems with this idea, as I see them, are set out in
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.


Many Worlds at 50 With Jonathan Barrett and David Wallace, I coorganised the "Many Worlds at 50" conference held at Perimeter Institute in September 2007 to mark the 50th anniversary of Everett's original paper.

The conference details are here.

The talks are archived here.

Generalizations of Quantum Theory and Experimental Tests


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

Related Experimental Work

from Salart et al. (op. cit.)
from Salart et al. (op. cit.)
from Salart et al. (op. cit.)

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 .


The Quantum Landscape

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-Locality and Experimental Tests

Anti-correlations produced by the quantum singlet compared
to bounds on the anti-correlations obtainable from local hidden
variable theories and to specific LHV theories

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

Quantum Non-Contextuality and the Finite Precision Loophole

Noncontextuality, Finite Precision Measurement and the Kochen-Specker Theorem
Simulating Quantum Mechanics by Non-Contextual Hidden Variables
Non-Contextual Hidden Variables and Physical Measurements

The Consistent or Decoherent Histories Approach to Quantum Theory and Its Problems

Quantum Histories
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

Work on the Quantum Reality Problem

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

Popular Articles

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

Other topics

Risk Analysis for hypothetical extinction catastrophes

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.

Speculative Exobiology and the Fermi Problem


Too Damned Quiet?


War of the Worlds The MIT Technology Review blog's report.

Earlier research: Conformal field theory, representation theory and integrable models

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.

Research Students

Former PhD students:

Matthias Doerrzapf, Senior Tutor and College Lecturer in Mathematics, St. John's College, Cambridge.
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.

Current PhD students:

Emily Adlam joined our group in October 2014.


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).

Science and Literature

Copenhagen 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.

A.P.A.Kent at


Department of Applied Mathematics & Theoretical Physics
Centre for Mathematical Sciences
Wilberforce Road
Cambridge CB3 0WA
United Kingdom

Last updated September 2014.