I am a theoretical physicist who has worked mainly on the quantum properties of the electromagnetic field and its interaction with atomic systems. My field of study is termed quantum optics and is shaped for the most part by the properties of laser light.Optics before the laser was concerned entirely with the production, manipulation and detection of noise.The light which our eyes receives from natural processes is wildly fluctuating in amplitude and phase, reflecting the chaotic, random environment which gave birth to these photons.Each atom in a natural light source is excited independently. Once excited, an atom emits radiation in two ways, either spontaneously or by stimulated emission, induced by the surrounding radiation. In the natural world outside the laboratory, photons are spontaneously emitted and are completely uncorrelated in phase with any of their neighbours.In a laser, a cooperative phase transition is possible in which a collective, ordered light field is established.Quantum optics is concerned with the nature of optical correlations, the description of coherence and the properties of photons and their interaction with atoms. Laser light can be extraordinarily intense and is responsible for dramatic nonlinearities, which are exploited in important new optical technologies (long distance communications, ultrafast optical logic, isotope separation and many more).But the subject also is concerned with fundamental and generic ideas of theoretical physics: quantum coherence, nonlinearities and phase transitions from disorder to order.
In what follows, I describe some of my recent research:
In strong field physics: I have worked in this area for the past 30 years. I (with Shore) was the first to predict that atoms driven by intense laser fields would radiate high harmonics; and produced the first code to describe atomic wave-packets driven by laser fields of arbitrary polarisation. Together with Keith Burnett I formulated the very successful “CRAPOLA” model which explains non-sequential ionisation of multi-electron atoms; and made a detailed analysis of the nature of stabilisation of atoms in super-intense fields. My work on strong fields has been done in close collaboration with Keith Burnett since the early 1980’s. Many papers were published, all in international journals, and have received excellent citations. The Science Citation Index lists me as the world’s highest-cited quantum optician for the 16 years to 1997. I have been interviewed several times on Radio 4 and by the BBC World Service on work connected to this work. Many seminars were given at other institutions, both in the UK and abroad. I was invited to review the field by Reports on Progress in Physics, and this article has been one of the most-accessed electronic articles within the IOP data base.
A very considerable degree of success has been achieved on some of the outstanding problems in the theory of laser-atom interactions at high intensity.For many years I have been involved in the development of entirely non-perturbative descriptions of atoms in intense laser fields, using dressed state techniques for resonant situations and wave-packet methods for non-resonant and more general excitation. In the last 5 years I have also been interested in electron correlation effects in intense field physics. These include the development of a simplified approach to dealing with correlations in 2-electron systems (the CRAPOLA model). I have also studied high harmonic generation (HHG), and the properties of atoms in intense fields in the relativistic regime. I have developed the first non-perturbative relativistic theories of atoms in very strong fields using both Monte-Carlo classical and fully quantum mechanical methods. I have also studied the way in which phase control can be used to steer coherently the outcome of multi-photon reactions. One of the most intense debates in multi-photon physics in recent years has been the physical nature of non-sequential ionisation of multi-electron atoms. I have developed in collaboration with Keith Burnett a simplified model of non-sequential ionisation, which treats the interaction between electrons using a version of unrestricted time-dependent Hartree-Fock theory. This has provided important insights into the nature of the non-sequential ionisation process and agrees with experimental results. The model strongly supports the re-interaction picture of the re-colliding electron knocking out the inner electron. We have also studied the correlations between harmonic generation and non-sequential ionisation. We extended the original model to handle elliptically polarised light. There has been a good deal of interest in the model and has resulted in a number of invited and plenary conference presentations. One of the potential limits to this “Crapola” method relates to the fact that by ignoring part of the electron correlation it may screen the inner electron too thoroughly at low intensities. This would lead to too low a threshold for the non-sequential ionisation (at least for the lower intensities data). To examine this issue we have looked into the role of electron correlation, through the use of a fully quantal treatment of the re-collision of the electron wave-packet. This supports the re-collision picture and the crapola method and is in full agreement with experiment.
In quantum optics and quantum information processing (QIP), my group and my former students (eg Ekert, Barnett, Phoenix, Kim, Vedral, Bose, Jonathan) have played a major role in establishing the UK as a world-class centre of research. I have concentrated on quantum optical realizations of QIP), especially those in a realistically hostile dissipative environment. In recent years a new fundamental concept of quantum computation has been developed. Instead of using classical bits that can represent either the values 0 or 1, the basic unit of a quantum computer is a quantum mechanical two-level system (qubit) that can exist in coherent superpositions of the logical values 0 and 1. A set of n qubits can then be in a superposition of up to 2n different states, each representing a binary number. Were we able to control and manipulate say 1500 qubits, we could access more states than there are particles in the visible universe. Computations are implemented by unitary transformations, which act on all state of a superposition simultaneously. Quantum gates form the basic units from which these unitary transformations are build up. I have studied the limitations decoherence place on such manipulations (especially for ion trap realizations) and have proposed ways in which decoherence can be controlled and utilized. The use of the quantum mechanical superpositions and entanglement results in a high degree of parallelism, which can increase the speed of computation exponentially. A number of problems which cannot be tackled on a classical computer can be solved efficiently on a quantum computer. In 1994 a quantum algorithm was discovered by Peter Shor which allows the solution of a practically important problem, namely factorization, with such an exponential increase of speed. Subsequently, possible experimental realizations of a quantum computer have been proposed, e.g. in linear ion traps and nuclear magnetic resonance schemes. Presently we are at a stage where quantum gates have been demonstrated in these two implementations. Quantum computation is closely related to quantum cryptography and quantum communication. Basic experiments demonstrating the in-principle possibility of these ideas have been carried out in various labs which have strong links to my theory research group (I lead a European collaboration funded by the European Union in this area).
The linear ion trap is one of the most promising systems for quantum computation and is one I have studied in detail. The quantum state preparation (laser cooling and optical pumping) in this system is a well-established technique, as is the state measurement by electron shelving and fluorescence. Singly-charged ions of an atom such as calcium or beryllium are trapped and laser-cooled to micro Kelvin temperatures, where they form a string lying along the axis of a linear rf Paul trap. The internal state of any one ion can be exchanged with the quantum state of motion of the whole string. This can be achieved by illuminating the ion with a pulse of laser radiation at a frequency tuned below the ion's internal resonance by the vibrational frequency of one of the normal modes of oscillation of the string. This couples single phonons into and out of the vibrational mode. The motional state can then be coupled to the internal state of another ion by directing the laser onto the second ion and applying a similar laser pulse. In this way general transformations of the quantum state of all the ions can be generated. The ion trap has several features to recommend it. It can achieve processing on quantum bits without the need for any new technological breakthroughs, such as micro-fabrication techniques or new cooling methods. The state of any ion can be measured and re-prepared many times without problem, which is an important feature for implementing quantum error correction protocols. I have investigated in detail the construction of non-classical states of trapped ions. I have also proposed recently a way of using light shifts to speed up the processor. The trapped ions can be strongly coupled to an electromagnetic field mode in a cavity, which permits the powerful combination of quantum processing and long-distance quantum communication, and again I have made a detailed study of such effects. This suggests ways in which we may construct quantum memories. The upper limit to the number of qubits which ion traps might manipulate is known from my work with Martin Plenio to be limited by spontaneous emission, but these systems can almost certainly realise a quantum processor larger than any which could be thoroughly simulated by classical computing.
"Linear Quantum Trajectories: Applications to Continuous Projection
Measurements"
K Jacobs and P L Knight
Phys Rev A57, 2301, (1998);P
"Generation and Preservation of Coherence in Dissipative Quantum
Optical Environments"
B M Garraway, P L Knight and M B Plenio
Physica Scripta T76, 152 (1998), Proc Nobel Symposium 1998.This
volume appeared in book form as Modern Studies of Basic Quantum
Concepts and Phenomena eds E B Karlsson and E Brandas, World
Scientific, Singapore (1999);S
"Radiation Back-Reaction in Ultra-Intense Laser-Atom Interactions"
C H Keitel, C Szymanowski, P L Knight and A Maquet
J Phys B 31, L75, (1998);P
"Squeezed States: Basic Principles"
P L Knight and V Buzek
in " Squeezed Light " edited by P D Drummond (Springer-Verlag, 2000)
in press;R
"A Multiparticle Generalization of Entanglement Swapping"
S Bose, V Vedral and P L Knight
Phys Rev A 57, 822, (1998);P
"Flocks of Quantum Clones: Multiple Copying of Qubits"
V Buzek, M Hillery and P L Knight
Fortschritte Physik Special Issue on Quantum Computation 46, 521
(1998);P
"Phase Control of a Two Channel Ionization System"
E Paspalakis, A Patel, M Protopapas and P L Knight
J Phys 1, 761, (1998);P
"Phase Control of Spontaneous Emission"
E Paspalakis and P L Knight
Phys Rev Lett 81, 293, (1998);P
Implementations of Quantum Logic: Fundamental and Experimental
Limits"
S Bose, P L Knight, M Murao, M Plenio, V Vedral
Phil Trans Roy Soc 356, 1823, (1998);S
"Multi-particle Entanglement Purification Protocols"
M Murao, M B Plenio, S Popescu, V Vedral and P L Knight
Phys Rev A57, R4075, Rapid Communication, (1998);P
"A Scheme to Probe the Decoherence of a Macroscopic Object"
S Bose, K Jacobs and P L Knight
Phys Rev A59, 3204, (1999);P
311. "Trapped Mode Contribution to Spontaneous Emission in a
Planar Fabry-Perot Microcavity"
B J Dalton, M Babiker and P L Knight
Optics Communications 152, 36, (1998);P
"Population Transfer via an Autoionizing State with Temporally
Delayed Chirped Laser Pulses"
E Paspalakis and P L Knight
J Phys B 31, 2753, (1998);P
"Stabilization with Arbitrary Laser Polarization"
A Patel, M Protopapas, D G Lappas and P L Knight
Phys Rev A58, Rapid Communication R2652, (1998);P
"Spontaneous Emission-Induced Coherent Effects on Absorption and
Dispersion of a V-Type Three Level Atom "
E Paspalakis, S-Q Gong and P L Knight
Opt Comm 152, 293, (1998);P
"Decoherence in Nonclassical Motional States of a Trapped ion"
M Murao and P L Knight
Phys Rev A58, 663 (1998);P
"Total Fluorescence Inhibition via Incoherent Dressed Population
Transfer"
C H Keitel, S-Y Zhu and P L Knight
Laser Physics 9, 826, (1999);P
"Fluorescence Control through Multiple Interference Mechanisms"
E Paspalakis, C H Keitel and P L Knight
Phys Rev A58, 4868 (1998);P
"Laser-Induced Continuum Structure in Helium: ab initio,
non-perturbative calculations"
N J Kylstra, E Paspalakis and P L Knight
J Phys 1, L719 (1998);P
"Can Harmonic Generation cause Non-Sequential Ionisation?"
A Sanpera, J B Watson, S E J Shaw, P L Knight, K Burnett and M
Lewenstein
J Phys 1, L841 (1998);P
"Ion Trap Quantum Gates, Decoherence and Error Correction"
P L Knight, M Murao, M B Plenio and V Vedral
in Special Issue on Quantum Computing of the Interdisciplinary
Journal of Nonlinear Science 'Chaos, Solitons and Fractals 10, 1621
Elsevier (1999);P
"High-Order Nonlinearities in the Motion of a Trapped Ion"
S Wallentowitz, W Vogel and P L Knight
Phys Rev A59, 531 (1999);P
"Effects of Spontaneous Emission Interference on Population
Inversions of a V-type Atom"
S-Q Gong, E Paspalakis and P L Knight
J Mod Opt 45, 2433 (1998);P
" Entanglement in Frequency Standards and Quantum Information Theory"
S F Huelga, P L Knight, C Macchiavello, M B Plenio and V Vedral
Applied Physics B 67, 723 (1998);P
The Construction and Detection of Non-Classical States in Quantum
Optics , P L Knight, Plenary Talk (extended abstract of ref 301), in
Proceedings of the 5th Wigner Symposium, eds P Kasperkovitz and D
Grau, (World Scientific, Singapore, 1998);S
Quasi Mode Theory of Macroscopic Canonical Quantization in Quantum
Optics and Cavity Quantum Electrodynamics,
B J Dalton, S M Barnett and P L Knight
J Mod Opt 46,1315 (1999);P
Recollisions and High Harmonic Generation
P L Knight, A Patel, M Protopapas, N J Kylstra, D G Lappas, K
Burnett, A Sanpera, S J Shaw and J Watson
Proc International Conference on Atomic Physics (1998), Windsor,
Canada, AIP ed W E Baylis and G Drake , (1998);S
Manipulation of Entangled States for Quantum Information
Processing , S Bose, S F Huelga, D Jonathan, P L Knight, M Murao, M
B Plenio and V Vedral,
in Quantum Communication, Computing and Measurement 2 , Proc 5th
International Conference on Quantum Communication, Measurement and
Computation, eds P Kumar, G M D B9Ariano and O Hirota ( Plenum 1998);S
Copying of Entangled States and the Degradation of Correlations
P Masiak and P L Knight
quant-ph/9808043;C'
Purification of Multi-Particle Entanglement
M Murao, M B Plenio, S Popescu, V Vedral and P L Knight
In Quantum Coherence and Decoherence , Proc ISQM, Tokyo B998
eds K Fujikawa and Y A Ono ( Elsevier, Amsterdam, 1998);S
Transparency induced via Decay Inteference
E Paspalakis, N Kylstra and P L Knight
Phys Rev Lett 82, 2079, (1999);P
Cavity Loss Induced Generation of Entangled Atoms
M B Plenio, S F Huelga, A Beige and P L Knight
Phys Rev A59, 2468 (1998);P
Survival of Entanglement after Quantum Copying,
P Masiak and P L Knight
In Mysteries, Puzzles and Paradoxes in Quantum Mechanics , AIP
Conference proceedings 461, (1999), p324
Proc Garda Conference on Quantum Mysteries ed R Bonifacio
(1998);S
Propagation Dynamics in Media with Interfering Dissipation
Mechanisms
E Paspalakis, N J Kylstra and P L Knight
Laser Physics 9, 819 (1999);P
Coherent Manipulation of Two Dipole-Dipole Interacting Ions
A Beige, S F Huelga, M B Plenio, R C Thompson and P L Knight
J Mod Opt 47, 401 (2000);P
Purification via Entanglement Swapping and Conserved Entanglement
S Bose, V Vedral and P L Knight
Phys Rev A60, 194 (1998);P
Quantum Scattering Theory Approach to Quantum Optical
Measurements
B J Dalton, S M Barnett and P L Knight
J Mod Opt 46, 1107, (1999);P
Macroscopic Canonical Quantization in Quantum Optics: Properties of
Quasi-Mode Annihilation and Creation Operators
B J Dalton, S M Barnett and P L Knight
J Mod Opt 46, 1495, (1999);P
Quasi-mode Theory of the Beam Splitter: a Quantum Scattering Theory
Approach
B J Dalton, S M Barnett and P L Knight
J Mod Opt 46, 1559,(1999) ;P
Numerical Simulations of Fundamental Processes in cavity QED:
Atomic Decay
V Buzek, G Drobny, Min Gyu Kim, M Havukainen and P L Knight
Phys Rev A60, 582, (1999);P
Double Ionisation of Helium in an Elliptically Polarised Laser
Field
K Burnett, J B Watson and P L Knight
J Phys 3, L103 (2000);P
Ellipticity and Pulse Shape Dependence of Localized Wavepackets
A Patel, N J Kylstra and P L Knight
Optics Express 4, 496 (1999);P
Propagation Dynamics in an Autoionization Medium
E Paspalakis, N Kylstra and P L Knight
Phys Rev A60, 642, (1999);P
Transparency near a Photonic Band Edge
E Paspalakis, N Kylstra and P L Knight
Phys Rev A (Rapid Commun) 60, R33, (1999);P
Time-dependent effects in the Non-Sequential Double Ionization of
Helium at Various Wavelengths
P P Corso, D G Lappas and P L Knight
Laser and Particle Beams 18,1(2000);P
Coherent Control of Spontaneous Emission in a Four Level System
E Paspalakis and P L Knight
J Mod Opt 47, 1025 (2000);P
Spontaneous Emission Properties of a Quasi-Continuum
E Paspalakis and P L Knight
Optics Communications 179, 257 (2000);P
Ab Initio, Non-Perturbative Calculations of Laser-Induced Continuum
Structure in Helium
E Paspalakis, N Kylstra and P L Knight
Lasers and Particle beams, Proc ULIA Conference, Crete (1999);S
The Standard Model in Cavity Quantum Electrodynamics: I General
Features of Mode Functions for a Fabry Perot Cavity
B J Dalton and P L Knight
J Mod Opt 46, 1817 (1999);P
The Standard Model in Cavity Quantum Electrodynamics: II Coupling
Constants and Atom-Field Coupling
B J Dalton and P L Knight
J Mod Opt 46, 1839, (1999);P
The Influence of Density of Modes on Dark Lines in Spontaneous
Emission
E Paspalakis, D G Angelakis and P L Knight
Optics Communications 172, 229 (1999);P
Proposal for Teleportation of an Atomic State via Cavity Decay
S Bose, P L Knight, M B Plenio and V Vedral
Phys Rev Letters 83, 5158 (1999); quant-ph/9908004;P
"Quantum Computation using Dissipation to remain in a decoherence-free
subspace"
A Beige, D Braun, B Tregenna and P L Knight
Phys Rev Lett 85, 1762, (2000); quant-ph/9912004;P
"Effect of Laser Pulse s on the Stabilization of a Model Atom"
A Patel, N J Kylstra and P L Knight
J Phys 2, 5759 (1999);P
"A Bell's Inequality Test with Entangled Atoms"
A Beige, P L Knight and W J Munro
Phys Rev A62, 052102 (2000); quant-ph0006054;P
"Fast Quantum Gates for Cold Trapped Ions"
D Jonathan, M B Plenio and P L Knight
Phys Rev A62, 042307(2000); quant-ph/0002092;P
"Effects of Pulse duration and Wavelength on the Non-Sequential
Ionization of Helium"
P P Corso, D Lappas and P L Knight
In "Multiphoton Processes," eds L DiMauro, R R Freeman and K C
Kulander, AIP Conference Proceedings 525, (2000),Proc ICOMP,
Monterey (1999);S
"Transient Properties of Modified Reservoir-Induced Transparency"
D G Angelakis, E Paspalakis and P L Knight
Phys Rev A61, 5802 (2000);P
"Transparency of a short laser pulse via decay interference in a
closed V system"
E Paspalakis, N J Kylstra, and P L Knight,
Phys Rev A61, 5802 (2000);P
"Breakdown of Stabilization of Atoms Interacting with Intense, High
Frequency Laser Pulses"
N J Kylstra, R A Worthington, A Patel, P L Knight, J R Vazquez de
Aldana and L Roso
Phys Rev Letters 85, 1835 (2000);P
"Recycling of Quantum Information: Multiple Observations of Quantum
Clocks"
V Buzek, P L Knight and N Imoto
Phys Rev A62, 062309 (2000); quant-ph0006048;P
"Driving Atoms into Decoherence-free States
A Beige, D Braun and P L Knight
New Journal of Physics, 2, 22.1 (2000);P
"Entangling Atoms and Ions in Dissipative Environments"
A Beige, S Bose, D Braun, S F Huelga, P L Knight, M B Plenio and V
Vedral
J Mod Opt 47, 2583 (2000);P
"Entanglement Quantification"
V Vedral, M B Plenio and P L Knight
In "The Physics of Quantum Information", edited by D Bouwmeester, A
Ekert and A Zeilinger, Springer (2000);C
"Limits to Quantum Computation due to Decoherence""
M B Plenio and P L Knight
In "The Physics of Quantum Information", edited by D Bouwmeester, A
Ekert and A Zeilinger, Springer (2000);C
"Generalisation of Purification to Multiparticle Entanglement""
M Murao, M B Plenio, S Popescu, V Vedral and P L Knight
In "The Physics of Quantum Information", edited by D Bouwmeester, A
Ekert and A Zeilinger, Springer (2000);C
"Coherent Phenomena in Photonic Crystals"
D G Angelakis, E Paspalakis and P L Knight,
Phys Rev A in press (2001) quant-ph/00
Intense Laser-Atom Interactions: Beyond the Dipole Approximation
N J Kylstra, R M Potvliege, R A Worthington, A S Patel,P L Knight, J
R Vaquezde Aldana, L Roso and C J Joachain, Proc SILAP, ed B Piraux
Localizing an Atom via Quantum Interference
E Paspalakis and P L Knight
Submitted to Phys Rev A (2000)
Transparency and Parametric Generation in a Four-Level System
E Paspalakis and P L Knight, Phys Rev A (Rapid) (2000)
Mechanism of the Double Ionization of Helium
D G Lappas, P L Knight and P P Corso
Rutherford Lab Central Laser Facility Annual Report 2000
Mechanism of the Non-Sequential Double Ionization of Helium
D G Lappas, P L Knight and P P Corso, Proc SILAP (2000) ed B Piraux.
Testing Bell Inequalities in Photonic Crystals
D G Angelakis and P L Knight
To be submitted for publication (2000).
Quantum Computing in the Dark
B Tregenna, A Beige and P L Knight
To be submitted for publication
Where the Weirdness Comes from
P L Knight
Nature 395 12 (1998) cited in Michael Frayn, Copenhagen ,
(Methuen, 2000)
Atoms Interacting with Intense, High Frequency Laser Pulses: Effect
of the Magnetic Field Component on Atomic Stabilization
J R Vazquez de Aldana, N J Kylstra, L Roso, P L Knight, A Patel and R
A Worthington
Submitted to Physical Review A (2000)