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Prof Peter Knight FRS

Professor

Tel: +44 (0) 20 7594 7500
Fax: +44 (0) 20 7594 7714
Rm 918 Blackett Laboratory

 

 

 

 

 

 

Research Interests

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.

Recent Publications

Yudin, G. L., L. N. Gaier, et al. (2004). "Hole-assisted energy deposition in clusters and dielectrics in multiphoton regime." Laser Physics 14(1): 51-56.

Rekdal, P. K., B. S. K. Skagerstam, et al. (2004). "On the preparation of pure states in resonant microcavities." Journal of Modern Optics 51(1): 75-84.

Rekdal, P. K., S. Scheel, et al. (2004). "Thermal spin flips in atom chips." Physical Review A 70(1): art. no.-013811.

Knight, P. L., E. Roldan, et al. (2004). "Optical cavity implementations of the quantum walk (vol 227, pg 147, 2003)." Optics Communications 232(1-6): 443-443.

Knight, P. L., E. Roldan, et al. (2004). "Propagating quantum walks: the origin of interference structures." Journal of Modern Optics 51(12): 1761-1777.

Gaier, L. N., M. Lein, et al. (2004). "Ultrafast multiphoton forest fires and fractals in clusters and dielectrics." Journal of Physics B-Atomic Molecular and Optical Physics 37(3): L57-L67.

Berry, D. W., S. Scheel, et al. (2004). "Improving single-photon sources via linear optics and photodetection." Physical Review A 69(3): art. no.-031806.

Berry, D. W., S. Scheel, et al. (2004). "Post-processing with linear optics for improving the quality of single-photon sources." New Journal of Physics 6: art. no.-93.

Angelakis, D. G., P. L. Knight, et al. (2004). "Photonic crystals and inhibition of spontaneous emission: an introduction." Contemporary Physics 45(4): 303-318.

Scheel, S., K. Nemoto, et al. (2003). "Measurement-induced nonlinearity in linear optics." Physical Review A 68(3): art. no.-032310.

Scheel, S., J. Eisert, et al. (2003). "Hot entanglement in a simple dynamical model." Journal of Modern Optics 50(6-7): 881-889.

Sanders, B. C., S. D. Bartlett, et al. (2003). "Quantum quincunx in cavity quantum electrodynamics." Physical Review A 67(4): art. no.-042305.

Sanders, B. C., S. D. Bartlett, et al. (2003). "Photon-number superselection and the entangled coherent-state representation." Physical Review A 68(4): art. no.-042329.

Pachos, J. K. and P. L. Knight (2003). "Quantum computation with a one-dimensional optical lattice." Physical Review Letters 91(10): art. no.-107902.

Moya-Cessa, H., D. Jonathan, et al. (2003). "A family of exact eigenstates for a single trapped ion interacting with a laser field." Journal of Modern Optics 50(2): 265-273.

Maruyama, K. and P. L. Knight (2003). "Upper bounds for the number of quantum clones under decoherence." Physical Review A 67(3): art. no.-032303.

Lein, M., P. P. Corso, et al. (2003). "Orientation dependence of high-order harmonic generation in molecules." Physical Review A 67(2): art. no.-023819.

Knight, P. L., E. Roldan, et al. (2003). "Quantum walk on the line as an interference phenomenon." Physical Review A 68(2): art. no.-020301.

Knight, P. L., E. Roldan, et al. (2003). "Optical cavity implementations of the quantum walk." Optics Communications 227(1-3): 147-157.

Knight, P. L., E. A. Hinds, et al. (2003). "Practical realizations of quantum information processing - Papers of a discussion meeting held at the Royal Society on 13 and 14 November 2002 - Preface." Philosophical Transactions of the Royal Society of London Series a-Mathematical Physical and Engineering Sciences 361(1808): 1321-1322.

Hay, N., M. Lein, et al. (2003). "Investigations of electron wave-packet dynamics and high-order harmonic generation in laser-aligned molecules." Journal of Modern Optics 50(3-4): 561-577.

Tregenna, B., A. Beige, et al. (2002). "Quantum computing in a macroscopic dark period." Physical Review A 65(3): art. no.-032305.

Paspalakis, E. and P. L. Knight (2002). "On pulse propagation in a coherently prepared multi-level medium." Journal of Modern Optics 49(1-2): 201-206.

Paspalakis, E. and P. L. Knight (2002). "Transparency and parametric generation in a four-level system." Journal of Modern Optics 49(1-2): 87-95.

Paspalakis, E., N. J. Kylstra, et al. (2002). "Propagation and nonlinear generation dynamics in a coherently prepared four-level system." Physical Review A 65(5): art. no.-053808.

Paspalakis, E. and P. L. Knight (2002). "Electromagnetically induced transparency and controlled group velocity in a multilevel system." Physical Review A 66(1): art. no.-015802.

Paspalakis, E. and P. L. Knight (2002). "Transparency, slow light and enhanced nonlinear optics in a four-level scheme." Journal of Optics B-Quantum and Semiclassical Optics 4(4): S372-S375.

Lein, M., N. Hay, et al. (2002). "Role of the intramolecular phase in high-harmonic generation." Physical Review Letters 88(18): art. no.-183903.

Lein, M., N. Hay, et al. (2002). "Interference effects in high-order harmonic generation with molecules." Physical Review A 66(2): art. no.-023805.

Lein, M., J. P. Marangos, et al. (2002). "Electron diffraction in above-threshold ionization of molecules." Physical Review A 66(5): art. no.-051404.

Kim, M. S., W. Son, et al. (2002). "Entanglement by a beam splitter: Nonclassicality as a prerequisite for entanglement." Physical Review A 65(3): art. no.-032323.

Kim, M. S., J. Lee, et al. (2002). "Entanglement induced by a single-mode heat environment." Physical Review A 65(4): art. no.-040101.

Angelakis, D. G. and P. L. Knight (2002). "Testing Bell inequalities in photonic crystals." European Physical Journal D 18(2): 247-250.

Paspalakis, E. and P. L. Knight (2001). "Localizing an atom via quantum interference." Physical Review A 6306(6): art. no.-065802.

Masiak, P. and P. L. Knight (2001). "Copying of entangled states and the degradation of correlations." Fortschritte Der Physik-Progress of Physics 49(10-11): 1001-1009.

de Aldana, J. R. V., N. J. Kylstra, et al. (2001). "Atoms interacting with intense, high-frequency laser pulses: Effect of the magnetic-field component on atomic stabilization." Physical Review A 6401(1): art. no.-013411.

Dalton, B. J. and P. L. Knight (2001). "The standard model in cavity quantum electrodynamics. II. Coupling constants and atom field integration (vol 46, pg 1839, 1999)." Journal of Modern Optics 48(4): 749-749.

Bose, S., P. L. Knight, et al. (2001). "Teleportation via decay." Pramana-Journal of Physics 56(2-3): 383-391.

Bose, S., I. Fuentes-Guridi, et al. (2001). "Subsystem purity as an enforcer of entanglement." Physical Review Letters 8705(5): art. no.-050401.

Bose, S., I. Fuentes-Guridi, et al. (2001). "Subsystem parity as an enforcer of entanglement (vol 87, art. no. 050401, 2001)." Physical Review Letters 8727(27): art. no.-279901.

Beige, A., W. J. Munro, et al. (2001). "Bell's inequality test with entangled atoms (vol A 62, art. no. 052102, 2000)." Physical Review A 6404(4): art. no.-049901.

Angelakis, D. G., E. Paspalakis, et al. (2001). "Coherent phenomena in photonic crystals." Physical Review A 6401(1): art. no.-013801.

Angelakis, D. G., A. Beige, et al. (2001). "Verifying atom entanglement schemes by testing Bell's inequality." Zeitschrift Fur Naturforschung Section a-a Journal of Physical Sciences 56(1-2): 27-34.

Watson, J. B., K. Burnett, et al. (2000). "Double ionization of helium in an elliptically polarized laser field." Journal of Physics B-Atomic Molecular and Optical Physics 33(4): L103-L109.

Paspalakis, E., N. J. Kylstra, et al. (2000). "Transparency of a short laser pulse via decay interference in a closed V-type system." Physical Review A 6104(4): art. no.-045802.

Paspalakis, E. and P. L. Knight (2000). "Coherent control of spontaneous emission in a four-level system." Journal of Modern Optics 47(6): 1025-1041.

Paspalakis, E. and P. L. Knight (2000). "Spontaneous emission properties of a quasi-continuum." Optics Communications 179(1-6): 257-265.

Paspalakis, E., N. J. Kylstra, et al. (2000). "Ab initio, nonperturbative calculations of laser-induced continuum structure in helium." Laser and Particle Beams 18(3): 461-466.

Kylstra, N. J., R. A. Worthington, et al. (2000). "Breakdown of stabilization of atoms interacting with intense, high-frequency laser pulses." Physical Review Letters 85(9): 1835-1838.

Jonathan, D., M. B. Plenio, et al. (2000). "Fast quantum gates for cold trapped ions." Physical Review A 6204(4): art. no.-042307.

Corso, P. P., D. G. Lappas, et al. (2000). "Time-dependent effects in the nonsequential ionization of helium at various wavelengths." Laser and Particle Beams 18(3): 433-441.

Buzek, V., P. L. Knight, et al. (2000). "Multiple observations of quantum clocks." Physical Review A 6206(6): art. no.-062309.

Beige, A., S. F. Huelga, et al. (2000). "Coherent manipulation of two-dipole-dipole interacting ions." Journal of Modern Optics 47(2-3): 401-414.

Beige, A., D. Braun, et al. (2000). "Quantum computing using dissipation to remain in a decoherence- free subspace." Physical Review Letters 85(8): 1762-1765.

Beige, A., W. J. Munro, et al. (2000). "Bell's inequality test with entangled atoms." Physical Review A 6205(5): art. no.-052102.

Beige, A., S. Bose, et al. (2000). "Entangling atoms and ions in dissipative environments." Journal of Modern Optics 47(14-15): 2583-2598.

Beige, A., D. Braun, et al. (2000). "Driving atoms into decoherence-free states." New Journal of Physics 2: art. no.-22.

Angelakis, D. G., E. Paspalakis, et al. (2000). "Transient properties of modified reservoir-induced transparency." Physical Review A 6105(5): art. no.-055802.