Tomographic quantum cryptography
(2003-08) Liang, Y.C.; Kaszlikowski, D.; Englert, B.-G.; Kwek, L.C.; Oh, C.H.; PHYSICS; DEAN'S OFFICE (SCIENCE)
A report on a protocol for quantum cryptogrphy, used for the purpose of quantum state tomography, was presented. The incoherence was found in eavesdropping attacks. The conditions under which a secure key cryptographic key was generated were also explained. The investigation was carried out for the channels, transmitting quantum systems of any finite dimension.
Ascertaining the values of σx, σy, and σz of a polarization qubit
(2003-05-02) Schulz, O.; Steinhübl, R.; Weber, M.; Englert, B.-G.; Kurtsiefer, C.; Weinfurter, H.; PHYSICS
The 1987 spin-retrodiction puzzle by Vaidman, Aharonov, and Albert - the King's problem - was realized in the form of a quantum-optical analog. Success probabilities that exceed, in each channel, the single-qubit optimum were achieved. Furthermore, the mean king's second challenge, in which unitary polarizations changes are performed, was successfully implemented.
Information-theoretic approach to single-particle and two-particle interference in multipath interferometers
(2003-07-18) Kaszlikowski, D.; Kwek, L.C.; Zukowski, M.; Englert, B.-G.; PHYSICS
An information-theoretic definition of the measure of the strength of two-particle interference is presented. It exploits the mutual information contained in the coincidence probabilities. The corresponding single-particle measure is, in some sense, the fringe-contrast analog of the entropic measure for path knowledge.
Tomographic Quantum Cryptography: Equivalence of Quantum and Classical Key Distillation
(2003-08-29) Bruß, D.; Christandl, M.; Ekert, A.; Englert, B.-G.; Kaszlikowski, D.; Macchiavello, C.; PHYSICS
The class of protocols which exploit tomographically complete measurement on entangled pairs of any dimension was discussed. It was found that the noise threshold for classical advantage distillation was identical with the threshold for quantum entanglement distillation. The analysis showed that the two distillation procedures were equivalent and neither offered a security advantage over the other.
Path distinguishability in double scattering of light by atoms
(2007-08-08) Miniatura, C.; Müller, C.A.; Lu, Y.; Wang, G.; Englert, B.-G.; PHYSICS
Wave-particle duality finds a natural application for electrons or light propagating in disordered media where coherent corrections to transport are given by two-wave interference. For scatterers with internal degrees of freedom, these corrections are observed to be much smaller than would be expected for structureless scatterers. By examining the basic example of the scattering of one photon by two spin-1/2 atoms-a case study for coherent backscattering-we demonstrate that the loss of interference strength is associated with which-path information stored by the scattering atoms. © 2007 The American Physical Society.
A simple minimax estimator for quantum states
(2012-06) Ng, H.K.; Englert, B.-G.; PHYSICS
Quantum tomography requires repeated measurements of many copies of the physical system, all prepared by a source in the unknown state. In the limit of very many copies measured, the often-used maximum-likelihood (ML) method for converting the gathered data into an estimate of the state works very well. For smaller data sets, however, it often suffers from problems of rank deficiency in the estimated state. For many systems of relevance for quantum information processing, the preparation of a very large number of copies of the same quantum state is still a technological challenge, which motivates us to look for estimation strategies that perform well even when there is not much data. After reviewing the concept of minimax state estimation, we use minimax ideas to construct a simple estimator for quantum states. We demonstrate that, for the case of tomography of a single qubit, our estimator significantly outperforms the ML estimator for small number of copies of the state measured. Our estimator is always full-rank, and furthermore, has a natural dependence on the number of copies measured, which is missing in the ML estimator. © 2012 World Scientific Publishing Company.
Wave-particle duality in multi-path interferometers: General concepts and three-path interferometers
(2008-02) Englert, B.-G.; Kaszlikowski, D.; Kwek, L.C.; Chee, W.H.; CENTRE FOR QUANTUM TECHNOLOGIES; PHYSICS
For two-path interferometers, the which-path predictability P and the fringe visibility ν are familiar quantities that are much used to talk about wave-particle duality in a quantitative way. We discuss several candidates that suggest themselves as generalizations P of P for multi-path interferometers, and treat the case of three paths in considerable detail. To each choice for the path knowledge P , the interference strength V - the corresponding generalization of ν - is found by a natural, operational procedure. In experimental terms, it amounts to finding those equal-weight superpositions of the path amplitudes which maximize P for the emerging intensities. Mathematically speaking, one needs to identify a certain optimum one among the Fourier transforms of the state of the interfering quantum object. Wave-particle duality is manifest, inasmuch as P = 1 implies V = 0 and V = 1 implies P = 0, whatever definition is chosen. The possible values of the pair (P,V) are restricted to an area with corners at (P,V) = (0,0) , (P,V) = (1,0), and (P,V) = (0,1) , with the shape of the border line from (1,0) to (0,1), depending on the particular choice for P and the induced definition of V. © 2008 World Scientific Publishing Company.
Solvable model of a strongly driven micromaser
(2004-02) Lougovski, P.; Casagrande, F.; Lulli, A.; Englert, B.-G.; Solano, E.; Walther, H.; PHYSICS
The dynamics of a strongly driven micromaser based on the resonant interaction of one mode of a high-Q cavity with Poissonian low density beam of two-level atoms strongly driven by a resonant classical field, was investigated. The time-dependent solution of the master equation and the expressions of the main statistical quantities for cavity field and the detected atoms, was presented. The theory of a strongly driven micromaser was developed and the features of the field statistics and the atomic correlations were studied. The results show that in the strong coupling regime, superpositions of coherent states can be generated in the transient dynamics, whose coherence vanishes towards its superpoissonian steady state.
Codes for key generation in quantum cryptography
(2005-11) Englert, B.-G.; Fu, F.-W.; Niederreiter, H.; Xing, C.; TEMASEK LABORATORIES; MATHEMATICS; PHYSICS
As an alternative to the usual key generation by two-way communication in schemes for quantum cryptography, we consider codes for key generation by one-way communication. We study codes that could be applied to the raw key sequences that are ideally obtained in recently proposed scenarios for quantum key distribution, which can be regarded as communication through symmetric four-letter channels. © World Scientific Publishing Company.
Micromotion in trapped atom-ion systems
(2012-05-31) Nguyên, L.H.; Kalev, A.; Barrett, M.D.; Englert, B.-G.; CENTRE FOR QUANTUM TECHNOLOGIES; PHYSICS
We examine the validity of the harmonic approximation, where the radio-frequency ion trap is treated as a harmonic trap, in the problem regarding the controlled collision of a trapped atom and a single trapped ion. This is equivalent to studying the effect of the micromotion since this motion must be neglected for the trapped ion to be considered as a harmonic oscillator. By applying the transformation of Cook and Shankland we find that the micromotion can be represented by two periodically oscillating operators. In order to investigate the effect of the micromotion on the dynamics of a trapped atom-ion system, we calculate (i) the coupling strengths of the micromotion operators by numerical integration and (ii) the quasienergies of the system by applying the Floquet formalism, a useful framework for studying periodic systems. It turns out that the micromotion is not negligible when the distance between the atom and the ion traps is shorter than a characteristic distance. Within this range the energy diagram of the system changes remarkably when the micromotion is taken into account, which leads to undesirable consequences for applications that are based on an adiabatic process of the trapped atom-ion system. We suggest a simple scheme for bypassing the micromotion effect in order to successfully implement a quantum controlled phase gate proposed previously and create an atom-ion macromolecule. The methods presented here are not restricted to trapped atom-ion systems and can be readily applied to studying the micromotion effect in any system involving a single trapped ion. © 2012 American Physical Society.

Berthold-Georg Englert

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