Results for "Berthold-Georg Englert"

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The Statistical AtomJul 26 2018This review is the updated and enlarged version of a talk delivered by J. S. on the occasion of the 1982 meeting of Nobel laureates at Lindau, and of talks given by B.-G. E. at several West German universities and Max Planck institutes in 1984.
Mutually unbiased bases in dimension six: The four most distant basesMar 05 2011We consider the average distance between four bases in dimension six. The distance between two orthonormal bases vanishes when the bases are the same, and the distance reaches its maximal value of unity when the bases are unbiased. We perform a numerical ... More
Minimal tomography with entanglement witnessesJun 22 2009We introduce informationally complete measurements whose outcomes are entanglement witnesses and so answer the question of how many witnesses need to be measured to decide whether an arbitrary state is entangled or not: as many as the dimension of the ... More
Structure of Two-qubit Symmetric Informationally Complete POVMsAug 06 2010In the four-dimensional Hilbert space, there exist 16 Heisenberg--Weyl (HW) covariant symmetric informationally complete positive operator valued measures (SIC~POVMs) consisting of 256 fiducial states on a single orbit of the Clifford group. We explore ... More
Test-State Approach to the Quantum Search ProblemFeb 17 2011The search for "a quantum needle in a quantum haystack" is a metaphor for the problem of finding out which one of a permissible set of unitary mappings---the oracles---is implemented by a given black box. Grover's algorithm solves this problem with quadratic ... More
One-atom maser: Non-separable atom pairsSep 24 2002When a one-atom maser is operated in the standard way -- excited, resonant two-level atoms traverse the resonator at random times -- the emerging atoms are entangled with the cavity field. As a consequence, the results of measurements on different atoms ... More
Measuring irreversible dynamics of a quantum harmonic oscillatorAug 17 2001We show that the unitary evolution of a harmonic oscillator coupled to a two-level system can be undone by a suitable manipulation of the two-level system -- more specifically: by a quasi-instantaneous phase change. This enables us to isolate the dissipative ... More
Experimental detection of entanglement with optimal-witness familiesFeb 24 2014Sep 28 2014We report an experiment in which one determines, with least tomographic effort, whether an unknown two-photon polarization state is entangled or separable. The method measures whole families of optimal entanglement witnesses. We introduce adaptive measurement ... More
Controllable generation of mixed two-photon statesApr 08 2013We report a controllable method for producing mixed two-photon states via Spontaneous Parametric Down-Conversion with a two-type-I crystal geometry. By using variable polarization rotators (VPRs), one obtains mixed states of various purities and degrees ... More
On Quantum TheoryAug 24 2013Nov 21 2013Quantum theory is a well-defined local theory with a clear interpretation. No "measurement problem" or any other foundational matters are waiting to be settled.
Comment on "Minimum Uncertainty and Entanglement"Aug 04 2011Sep 03 2011Dass, Qureshi, and Sheel conjecture that the lower bound in the Heisenberg-Robertson uncertainty relation cannot be reached in mixed states. The conjecture is wrong.
Julian Schwinger and the Semiclassical AtomJul 10 2019In the early 1980s, Schwinger made seminal contributions to the semiclassical theory of atoms. There had, of course, been earlier attempts at improving upon the Thomas--Fermi model of the 1920s. Yet, a consistent derivation of the leading and next-to-leading ... More
Elements of Micromaser PhysicsMar 13 2002The elements of micromaser physics are reviewed in a tutorial way. The emphasis is on the basic theoretical concepts, not on technical details or experimental subtleties. After a brief treatment of the atom-photon interaction according to the Jaynes-Cummings ... More
The mean king's problem: Spin 1Jan 16 2001We show how one can ascertain the values of four mutually complementary observables of a spin-1 degree of freedom.
Quantum State Tomography with Joint SIC POMs and Product SIC POMsMay 23 2011We introduce random matrix theory to study the tomographic efficiency of a wide class of measurements constructed out of weighted 2-designs, including symmetric informationally complete (SIC) probability operator measurements (POMs). In particular, we ... More
Remarks on 2-q-bit statesJul 17 2000We distinguish six classes of families of locally equivalent states in a straightforward scheme for classifying all 2-q-bit states; four of the classes consist of two subclasses each. The simple criteria that we stated recently for checking a given state's ... More
Density Functional of a Two-Dimensional Gas of Dipolar Atoms: Thomas-Fermi-Dirac TreatmentAug 06 2010Mar 06 2011We derive the density functional for the ground-state energy of a two-dimensional, spin-polarized gas of neutral fermionic atoms with magnetic-dipole interaction, in the Thomas-Fermi-Dirac approximation. For many atoms in a harmonic trap, we give analytical ... More
Efficient quantum key distribution with trines of reference-frame-free qubitsOct 28 2009We propose a rotationally-invariant quantum key distribution scheme that uses a pair of orthogonal qubit trines, realized as mixed states of three physical qubits. The measurement outcomes do not depend on how Alice and Bob choose their individual reference ... More
Five Lectures On Dissipative Master EquationsJun 18 20021 First Lecture: Basics 1.1 Physical Derivation of the Master Equation 1.2 Some Simple Implications 1.3 Steady State 1.4 Action to the Left 2 Second Lecture: Eigenvalues and Eigenvectors of L 2.1 A Simple Case First 2.2 The General Case 3 Third Lecture: ... More
Symmetric coupling of four spin-1/2 systemsApr 12 2012We address the non-binary coupling of identical angular momenta based upon the representation theory for the symmetric group. A correspondence is pointed out between the complete set of commuting operators and the reference-frame-free subsystems. We provide ... More
How well can you know the edge of a quantum pyramid?May 04 2009We consider a symmetric quantum communication scenario in which the signal states are edges of a quantum pyramid of arbitrary dimension and arbitrary shape, and all edge states are transmitted with the same probability. The receiver could employ different ... More
Tutorial Notes on One-Party and Two-Party Gaussian StatesJul 28 2003Aug 22 2003Gaussian states -- or, more generally, Gaussian operators -- play an important role in Quantum Optics and Quantum Information Science, both in discussions about conceptual issues and in practical applications. We describe, in a tutorial manner, a systematic ... More
Separability of Two-Party Gaussian StatesJul 26 2001We investigate the separability properties of quantum two-party Gaussian states in the framework of the operator formalism for the density operator. Such states arise as natural generalizations of the entangled state originally introduced by Einstein, ... More
The mean king's problem: Prime degrees of freedomJan 29 2001Mar 21 2001We show how one can ascertain the values of a complete set of mutually complementary observables of a prime degree of freedom.
Separability of entangled q-bit pairsDec 18 1999Jan 27 2000The state of an entangled q-bit pair is specified by 15 numerical parameters that are naturally regarded as the components of two 3-vectors and a $3\times3$-dyadic. There are easy-to-use criteria to check whether a given pair of 3-vectors plus a dyadic ... More
Semiclassical theory of trapped fermionic dipolesOct 13 2000Jan 19 2001We investigate the properties of a degenerate dilute gas of neutral fermionic particles in a harmonic trap that interact via dipole-dipole forces. We employ the semiclassical Thomas-Fermi method and discuss the Dirac correction to the interaction energy. ... More
Average transmission probability of a random stackJul 31 2009The transmission through a stack of identical slabs that are separated by gaps with random widths is usually treated by calculating the average of the logarithm of the transmission probability. We show how to calculate the average of the transmission ... More
Minimal qubit tomographyMay 16 2004Nov 23 2004We present, and analyze thoroughly, a highly symmetric and efficient scheme for the determination of a single-qubit state, such as the polarization properties of photons emitted by a single-photon source. In our scheme there are only four measured probabilities, ... More
A simple minimax estimator for quantum statesFeb 23 2012Mar 28 2012Quantum 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 ... More
Universal unitary gate for single-photon 2-qubit statesJan 16 2001Upon entangling a spatial binary alternative of a photon with its polarization, one can use single photons to study arbitrary 2-qubit states. Sending the photon through a Mach-Zehnder interferometer, equipped with sets of wave plates that change the polarization, ... More
Iterative procedure for computing accessible information in quantum communicationAug 20 2004We present an iterative algorithm that finds the optimal measurement for extracting the accessible information in any quantum communication scenario. The maximization is achieved by a steepest-ascent approach toward the extremal point, following the gradient ... More
Hybrid Quantum ComputationAug 06 2010Sep 25 2010We present a hybrid model of the unitary-evolution-based quantum computation model and the measurement-based quantum computation model. In the hybrid model part of a quantum circuit is simulated by unitary evolution and the rest by measurements on star ... More
Experimental proposal for symmetric minimal two-qubit state tomographyMar 08 2012We propose an experiment that realizes a symmetric informationally complete (SIC) probability-operator measurement (POM) in the four-dimensional Hilbert space of a qubit pair. The qubit pair is carried by a single photon as a polarization qubit and a ... More
Mutually unbiased bases for the rotor degree of freedomMar 23 2012May 23 2012We consider the existence of a continuous set of mutually unbiased bases for the continuous and periodic degree of freedom that describes motion on a circle (rotor degree of freedom). By a singular mapping of the circle to the line, we find a first, but ... More
Symmetric minimal quantum tomography by successive measurementsMar 08 2012We consider the implementation of a symmetric informationally complete probability-operator measurement (SIC POM) in the Hilbert space of a d-level system by a two-step measurement process: a diagonal-operator measurement with high-rank outcomes, followed ... More
One-dimensional transport revisited: A simple and exact solution for phase disorderDec 10 2012Apr 15 2013Disordered systems have grown in importance in the past decades, with similar phenomena manifesting themselves in many different physical systems. Because of the difficulty of the topic, theoretical progress has mostly emerged from numerical studies or ... More
Checking for prior-data conict using prior to posterior divergencesNov 01 2016When using complex Bayesian models to combine information, the checking for consistency of the information being combined is good statistical practice. Here a new method is developed for detecting prior-data conflicts in Bayesian models based on comparing ... More
Checking for prior-data conflict using prior to posterior divergencesNov 01 2016Nov 04 2016When using complex Bayesian models to combine information, the checking for consistency of the information being combined is good statistical practice. Here a new method is developed for detecting prior-data conflicts in Bayesian models based on comparing ... More
Encoding many qubits in a rotorMar 05 2010Mar 11 2010We propose a scheme for encoding many qubits in a single rotor, that is, a continuous and periodic degree of freedom. A key feature of this scheme is its ability to manipulate and entangle the encoded qubits with a single operation on the system. We also ... More
On mutually unbiased basesApr 20 2010Oct 20 2010Mutually unbiased bases for quantum degrees of freedom are central to all theoretical investigations and practical exploitations of complementary properties. Much is known about mutually unbiased bases, but there are also a fair number of important questions ... More
Quantum state tomography: Mean squared error matters, bias does notMay 21 2014Because of the constraint that the estimators be bona fide physical states, any quantum state tomography scheme - including the widely used maximum likelihood estimation - yields estimators that may have a bias, although they are consistent estimators. ... More
Adaptive schemes for incomplete quantum process tomographyOct 06 2011We propose an iterative algorithm for incomplete quantum process tomography, with the help of quantum state estimation, based on the combined principles of maximum-likelihood and maximum-entropy. The algorithm yields a unique estimator for an unknown ... More
Periodic and discrete Zak basesNov 25 2005Weyl's displacement operators for position and momentum commute if the product of the elementary displacements equals Planck's constant. Then, their common eigenstates constitute the Zak basis, each state specified by two phase parameters. Upon enforcing ... More
SeCQC: An open-source program code for the numerical Search for the classical Capacity of Quantum ChannelsAug 01 2011SeCQC is an open-source program code which implements a Numerical Search for the classical Capacity of Quantum Channels (SeCQC) by using an iterative method. Given a quantum channel, SeCQC finds the statistical operators and POVM outcomes that maximize ... More
On mechanisms that enforce complementarityOct 08 1999In a recent publication Luis and Sanchez-Soto arrive at the conclusion that complementarity is universally enforced by random classical phase kicks. We disagree. One could just as well argue that quantum entanglement is the universal mechanism. Both claims ... More
Product measurements and fully symmetric measurements in qubit-pair tomography: A numerical studyJul 24 2009State tomography on qubit pairs is routinely carried out by measuring the two qubits separately, while one expects a higher efficiency from tomography with highly symmetric joint measurements of both qubits. Our numerical study of simulated experiments ... More
Spin orbit interaction induced spin-separation in platinum nanostructuresApr 01 2008Hirsch (1999) proposed a mechanism and geometry for the observation of the spin-Hall effect. In this work, we present a novel realization of the Hirsch geometry in a platinum (Pt) nanostructure, which is an increasingly important material for spintronics ... More
Quantitative wave-particle duality and non-erasing quantum erasureAug 23 1999The notion of wave-particle duality may be quantified by the inequality V^2+K^2 <=1, relating interference fringe visibility V and path knowledge K. With a single-photon interferometer in which polarization is used to label the paths, we have investigated ... More
Optimal error intervals for properties of the quantum stateFeb 18 2016Aug 11 2016Quantum state estimation aims at determining the quantum state from observed data. Estimating the full state can require considerable efforts, but one is often only interested in a few properties of the state, such as the fidelity with a target state, ... More
Giant spin Hall conductivity in platinum at room temperatureApr 08 2010Jun 10 2010We have demonstrated the electrical generation and detection of spin polarization by the spin Hall effect (SHE) in platinum. The experiment was performed in a non-local geometry without the use of ferromagnetic materials or magnetic field. We designed ... More
Information theoretic approach to single-particle and two-particle interference in multi-path interferometersFeb 19 2003We propose entropic measures for the strength of single-particle and two-particle interference in interferometric experiments where each particle of a pair traverses a multi-path interferometer. Optimal single-particle interference excludes any two-particle ... More
Symmetric construction of reference-frame-free quditsFeb 12 2008Oct 17 2008By exploiting a symmetric scheme for coupling $N$ spin-1/2 constituents (the physical qubits) to states with total angular momentum $N/2-1$, we construct rotationally invariant logical qudits of dimension $d=N-1$. One can encode all qudit states, and ... More
Raw-data attacks in quantum cryptography with partial tomographySep 22 2006We consider a variant of the BB84 protocol for quantum cryptography, the prototype of tomographically incomplete protocols, where the key is generated by one-way communication rather than the usual two-way communication. Our analysis, backed by numerical ... More
Implementing a neutral-atom controlled-phase gate with a single Rydberg pulseJul 30 2014Feb 24 2016One can implement fast two-qubit entangling gates by exploiting the Rydberg blockade. Although various theoretical schemes have been proposed, experimenters have not yet been able to demonstrate two-atom gates of high fidelity due to experimental constraints. ... More
Minimax mean estimator for the trineJul 01 2012We explore the question of state estimation for a qubit restricted to the $x$-$z$ plane of the Bloch sphere, with the trine measurement. In our earlier work [H. K. Ng and B.-G. Englert, eprint arXiv:1202.5136[quant-ph] (2012)], similarities between quantum ... More
Optimal Lewenstein-Sanpera decomposition of two-qubit states using Semidefinite ProgrammingSep 25 2009We use the language of semidefinite programming and duality to derive necessary and sufficient conditions for the optimal Lewenstein-Sanpera Decomposition (LSD) of 2-qubit states. We first provide a simple and natural derivation of the Wellens-Kus equations ... More
Statistics of dressed modes in a thermal stateMay 05 2002By a Wigner-function calculation, we evaluate the trace of a certain Gaussian operator arising in the theory of a boson system subject to both finite temperature and (weak) interaction. Thereby we rederive (and generalize) a recent result by Kocharovsky, ... More
Secure communication with a publicly known keyNov 20 2001May 09 2002We present a scheme for direct and confidential communication between Alice and Bob, where there is no need for establishing a shared secret key first, and where the key used by Alice even will become known publicly. The communication is based on the ... More
Secure communication with single-photon two-qubit statesJan 16 2001Jul 23 2002We propose a cryptographic scheme that is deterministic: Alice sends single photons to Bob, and each and every photon detected supplies one key bit -- no photon is wasted. This is in marked contrast to other schemes in which a random process decides whether ... More
Checking for prior-data conflict using prior to posterior divergencesNov 01 2016Nov 28 2016When using complex Bayesian models to combine information, the checking for consistency of the information being combined is good statistical practice. Here a new method is developed for detecting prior-data conflicts in Bayesian models based on comparing ... More
Very strong evidence in favor of quantum mechanics and against local hidden variables from a Bayesian analysisAug 21 2018Jan 22 2019The data of four recent experiments --- conducted in Delft, Vienna, Boulder, and Munich with the aim of refuting nonquantum hidden-variables alternatives to the quantum-mechanical description --- are evaluated from a Bayesian perspective of what constitutes ... More
Codes for Key Generation in Quantum CryptographyApr 12 2005As 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 ... More
Optimal error intervals for properties of the quantum stateFeb 18 2016Dec 14 2016Quantum state estimation aims at determining the quantum state from observed data. Estimating the full state can require considerable efforts, but one is often only interested in a few properties of the state, such as the fidelity with a target state, ... More
Measuring Quantum Correlations using Lossy Photon-Number-Resolving Detectors with SaturationOct 30 2012Sep 12 2015The variance of difference of photocounts (VDPs) is an established measure of quantum correlations for quantum states of light. It enables us to discriminate between the classical correlation of a two-mode coherent state and the quantum correlation of ... More
Beyond adiabatic elimination: A hierarchy of approximations for multi-photon processesSep 28 2012Apr 02 2013In multi-level systems, the commonly used adiabatic elimination is a method for approximating the dynamics of the system by eliminating irrelevant, non-resonantly coupled levels. This procedure is, however, somewhat ambiguous and it is not clear how to ... More
Long-lived qubit from three spin-1/2 atomsAug 09 2010Mar 10 2011A system of three spin-1/2 atoms allows the construction of a reference-frame-free (RFF) qubit in the subspace with total angular momentum $j=1/2$. The RFF qubit stays coherent perfectly as long as the spins of the three atoms are affected homogeneously. ... More
Raman transitions without adiabatic elimination: A simple and accurate treatmentSep 28 2012Driven Raman processes --- nearly resonant two-photon transitions through an intermediate state that is non-resonantly coupled and does not acquire a sizeable population --- are commonly treated with a simplified description in which the intermediate ... More
How well can you know the edge of a quantum pyramid?Jul 11 2003The methods of quantum cryptography enable one to have perfectly secure communication lines, whereby the laws of quantum physics protect the privacy of the data exchanged. Each quantum-cryptography scheme has its own security criteria that need to be ... More
Quantum and classical advantage distillation are not equivalentOct 26 2003We report that, for the generation of a secure cryptographic key from correlations established through a noisy quantum channel, the quantum and classical advantage distillation procedures are not equivalent, when coherent eavesdropping attacks are duly ... More
Single-loop interferometer for minimal ellipsometrySep 02 2004We present a simple polarizing Mach-Zehnder interferometer that can be used for optimal minimal ellipsometry: Only four intensities are measured to determine the three Stokes parameters, and an optimal choice for the four polarization projections can ... More
Optimal error regions for quantum state estimationFeb 17 2013Mar 30 2013Rather than point estimators, states of a quantum system that represent one's best guess for the given data, we consider optimal regions of estimators. As the natural counterpart of the popular maximum-likelihood point estimator, we introduce the maximum-likelihood ... More
Using prior expansions for prior-data conflict checkingFeb 27 2019Any Bayesian analysis involves combining information represented through different model components, and when different sources of information are in conflict it is important to detect this. Here we consider checking for prior-data conflict in Bayesian ... More
Past of a quantum particle: Common sense prevailsApr 12 2017Aug 27 2017We analyze Vaidman's three-path interferometer with weak path marking [Phys. Rev. A 87, 052104 (2013)] and find that common sense yields correct statements about the particle's path through the interferometer. This disagrees with the original claim that ... More
Ascertaining the Values of $σ_x$, $σ_y$, and $σ_z$ of a Polarization QubitSep 24 2002Mar 26 2003In the 1987 spin retrodiction puzzle of Vaidman, Aharonov, and Albert one is challenged to ascertain the values of $\sigma_x$, $\sigma_y$, and $\sigma_z$ of a spin-1/2 particle by utilizing entanglement. We report the experimental realization of a quantum-optical ... More
Checking the Model and the Prior for the Constrained MultinomialApr 18 2018The multinomial model is one of the simplest statistical models. When constraints are placed on the possible values for the probabilities, however, it becomes much more difficult to deal with. Model checking and checking for prior-data conflict is considered ... More
Monte Carlo sampling from the quantum state space. IJul 29 2014Apr 27 2015High-quality random samples of quantum states are needed for a variety of tasks in quantum information and quantum computation. Searching the high-dimensional quantum state space for a global maximum of an objective function with many local maxima or ... More
Ultracold Fermions in a Graphene-Type Optical LatticeJun 23 2009Some important features of the graphene physics can be reproduced by loading ultracold fermionic atoms in a two-dimensional optical lattice with honeycomb symmetry and we address here its experimental feasibility. We analyze in great details the optical ... More
Quantum Cryptography: Security Criteria ReexaminedOct 23 2003Sep 06 2004We find that the generally accepted security criteria are flawed for a whole class of protocols for quantum cryptography. This is so because a standard assumption of the security analysis, namely that the so-called square-root measurement is optimal for ... More
SOMIM: An open-source program code for the numerical Search for Optimal Measurements by an Iterative MethodMay 19 2008Mar 12 2011SOMIM is an open-source code that implements a Search for Optimal Measurements by using an Iterative Method. For a given set of statistical operators, SOMIM finds the POVMs that maximizes the accessed information, and thus determines the accessible information ... More
Reply to "Comment on `Past of a quantum particle revisited' "Jan 17 2019We stand by our findings in Phys. Rev A. 96, 022126 (2017). In addition to refuting the invalid objections raised by Peleg and Vaidman, we report a retrocausation problem inherent in Vaidman's definition of the past of a quantum particle.
Proper error bars for self-calibrating quantum tomographyApr 25 2019Self-calibrating quantum state tomography aims at reconstructing the unknown quantum state and certain properties of the measurement devices from the same data. Since the estimates of the state and device parameters come from the same data, one should ... More
User-specified random sampling of quantum channels and its applicationsMay 02 2019Random samples of quantum channels have many applications in quantum information processing tasks. Due to the Choi--Jamio\l{}kowski isomorphism, there is a well-known correspondence between channels and states, and one can imagine adapting \emph{state} ... More
The Singapore Protocol: Incoherent Eavesdropping AttacksMay 10 2005We thoroughly analyse the novel quantum key distribution protocol introduced recently in quant-ph/0412075, which is based on minimal qubit tomography. We examine the efficiency of the protocol for a whole range of noise parameters and present a general ... More
Airy-averaged gradient corrections for two-dimensional fermion gasesDec 13 2016Apr 17 2017Building on the discussion in PRA 93, 042510 (2016), we present a systematic derivation of gradient corrections to the kinetic-energy functional and the one-particle density, in particular for two-dimensional systems. We derive the leading gradient corrections ... More
Leading gradient correction to the kinetic energy for two-dimensional fermion gasesDec 23 2015Feb 26 2016Density functional theory (DFT) is notorious for the absence of gradient corrections to the two-dimensional (2D) Thomas-Fermi kinetic-energy functional; it is widely accepted that the 2D analog of the 3D von Weizs\"acker correction vanishes, together ... More
Coherent Eavesdropping Attacks in Quantum Cryptography: Nonequivalence of Quantum and Classical Key DistillationDec 20 2003The security of a cryptographic key that is generated by communication through a noisy quantum channel relies on the ability to distill a shorter secure key sequence from a longer insecure one. We show that -- for protocols that use quantum channels of ... More
A Method to Produce Intense Positron Beams via Electro Pair Production on ElectronsJul 13 2016Intense positron beams can be prepared via electro production with the reaction $e^{-} + e^{-} \rightarrow e^{-}+e^{+}+e^{-}+e^{-}$ due to the availability of high current electron beams. Head on collisions inside of a magnetic field of a solenoid are ... More
Tomographic Quantum Cryptography: Equivalence of Quantum and Classical Key DistillationMar 31 2003The security of a cryptographic key that is generated by communication through a noisy quantum channel relies on the ability to distill a shorter secure key sequence from a longer insecure one. For an important class of protocols, which exploit tomographically ... More
Monte Carlo sampling from the quantum state space. IIJul 29 2014Apr 27 2015High-quality random samples of quantum states are needed for a variety of tasks in quantum information and quantum computation. Searching the high-dimensional quantum state space for a global maximum of an objective function with many local maxima or ... More
Random samples of quantum states: Online resourcesDec 15 2016Dec 19 2016This is the documentation for generating random samples from the quantum state space in accordance with a specified distribution, associated with this webpage: http://tinyurl.com/QSampling . Ready-made samples (each with at least a million points) from ... More
Efficient and Robust Quantum Key Distribution With Minimal State TomographyDec 09 2004May 19 2008We introduce the Singapore protocol, a qubit protocol for quantum key distribution that is fully tomographic, more efficient than other tomographic protocols, and very robust. Under ideal circumstances the efficiency is log_2(4/3)=0.415 key bits per qubit ... More
Micromotion in trapped atom-ion systemsMar 05 2012Jul 09 2012We 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 ... More
Wave-particle duality in multi-path interferometers: General concepts and three-path interferometersSep 30 2007For two-path interferometers, the which-path predictability $\mathcal{P}$ and the fringe visibility $\mathcal{V}$ are familiar quantities that are much used to talk about wave-particle duality in a quantitative way. We discuss several candidates that ... More
Tomographic Quantum CryptographyMay 05 2003We present a protocol for quantum cryptography in which the data obtained for mismatched bases are used in full for the purpose of quantum state tomography. Eavesdropping on the quantum channel is seriously impeded by requiring that the outcome of the ... More
Path Distinguishability in Double Scattering of Light by AtomsApr 15 2007Apr 17 2007Wave-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 ... More
Systematic corrections to the Thomas-Fermi approximation without a gradient expansionSep 06 2017Mar 24 2018We improve on the Thomas-Fermi approximation for the single-particle density of fermions by introducing inhomogeneity corrections. Rather than invoking a gradient expansion, we relate the density to the unitary evolution operator for the given effective ... More
Verification of state and entanglement with incomplete tomographyJul 23 2012There exists, in general, a convex set of quantum state estimators that maximize the likelihood for informationally incomplete data. We propose an estimation scheme, catered to measurement data of this kind, to search for the exact maximum-likelihood-maximum-entropy ... More
Incomplete quantum state estimation: a comprehensive studyFeb 08 2012We present a detailed account of quantum state estimation by joint maximization of the likelihood and the entropy. After establishing the algorithms for both perfect and imperfect measurements, we apply the procedure to data from simulated and actual ... More
Optimality of entropic uncertainty relationsSep 01 2015The entropic uncertainty relation proven by Maassen and Uffink for arbitrary pairs of two observables is known to be non-optimal. Here, we call an uncertainty relation optimal, if the lower bound can be attained for any value of either of the corresponding ... More
Experimental investigation of high-dimensional quantum key distribution protocols with twisted photonsFeb 15 2018Nov 29 2018Quantum key distribution is on the verge of real world applications, where perfectly secure information can be distributed among multiple parties. Several quantum cryptographic protocols have been theoretically proposed and independently realized in different ... More