Denis Sych

A generator for unique quantum random numbers based on vacuum states

Researchers demonstrate random-number generation by exploiting the intrinsic randomness of vacuum states. The approach may lead to reliable and high-speed quantum random-number generators for applications ranging from gambling to cryptography.

Feasibility of free space quantum key distribution with coherent polarization states

Free space QKD over an atmospheric channel was demonstrated in 1996 for the first time [1]. Since then, several prepare-and-measure and entanglement-based schemes have been implemented (for a detailed overview, see [2]). All of these systems use single-photon detectors, and therefore have to employ spatial, spectral and/or temporal filtering in order to reduce background light. In our system, we use an alternative approach: with the help of a bright local oscillator (LO), we perform homodyne measurements on weak coherent polarization states [3].

Discrimination of binary coherent states using a homodyne detector and a photon number resolving detector

We investigate quantum measurement strategies capable of discriminating two coherent states probabilistically with significantly smaller error probabilities than can be obtained using nonprobabilistic state discrimination. We apply a postselection strategy to the measurement data of a homodyne detector as well as a photon number resolving detector in order to lower the error probability. We compare the two different receivers with an optimal intermediate measurement scheme where the error rate is minimized for a fixed rate of inconclusive results. The photon number resolving (PNR) receiver is experimentally demonstrated and compared to an experimental realization of a homodyne receiver with postselection. In the comparison, it becomes clear that the performance of the PNR receiver surpasses the performance of the homodyne receiver, which we prove to be optimal within any Gaussian operations and conditional dynamics.

Demonstration of coherent-state discrimination using a displacement-controlled photon-number-resolving detector.

We experimentally demonstrate a new measurement scheme for the discrimination of two coherent states. The measurement scheme is based on a displacement operation followed by a photon-number-resolving detector, and we show that it outperforms the standard homodyne detector which we, in addition, prove to be optimal within all Gaussian operations including conditional dynamics. We also show that the non-Gaussian detector is superior to the homodyne detector in a continuous variable quantum key distribution scheme.

Coherent state quantum key distribution with multi letter phase-shift keying

We present a protocol for quantum key distribution using discrete modulation of coherent states of light. Information is encoded in the variable phase of coherent states which can be chosen from a regular discrete set ranging from binary to continuous modulation similar to phase-shift keying in classical communication. Information is decoded by simultaneous homodyne measurement of both quadratures and requires no active choice of basis. The protocol utilizes either direct or reverse reconciliation both with and without postselection. We analyze the security of the protocol and show how to enhance it by the optimal choice of all variable parameters of the quantum signal.

A complete basis of generalized Bell states

A generalization of the Bell states and Pauli matrices to dimensions which are powers of 2 is considered. A basis of maximally entangled multidimensional bipartite states (MEMBS) is chosen very similar to the standard Bell states and constructed of only symmetric and antisymmetric states. This special basis of MEMBS preserves all basic properties of the standard Bell states. We present a recursive and non-recursive method for the construction of MEMBS and discuss their properties. The antisymmetric MEMBS possess the property of rotationally invariant exclusive correlations which is a generalization of the rotational invariance of the antisymmetric singlet Bell state.

Informational completeness of continuous-variable measurements

We justify that homodyne tomography turns out to be informationally complete when the number of independent quadrature measurements is equal to the dimension of the density matrix in the Fock representation. Using this as our thread, we examine the completeness of other schemes, when continuous-variable observations are truncated to discrete finite-dimensional subspaces.

Triplet-like correlation symmetry of continuous variable entangled states

We report on a continuous variable analogue of the triplet two-qubit Bell states. We discuss the symmetry properties of entangled states of either kind, and theoretically and experimentally demonstrate a remarkable similarity of two-mode continuous variable entangled states with triplet Bell states with respect to their correlation patterns. Understanding the symmetry properties helps finding decoherence-free subspaces.

Practical Receiver for Optimal Discrimination of Binary Coherent Signals.

One of the most fascinating aspects of quantum mechanics is the principle impossibility of deterministic errorless discrimination of nonorthogonal signals, such as coherent states. On the one hand, it prevents perfect cloning of quantum states [1, 2] and enables secure communication [3]. On the other hand, it makes a grand challenge to reach the ultimate measurement precision. Although the minimum possible error rate (the Helstrom bound [4]) has been known for almost five decades, there is no practical way to achieve it [5–10]. Developing the realis tic optimal measurement strategies to attain the Helstrom bound is of utmost importance for high-precision applications, long-distance free-space and optical fiber communication, gravitational wave detection, optical sensing in b iology and medicine, to name a few. In this work, we show an optimal receiver for coherent states which admits a relatively simple technological implementation. The receive r is based on multichannel splitting of the signal, followed by feed-forward signal displacement and photon detection. Of all pure quantum states, coherent states are most robust against loss hence their importance for a wide range of appli cations and the need for the best possible detection strateg ies. In the simplest scenario, one has to discriminate between tw o coherent states |α〉 and |−α〉 that have equal a priori probabilities, i.e. to identify the binary signal [11, 12]. Due t o the nonzero overlap| 〈α| −α〉 |2 = e−4α, there is a certain error rate, which depends on the measurement strategy. In the middle 60’s, Helstrom found the minimum possible error rateεHelstrom= 2 (

Temporal shaping of single photons enabled by entanglement

We present a method to produce pure single photons with an arbitrary designed temporal shape in a heralded way. As an indispensable resource, the method uses pairs of time-energy entangled photons. One photon of a pair undergoes temporal amplitude-phase modulation according to the desired shape. Subsequent frequency-resolved detection of the modulated photon heralds its entangled counterpart in a pure quantum state. The temporal shape of the heralded photon is indirectly affected by the modulation in the heralding arm. We derive conditions for which the shape of the heralded photon is given by the modulation function. The method can be implemented with various sources of time-energy entangled photons. In particular, using entangled photons from parametric down-conversion the method provides a simple means to generate pure shaped photons with an unprecedented broad range of temporal durations, from tenths of femtoseconds to microseconds. This shaping of single photons will push forward the implementation of scalable multidimensional quantum information protocols, efficient photon-matter coupling, and quantum control at the level of single quanta.