Sergey A. Podoshvedov

Elementary quantum gates with Gaussian states

We study the question of converting initially Gaussian states into non-Gaussian ones by two- and three-photon subtraction to improve non-classical properties of the conditional optical fields. We show the photon subtraction may effectively generate non-Gaussian states only in case of small values of the mean values of the position and momentum operators. In particular, the photon-subtracted state can be made arbitrary close to Gaussian state in limiting case of large initial amplitude of displacement. Use of initial displacement in input Gaussian states opens wider prospects to manipulate them. In particular, realization of probabilistic Hadamard gate with input Gaussian states is discussed where photon subtraction is motive force able unevenly to increase measure of non-classicality of the output state. Subtraction of larger number of photons enables to increase fidelity and non-classical measure of the conditional states.

A simple scheme with coupled down converters with type-I phase matching as resource for conditional preparation of macroscopic entangled states

It is shown that macroscopic entangled states can be generated using an experimental arrangement consisting of coupled spontaneous parametric down-converters with type-I phase matching (SPDCI) pumped simultaneously by optical fields in coherent state and two beam splitters. Two beam splitters in auxiliary generated modes are used to conditionally prepare macroscopic entangled states in output pumping modes of the studied system. Identification of two macroscopic entangled states is produced by use of photon number resolving detection. In contrast to all previous schemes, our scheme does not need Kerr-type nonlinear interaction and is purely based on second-order susceptibility of the crystal which is stronger for the Kerr nonlinearity. We calculate concurrence of the states as a measure of the amount of entanglement stored in the states and present analysis concerning ‘separation’ between components forming studied entangled states.

Controlled sign gate through mode entangled states

We propose a model to perform controlled sign (CS) gate flip in two stages that is free of the use of detectors discriminating between one- and two-photon number states. The CS gate with mode entangled qubits and two-photon quantum channel is used at the first stage to produce a more perfect four-photon quantum channel. A two-photon quantum channel can be constructed using a spontaneous parametric down converter and methods of linear optics. We describe the CS gate performance with the help of a four-photon quantum channel at the second stage. The success probability of the CS gate performance with both two- and four-photon quantum channels is 1/4. The use of usual detectors at the first stage leads to the appearance of a noisy quantum channel that is shown not to have influence on the valid CS gate flip at the second stage whose outcome is characterized by fourfold coincidence registration of photons. The success probability of such a CS gate in two stages is 1/16.

Entanglement of quantum states between macroscopic and microscopic systems

We introduce a new entangled state composed of Fock states, a coherent state and a single-photon-added state. The nonclassical properties are demonstrated by phase squeezing, photon-number squeezing (sub-Poisson statistics), the violation of Cauchy-Schwarz inequality, and the negativity of Wigner function of the proposed entangled state.

Testing quantum mechanics against macroscopic realism using the output of {chi}{sup (2)} nonlinearity

We suggest an all-optical scheme to generate entangled superposition of a single photon with macroscopic entangled states for testing macroscopic realism. The scheme consists of source of single photons, a Mach-Zehnder interferometer in routes of which a system of coupled-down converters with type-I phase matching is inserted, and a beam splitter for the other auxiliary modes of the scheme. We use quantization of the pumping modes, depletion of the coherent states passing through the system, and interference effect in the pumping modes in the process of erasing which-path information of the single-photon on exit from the Mach-Zehnder interferometer. We show the macroscopic fields of the output superposition are distinguishable states. This scheme generates macroscopic entangled state that violates Bells inequality. Moreover, the detailed analysis concerning change of amplitudes of entangled superposition by means of repeating this process many times is accomplished. We show our scheme works without photon number resolving detection and it is robust to detector inefficiency.

Theoretical consideration of the use of mode entangled states to beat the minimal period of an interference pattern

We propose to use multi-photon mode entangled states to beat the minimal period of an interference pattern. Using the multi-photon mode entangled states, we show that it is possible to observe an interference effect with a period of minimum size λ/2N in an N-photon absorbing substrate. In the framework of the method developed, we propose a simple scheme for a quantum encoder with a two-photon quantum channel for producing a desired N-photon mode entangled state on which to write an interference pattern with a smaller period, as compared with the one in the case of the use of classical light.