Ivan V. Sokolov

Squeezed states of light and noise-free optical images

Abstract The regularity of photocount statistics in space, related to the wide-band squeezing, is demonstrated. The resolving power of low-noise spatial measurements is evaluated.

Multimode Squeezing, Antibunching in Space and Noise-Free Optical Images

Three-dimensional antibunching of photons by observation of multimode-squeezed light is considered. The phenomenon is of practical importance for producing photon beams which are regular not only in time but also in space. The possible experiment to observe the space correlations of photocounts is discussed.

Quantum memory for images: A quantum hologram

Matter-light quantum interface and quantum memory for light are important ingredients of quantum information protocols, such as quantum networks, distributed quantum computation, etc. [P. Zoller et al., Eur. Phys. J. D 36, 203 (2005)]. In this paper we present a spatially multimode scheme for quantum memory for light, which we call a quantum hologram. Our approach uses a multiatom ensemble which has been shown to be efficient for a single spatial mode quantum memory. Due to the multiatom nature of the ensemble and to the optical parallelism it is capable of storing many spatial modes, a feature critical for the present proposal. A quantum hologram with the fidelity exceeding that of classical hologram will be able to store quantum features of an image, such as multimode superposition and entangled quantum states, something that a standard hologram is unable to achieve.

Squeezed-light source for superresolving microscopy

We propose a source of multimode squeezed light that can be used for superresolving microscopy. This source is an optical parametric amplifier with a properly chosen diaphragm on its output and a Fourier lens. We demonstrate that such an arrangement produces squeezed prolate spheroidal waves that are the eigenmodes of the optical imaging scheme used in microscopy and discuss the conditions of the degree of squeezing and of the number of spatial modes in illuminating light.

Quantum volume hologram

We propose a scheme for parallel spatially multimode quantum memory for light. The scheme is based on a counterpropagating quantum signal wave and a strong classical reference wave as in a classical volume hologram and therefore can be called a quantum volume hologram. The medium for the hologram consists of a spatially extended ensemble of atoms placed in a magnetic field. The write-in and readout of this quantum hologram is as simple as that of its classical counterpart and consists of a single-pass illumination. In addition, we show that the present scheme for a quantum hologram is less sensitive to diffraction and therefore is capable of achieving a higher density of storage of spatial modes as compared to previous proposals. We present a feasibility study and show that experimental implementation is possible with available cold atomic samples. A quantum hologram capable of storing entangled images can become an important ingredient in quantum information processing and quantum imaging.

Quantum teleportation of optical images with frequency conversion

A new version of quantum teleportation of optical images in continuous variables is considered. The new scheme is based on quantum entanglement in continuous variables between light fields of different frequencies and allows wavelength conversion between input and teleported images. The protocol of quantum teleportation with frequency conversion can be used as a combined part of light/matter interface in quantum parallel data processing and in a parallel quantum memory.

Quantum parallel dense coding of optical images

We propose quantum dense coding protocol for optical images. This protocol extends the earlier proposed dense coding scheme for continuous variables [S.L. Braunstein and H.J. Kimble, Phys. Rev. A 61 042302 (2000)] to an essentially multimode in space and time optical quantum communication channel. This new scheme allows, in particular, for parallel dense coding of non-stationary optical images. Similar to some other quantum dense coding protocols, our scheme exploits the possibility of sending a message through only one of the two entangled spatially-multimode beams, using the other one as a reference system. We evaluate the Shannon mutual information for our protocol and find that it is superior to the standard quantum limit. Finally, we show how to optimize the performance of our scheme as a function of the spatio-temporal parameters of the multimode entangled light and of the input images.

Control of parameters of quantum memory for light in a cavity configuration

Control of time dependences of waveforms is important for the application of optical signals in the nonclassical state that are recorded in various quantum-memory devices. Matching of waveforms at the signal detectors is needed for measurements using optical mixing (homodyne detection), detection of entangled states, etc. Earlier results for cavity quantum memory on an ensemble of cold atoms show that the waveform of the strong control field can be changed in such a way that the profile of optical signal recorded and readout from a collective atomic spin is convenient for measurements. In the course of recording, the control field provides the suppression of the reflection loss of the input signal related to the destructive interference of the signal and local field at the coupling mirror (impedance matching). Using an example of memory reading, we show that impedance matching provides additional possibilities for variations in the control field and allows efficient generation of output quantum signals with predetermined waveforms convenient for experimental measurements.

The efficiency of parallel quantum memory for light in a cavity configuration

We present a new scheme of quantum memory for optical images (spatially multimode light fields) that allows mapping the quantum state of the signal onto the long-lived coherence of the ground state of an ensemble of stationary atoms or impurity centers. The memory medium is embedded in an optical cavity with degenerate transverse modes, which increases the effective optical thickness of the medium and allows one, in principle, to store information in optically thin atomic layers. Since, in reality, storage and retrieval of limited-duration signals, including signals shorter than the lifetime of the field in the cavity, is of interest, we do not use the low-Q cavity approximation. The influence of losses due to partial reflection of the nonstationary signal field incident on a coupling mirror on the storage efficiency is considered. We used the method of approximate impedance matching, wherein losses due to reflection can be minimized by controlling the coupling parameter of the light field with memory medium in time, thus creating conditions for destructive interference of the signal and local fields on the coupling mirror. The influence of diffraction on the transverse resolution of memory at the writing and readout stages is investigated, and the number of effectively stored transverse spatial modes of the signal is estimated.

Possibility of suppressing quantum light fluctuations when excess photon fluctuations occur inside a cavity

Using the optical excitation of a high-Q cavity as an example, it is shown that when light is observed at the output of this cavity, effective suppression of the photocurrent shot noise below the quantum limit is in general independent of the parameters of the stationary state of the field oscillator (in particular, it is independent of the rms photon fluctuations) inside the cavity and can occur not only at any allowed negative value but even at a positive value of the Mandel parameter. It was assumed in solving the problem that the cavity is optically excited by superimposing the radiation of a sub-Poisson laser and a laser with excess photon noise. A formal solution was obtained in terms of the kinetic equation for the density matrix of the actual fields (inside the laser cavities and the empty cavity), which is derived here on the basis of the Heisenberg-Langevin quantum equations, taking into account directed propagation of the field from the laser cavities inside the empty cavity. The resulting kinetic equation can also be used to solve other physical problems, since it is applicable to optical systems that contain, in principle, an arbitrary number of coupled cavities and interference mixers.