Generation of broadband biphotons and their compression in an optical fiber

The generation of biphoton wave packets with a broad spectrum owing to quasi-phase-matched spontaneous parametric down-conversion in a crystal with an aperiodic poling has been considered. Biphotons at the exit of such a sample are not Fourier limited; i.e., their correlation time is much larger than the inverse width of the spectrum. It has been shown that if the inverse grating vector varies linearly along the sample length, the generated biphoton wave packets can be compressed in time by transmitting two-photon light through an optical fiber.     

Quantum interference and diffraction of parametric down-converted biphotons

We present two-photon diffraction and interference experiments utilizing parametric down-converted photon pairs (biphotons) and a transmission grating. The biphoton exhibits a diffraction-interference pattern equivalent to an effective single particle with half wavelength of the constituent photons.     

Control of the spectrum of the biphoton field

The main methods for controlling the biphoton field, as well as the problems for which the width and the shape of the spectrum of the biphoton field are of decisive importance, are discussed. The method for controlling the spectrum of the spontaneous parametric downconversion of light based on the spatial modulation of the refractive indices of a nonlinear crystal in which the generation of biphotons has been analyzed. Modulation is due to the thermo-optic and electro-optic effects.     

Generation of narrow-band correlated photon pairs by spontaneous down conversion in a cavity

The problem of generating a narrow-band biphoton field during spontaneous parametric down conversion of light in a quadratic nonlinear medium in a cavity is discussed. It is shown that, in the case of double resonance (for signal and idle fields), the form of the single-photon wave packet is biexponential and has a half-width determined by the photon life time in the cavity. An experiment is carried out on the generation of narrow-band biphotons in a crystal of lithium iodine with I-type matching in a frequency degenerate collinear regime. The second-order correlation function of the cavity-enhanced biphotons is measured using the electric nanosecond time delay line.     

“Spreading” of a biphoton in a group-velocity-dispersion medium and two-photon interference

It is shown that two-photon Bell states can be prepared by “spreading” of a two-photon wave packet (biphoton) in a dispersive medium without compensating for group delays between photons with orthogonal polarizations or using narrow-band filters but by selecting the time correlation function. This is possible because two-photon interference effects are manifested in the shape of the time correlation function of intensity due to its spreading.     

The influence of the icosahedral percolation transition in supercooled liquid iron on the diffusion mobility of atoms

An experiment on preparation of entangled photon pairs (biphotons) in an arbitrary polarization state is described. The biphotons are qutrits (three-state quantum systems). They can be used in ternary quantum cryptography protocols. A theoretically derived orthogonality criterion for the prepared biphotons is validated experimentally. The criterion can be used to identify orthogonal biphoton states.     

Generation of arbitrary frequency-entangled states of two-photon light

We consider a new method for the generation of polarization-frequency entangled states of photon pairs. We use a frequency-nondegenerate regime of spontaneous parametric down conversion where the photon pairs (biphotons) are produced with identical polarizations, propagate mostly in the same direction, but differ in frequency. Entanglement is achieved by a coherent superposition of pairs emitted from two nonlinear crystals, with the polarization of the biphotons from the first crystal being changed by a transformer placed between the two crystals. We show that this scheme allows the degree of entanglement to be controlled by the choice of biphoton frequencies.     

Optimization of optical fiber parameters to reduce errors of quantum key distribution using entangled polarization states of biphotons

A phenomenological Hamiltonian of photons in single-mode stochastic fiber depending on the vector of random parameters is proposed. The time dynamics of single-photon density matrix in the basis of states with orthogonal polarizations is considered. The fiber-parameter-averaged quantum bit error rate (QBER) in a sifted quantum key distributed over the BB84 protocol using entangled polarization states of biphotons is found. It is shown that QBER can be significantly reduced even at large dispersions of random fiber parameters. To this end, identically fabricated fibers must be used for quantum channels A and B. The choice of pairs of fiber segments must be correlated, with a correlation coefficient close to unity. This approach is based on a remarkable property of the singlet biphoton state, which is “free of collective decoherence.” A correlated choice of fiber segments for channels A and B reduces significantly QBER, making its value below critical (i.e., equal to 0.11, a level below which a distributed key is accepted for cryptographic purposes).     

Polarization tomography of a narrow-band biphoton field

Narrow-band orthogonally polarized collinear frequency-degenerate biphotons are generated in the process of the spontaneous parametric down-conversion of light in a nonlinear BBO crystal placed in an optical resonator. Quantum polarization tomography and polarization transformation of the generated biphoton states are performed. The results from our experiment agree well with theoretical calculations.     

Observation of optical precursors at the biphoton level

We describe the observation of a sharp leading-edge spike in a biphoton wave packet that is produced using slow light and measured by two-photon correlation. Using the stationary-phase approximation we characterize this spike as a Sommerfeld-Brillouin precursor resulting from the interference of low- and high-frequency spectral components.     

Entangled two-photon wave packet in a dispersive medium

We report an experimental study of group-velocity dispersion effect on an entangled two-photon wave packet, generated via spontaneous parametric down-conversion and propagating through a dispersive medium. Even in the case of using cw laser beam for pump, the biphoton wave packet and the second-order correlation function spread significantly. The study and understanding of this phenomenon is of great importance for quantum information applications, such as quantum communication and distant clock synchronization.     

Operationalistic orthogonality condition for single-mode biphotons (qutrits)

An arbitrary polarization state of a single-mode biphoton is considered. The operationalistic criterion is formulated for the orthogonality of these states. It can be used to separate a biphoton with an arbitrary degree of polarization from a set of biphotons orthogonal to it. This is necessary for the implementation of quantum cryptography protocol based on the three-level systems. The experimental test of this criterion amounts to the observation of the anticorrelation effect for a biphoton with an arbitrary polarization state.     

Ultrabroadband biphotons generated via chirped quasi-phase-matched optical parametric down-conversion

We generate ultrabroadband biphotons via the process of spontaneous parametric down-conversion (SPDC) in quasi-phase-matched nonlinear gratings that have a linearly chirped wave vector. By using these ultrabroadband biphotons (300-nm bandwidth), we measure the narrowest Hong-Ou-Mandel dip to date, having a full width at half maximum of 7.1 fs. This enables the generation of a high flux of nonoverlapping biphotons with ultrabroad bandwidth, thereby promoting the use of SPDC light in many nonclassical applications.     

Biphoton light generation in polarization-frequency bell states

Four polarization-frequency Bell states are obtained experimentally for photon pairs (biphotons) emitted during spontaneous parametric scattering from continuous pumping in the collinear frequency-nondegenerate regime. The polarization properties of such states are investigated. It is shown that biphoton light in the singlet Bell state is not polarized in the second or fourth order in the field.     

Separate spatial soliton pairs in an unbiased series two-photon photorefractive crystal circuit

Dark (bright) steady-state spatial solitons are predicted in one dimension for a series circuit consisting of two two-photon photorefractive crystals of which at least one must be photovoltaic. Each crystal can support a spatial soliton. The two solitons are known collectively as separate spatial soliton pairs with three types: dark-dark, bright-dark and bright-bright. In the limit in which the optical wave has a spatial extent much less than the width of the crystal, the dark soliton can affect the other soliton by light-induced current, but the bright soliton cannot affect the other soliton in the soliton pair.     

脉冲光场作用下啁啾双光子量子特性研究

基于量子理论和非线性光学,研究了脉冲光场作用啁啾准相位匹配非线性晶体的第Ⅱ类自发参变下转换过程中,脉冲宽度和啁啾系数对产生的纠缠双光子特性及Hong-Ou-Mandel(HOM)量子干涉结果的影响。结果表明,啁啾系数一定时,随着脉冲宽度的增加,双光子谱的不可区分性降低,HOM量子干涉可见度下降。当脉冲宽度一定时,随着啁啾系数的增加使双光子谱带宽增加,HOM量子干涉陷落变窄,双光子谱的不可区分性增强,从而提高了相干精度和干涉可见度。理论上得到了超宽带的双光子谱和对应超窄的HOM量子干涉图。     

Control of the frequency spectrum of a biphoton field due to the electro-optical effect

A method for controlling the spectrum of spontaneous parametric down-conversion has been implemented. This method is based on the application of the gradient of an electrostatic field to a nonlinear crystal where down-conversion occurs. Due to the electro-optical effect, i.e., the dependence of the refractive indices on the applied field, the phase-matching conditions are different along the length of the crystal and, as a result, the spectrum of the emitted biphoton field is modified.     

Diffraction of entangled photon pairs by ultrasonic waves

In this paper, we have presented and established a new theoretical formulation of photon optics based on photon path and Feynman path integral idea. We have used Feynman path integral approach to discuss Fraunhofer, Fresnel diffraction of single photon and entangled photon pairs by ultrasonic wave and obtained the following results: i) quantum state and probability distribution of single photon and entangled photon pairs by Fraunhofer and Fresnel ultrasonic diffraction, ii) oblique incidence Raman-Nath and Bragg diffraction conditions, iii) total correlation state and its probability distribution. Our calculation results are in agreement with the experiment results. Comparing one-photon and two-photon diffraction effects by ultrasonic waves, we have found that two-photon diffraction by ultrasonic waves is also a sub-wavelength diffraction.     

State entanglement of biphoton qutrits and ququarts

Properties of biphoton qutrits and ququarts are characterized with allowance for the demands of the symmetry of wave functions of two-boson states and, in the case of ququarts, for the presence of two independent degrees of freedom (e.g., polarization and frequency) of the photons.     

Controlling the spectrum of a two-photon field: Inhomogeneous broadening due to a temperature gradient

A method is proposed and implemented for controlling the spectrum of spontaneous parametric down-conversion. The method is based on generating a temperature gradient along a nonlinear crystal in which biphotons are generated. In this case, the phase-synchronism conditions are modified due to the temperature dependence of the refractive indices. As a result, the spectral composition of the emitted two-photon field is changed.