On the distribution of 1550-nm photon pairs efficiently generated using a periodically poled lithium niobate waveguide

We report on efficient generation of 1550-nm photon pairs in a periodically poled lithium niobate waveguide using the spontaneous parametric down-conversion process. Such photon pairs are expected to find applications in fiber-based long-distance quantum communication. Pumping the waveguide with a pulsed semiconductor laser with a pulse rate of 800 kHz and a maximum average pump power of 50 μW, we obtain a coincidence rate of 600 s⁻¹. Despite only two single-photon detectors are used, we gain some information about the photon-number distribution. Our measurements are found to be in agreement with a Poissonian photon-pair distribution, but clearly differ from the expected outcomes for both conventional and two-mode squeezed states, the latter corresponding to a thermal photon-pair distribution. The Poissonian photon-pair distribution is also explained by comparing the coherence time of the pump light and of the detected photons. An average of 0.9 generated photon pairs per pulse can thus be inferred.     

Degenerate entangled photon pairs source based on PPLN waveguide for quantum computation

Integrated photonic devices are expected to play a promising role in the field of quantum information science. In this paper we propose two schemes for generating polarization-mode entangled photon pairs based on titanium-indiffused waveguide on periodically polled lithium niobate by using simultaneous quasi-phase-matching of Type-I and higher order Type-0 spontaneous parametric down conversion processes in one of them and Type-II in another. The photon pairs are emitted at the wavelength of 812 nm suitable for quantum computation applications within a bandwidth of 14 and 0.2 nm, and the generation rate of the degenerate sources is 44,360 and 91 pairs/(s GHz mW) respectively, in a 1-cm long waveguide. These degenerate sources can provide maximally entangled photon pairs as the Tangle of the sources is as high as 0.9999 and 1, accordingly.     

Spectral properties of entangled photon pairs generated via quasi-phased-matched spontaneous parametric down-conversion

The spectral properties of entangled photon pairs generated via quasi-phased matching in spontaneous parametric down-conversion are proposed and demonstrated experimentally. A general mathematical model for evaluating the spectral properties is developed to obtain the spectrum shape and range of down-converted photons. The model takes into account the effects of phase mismatching due to non-ideal pumping and the relationship between crystal periodic modulation function and the incidence angle of the pump beam. The spectrum curve shape is determined by the discrete Fourier transform of a Gaussian pump beam and the integration of parametric down-conversion generated by an individual plane wave. An experiment is carried out with a PPLN non-linear crystal and dispersing optics, which shows a good consistency in their spectral ranges and shapes with our model predictions within the spectrum of 600–633 nm. This therefore illustrates that both the simulation model and the experimental process are reasonable. This novel method has potential applications in high-accuracy calibration in the wide spectrum using correlated photons.     

Differential-phase quantum key distribution experiment using a series of quantum entangled photon pairs

We report what we believe to be the first differential-phase quantum key distribution experiment using a series of quantum entangled photon pairs. We employed two outstanding techniques. As an entangled photon source, we used a 1.5 microm band entangled photon pair source based on spontaneous four-wave mixing in a cooled dispersion-shifted fiber. As receivers, photon pairs were actively phase modulated with LiNbO3 phase modulators followed by very stable planar light-wave circuit Mach-Zehnder interferometers, which provided two nonorthogonal measurements. As a consequence, we successfully generated sifted keys with a quantum bit error rate of 8.3% and a key generation rate of 0.3 bit/s and revealed the feasibility of this QKD scheme.     

Observation of surface solitons in chirped waveguide arrays

We report the observation of surface solitons in chirped semi-infinite waveguide arrays whose waveguides exhibit exponentially decreasing refractive indices. We show that the power threshold for surface wave formation decreases with an increase of the array chirp and that for sufficiently large chirp values linear surface modes are supported.     

Vortex properties of a photon flux in a dielectric waveguide

It is shown theoretically and experimentally that the photon flux energy in a dielectric waveguide depends on the mode composition of the field and the ratio between the numbers of photons with left-and right-handed helicity.     

Generation of correlated photon pairs in a microstructure fiber

We propose and experimentally demonstrate a new method of generating correlated photons in a microstructure fiber by means of a reversed degenerate four-wave-mixing process. Here one photon is annihilated from each of the bichromatic pump pulses to generate a pair of photons at the mean frequency. For a microstructure fiber as short as 1.5 m the measured coincidence counting rate is approximately eight times that of the accidental coincidences with a peak pump power of 0.25 W.     

Monolithic source of photon pairs

The creation of monolithically integratable sources of single and entangled photons is a top research priority with formidable challenges: The production, manipulation, and measurement of the photons should all occur in the same material platform, thereby fostering stability and scalability. Here we demonstrate efficient photon pair production in a semiconductor platform, gallium arsenide. Our results show type-I spontaneous parametric down-conversion of laser light from a 2.2 mm long Bragg-reflection waveguide, and we estimate its internal pair production efficiency to be 2.0×10(-8) (pairs/pump photon). This is the first time that significant pair production has been demonstrated in a structure that can be electrically self-pumped and which can form the basis for passive optical circuitry, bringing us markedly closer to complete integration of quantum optical technologies.     

Generation of hyperentangled photon pairs

We experimentally demonstrate the first quantum system entangled in every degree of freedom (hyperentangled). Using pairs of photons produced in spontaneous parametric down-conversion, we verify entanglement by observing a Bell-type inequality violation in each degree of freedom: polarization, spatial mode, and time energy. We also produce and characterize maximally hyperentangled states and novel states simultaneously exhibiting both quantum and classical correlations. Finally, we report the tomography of a 2 x 2 x 3 x 3 system (36-dimensional Hilbert space), which we believe is the first reported photonic entangled system of this size to be so characterized.     

Effective generation of polarization-entangled photon pairs in a cavity-QED system

We propose a cavity-QED scheme to effectively generate Einstein-Podolsky-Rosen polarization-entangled photon pairs. Assisted by a classical π-polarized pump field, a tripod four-level atom successively couples to two high-Q optical cavities possessing polarization degeneracy. Through stimulated Raman adiabatic passage process the polarization-entangled photon pairs can be produced.     

Creating single time-bin-entangled photon pairs

When a single emitter is excited by two phase-coherent pulses with a time delay, each of the pulses can lead to the emission of a photon pair, thus creating a "time-bin-entangled" state. Double pair emission can be avoided by initially preparing the emitter in a metastable state. We show how photons from separate emissions can be made indistinguishable, permitting their use for multiphoton interference. Possible realizations are discussed. The method might also allow the direct creation of n-photon entangled states (n > 2).     

Single-photon diode by exploiting the photon polarization in a waveguide

A single-photon optical diode operates on individual photons and allows unidirectional propagation of photons. By exploiting the unique polarization configuration in a waveguide, we show here that a single-photon optical diode can be accomplished by coupling a quantum impurity to a passive, linear optical waveguide which possesses a locally planar, circular polarization. We further show that the diode provides a near unitary contrast for an input pulse with finite frequency bandwidth and can be implemented in a variety of types of waveguides. Moreover, the performance of the diode is not sensitive to the intrinsic dissipation of the quantum impurity.     

Spectral density matrix of a single photon measured

We propose and demonstrate a method for measuring the spectral density matrix of a single photon pulse. The method is based on registering Hong-Ou-Mandel interference between a photon to be measured and a pair of attenuated and suitably delayed laser pulses described by a known spectral amplitude. The density matrix is retrieved from a two-dimensional interferogram of coincidence counts. The method has been implemented for a type-I down-conversion source, pumped by ultrashort laser pulses. The experimental results agree well with a theoretical model which takes into account the temporal as well as spatial effects in the source.     

Spectral Talbot phenomena in sampled arbitrarily chirped Bragg gratings

We derive the general conditions for which spectral Talbot phenomena, both integer and fractional self-imaging, can be observed in sampled arbitrarily chirped fiber Bragg gratings. These results are a generalization of those previously reported in [C. Wang, J. Azaña, L.R. Chen, Opt. Lett. 29(14) (2004) 1590] in which spectral Talbot phenomena were observed in sampled linearly chirped fiber Bragg gratings. We confirm our theoretical derivations with numerical simulations.     

Generation of pulsed polarization-entangled photon pairs in a 1.55-microm band with a periodically poled lithium niobate waveguide and an orthogonal polarization delay circuit

We report a scheme for generating pulsed polarization-entangled photon pairs based on conversion from time-bin entanglement to polarization entanglement by use of an orthogonal polarization delay circuit and post-selection. We have experimentally demonstrated the scheme, using a periodically poled lithium niobate waveguide, and successfully obtained polarization entanglement in the 1.55-microm telecom wavelength band.     

Integrated optical temporal Fourier transformer based on a chirped Bragg grating waveguide

We experimentally demonstrate the first integrated temporal Fourier transformer based on a linearly chirped Bragg grating waveguide written in silica glass with a femtosecond laser. The operation is based on mapping the energy spectrum of the input optical signal to the output temporal waveform by making use of first-order chromatic dispersion. The device operates in reflection, has a bandwidth of 10 nm, and can be used for incident temporal waveforms as long as 20 ps. Experimental results, obtained through both temporal oscilloscope traces and Fourier transform spectral interferometry, display a successful Fourier transformation of in-phase and out-of-phase pairs of input optical pulses, and demonstrate the correct functionality of the device for both amplitude and phase of the temporal output.     

Efficient generation of non-classical photon pairs in a hot atomic ensemble

We demonstrate the generation of non-classical photon pairs in a warm ~(87)Rb atomic vapor cell with no buffer gas or polarization preserving coatings via spontaneous four-wave mixing. We obtain the photon pairs with a 1/e correlation time of 40 ns and the violation of Cauchy–Schwartz inequality by a factor of 23±3. This provides a convenient and efficient method to generate photon pair sources based on an atomic ensemble.     

Controllable scattering of a single photon inside a one-dimensional resonator waveguide

We analyze the coherent transport of a single photon, which propagates in a one-dimensional coupled-resonator waveguide and is scattered by a controllable two-level system located inside one of the resonators of this waveguide. Our approach, which uses discrete coordinates, unifies low and high energy effective theories for single-photon scattering. We show that the controllable two-level system can behave as a quantum switch for the coherent transport of a single photon. This study may inspire new electro-optical single-photon quantum devices. We also suggest an experimental setup based on superconducting transmission line resonators and qubits.     

Beamlike high-brightness source of polarization-entangled photon pairs

We report on an ultrabright beamlike source of polarization-entangled photon pairs that is suitable for the task of multiphoton quantum information processing. The photon pairs are generated from a beamlike type-II parametric downconversion process in two adjacently located but 180 degrees rotated beta-barium borate crystals. Approximately 30,000 s(-1) entangled photon pairs are recorded experimentally with only 100 mW pump power. The fidelity of the generated entangled state as compared with a Bell state is measured to be 0.974 with the method of quantum state tomography. With this source, we also obtain a violation of Bell's inequality by 61 standard deviations in just a few seconds.