Pulsed Parametric Down-conversion Research Articles

Few-mode squeezing in type-I parametric downconversion by complete group velocity matching.

Frequency-degenerate pulsed type-I parametric downconversion is a widely used source of squeezed light for numerous quantum optical applications. However, this source is typically spectrally multimode, and the generated squeezing is distributed between many spectral modes with a limited degree of squeezing per mode. We show that in a nonlinear crystal, where the condition of complete group velocity matching (GVM) for the pump and the signal is satisfied, the number of generated modes may be as low as two or three modes. We illustrate the general theory with the example of the MgO-doped lithium niobate crystal pumped at 775 nm and generating squeezed light at 1.55 µm. Our model includes the derivation of the degree of squeezing from the properties of the pump and the crystal and shows that 12 dB of squeezing can be obtained in a periodically poled crystal ata length of 80 mm.

Spectral characterization of SPDC-based single-photon sources for quantum key distribution

In our laboratory, we employ two biphoton sources for quantum key distribution. The first is based on cw parametric down-conversion of photons at 404 nm in PPKTP waveguide chips, while the second is based on the pulsed parametric down-conversion of 775 nm photons in PPLN waveguides. The spectral characterization is important for the determination of certain side-channel attacks. A Hong-Ou-Mandel experiment employing the first photon source revealed a complex structure of the common Hong-Ou-Mandel dip. By measuring the spectra of the single photons at 808 nm, we were able to associate these structures to the superposition of different transverse modes of the pump photons in our waveguide chips. The pulsed source was characterized by means of single-photon spectra measured by a sensitive spectrum analyzer as well as dispersion-based measurements. Finally, we also describe Hong-Ou-Mandel experiments using the photons from the second source.

Distillation of squeezing using an engineered pulsed parametric down-conversion source.

Hybrid quantum information processing combines the advantages of discrete and continues variable protocols by realizing protocols consisting of photon counting and homodyne measurements. However, the mode structure of pulsed sources and the properties of the detection schemes often require the use of optical filters in order to combine both detection methods in a common experiment. This limits the efficiency and the overall achievable squeezing of the experiment. In our work, we use photon subtraction to implement the distillation of pulsed squeezed states originating from a genuinely spatially and temporally single-mode parametric down-conversion source in non-linear waveguides. Due to the distillation, we witness an improvement of 0.17 dB from an initial squeezing value of -1.648 ± 0.002 dB, while achieving a purity of 0.58, and confirm the non-Gaussianity of the distilled state via the higher-order cumulants. With this, we demonstrate the source's suitability for scalable hybrid quantum network applications with pulsed quantum light.

On-chip III-V monolithic integration of heralded single photon sources and beamsplitters

We demonstrate a monolithic III-V photonic circuit combining a heralded single photon source with a beamsplitter, at room temperature and telecom wavelength. Pulsed parametric down-conversion in an AlGaAs waveguide generates counterpropagating photons, one of which is used to herald the injection of its twin into the beamsplitter. We use this configuration to implement an integrated Hanbury-Brown and Twiss experiment, yielding a heralded second-order correlation gher(2)(0)=0.10±0.02 that confirms single-photon operation. The demonstrated generation and manipulation of quantum states on a single III-V semiconductor chip opens promising avenues towards real-world applications in quantum information.

Local Sampling of the Wigner Function at Telecom Wavelength with Loss-Tolerant Detection of Photon Statistics.

We report the experimental point-by-point sampling of the Wigner function for nonclassical states created in an ultrafast pulsed type-II parametric down-conversion source. We use a loss-tolerant time-multiplexed detector based on a fiber-optical setup and a pair of photon-number-resolving avalanche photodiodes. By capitalizing on an expedient data-pattern tomography, we assess the properties of the light states with outstanding accuracy. The method allows us to reliably infer the squeezing of genuine two-mode states without any phase reference.

Femtosecond source of unbalanced polarization-entangled photons

We characterize a polarization-entangled photon source based on femtosecond pulsed parametric downconversion in a periodically poled χ(2) medium. Employing a Sagnac interferometer, we prepared different quantum states aiming at future investigations in quantum information and on the foundations of quantum mechanics. In this work we demonstrate the controlled generation of unbalanced entangled states, rather than the usual maximally entangled ones, by tuning the polarization of the system’s pumping field.

Giant narrowband twin-beam generation along the pump-energy propagation direction.

Walk-off effects, originating from the difference between the group and phase velocities, limit the efficiency of nonlinear optical interactions. While transverse walk-off can be eliminated by proper medium engineering, longitudinal walk-off is harder to avoid. In particular, ultrafast twin-beam generation via pulsed parametric down-conversion and four-wave mixing is only possible in short crystals or fibres. Here we show that in high-gain parametric down-conversion, one can overcome the destructive role of both effects and even turn them into useful tools for shaping the emission. In our experiment, one of the twin beams is emitted along the pump Poynting vector or its group velocity matches that of the pump. The result is markedly enhanced generation of both twin beams, with the simultaneous narrowing of angular and frequency spectrum. The effect will enable efficient generation of ultrafast twin photons and beams in cavities, waveguides and whispering-gallery mode resonators.

Characterizing entanglement in pulsed parametric down-conversion using chronocyclic Wigner functions

Pulsed parametric down-conversion (PDC) processes generate photon pairs with a rich spectral-temporal structure, which offer an attractive potential for quantum information and communication applications. We investigate the four-dimensional chronocyclic Wigner function ${W}_{\mathrm{PDC}}({\ensuremath{\omega}}_{\mathrm{s}},{\ensuremath{\omega}}_{\mathrm{i}},{\ensuremath{\tau}}_{\mathrm{s}},{\ensuremath{\tau}}_{\mathrm{i}})$ of the PDC state, which naturally lends itself to the pulsed characteristics of these states. From this function we derive the conditioned time-bandwidth product of one of the pair photons, a quantity which not only is a valid measure of entanglement between the PDC photons but also allows us to highlight a remarkable link between the discrete- and continuous-variable descriptions of PDC. We numerically analyze PDC processes with different conditions to demonstrate the versatility of our approach, which is applicable to a large number of current PDC sources.

Heralded single photon source at 1550 nm from pulsed parametric down conversion

Heralding of single photon at 1550 nm from pump pulsed nondegenerate spontaneous parametric downconversion is demonstrated. P(1) and P(2) of the source are 0.1871 and 2.4 × 10−3 respectively, with source triggering at 2.16 × 105 triggers per second. This source may be used in quantum key distribution systems.

Spatial and spectral mode selection of heralded single photons from pulsed parametric down-conversion

We describe an experiment in which photon pairs from a pulsed parametric down-conversion (PDC) source were coupled into single-mode fibers with heralding efficiencies as high as 70%. Heralding efficiency or mode preparation efficiency is defined as the probability of finding a photon in a fiber in a definite state, given the detection of its twin. Heralding efficiencies were obtained for a range of down-conversion beam-size configurations. Analysis of spatial and spectral mode selection, and their mutual correlation, provides a practical guide for engineering PDC-produced single photons in a definite mode and spectral emission band. The spectrum of the heralded photons were measured for each beam configuration, to determine the interplay between transverse momentum and spectral entanglement on the preparation efficiency.

Heralding single photons from pulsed parametric down-conversion

We describe an experiment in which photon pairs from a pulsed parametric down-conversion source were coupled into single-mode fibers. Detecting one of the photons heralded the presence of the other photon in its fiber with a probability of 83%. The heralded photons were then used in a simple multi-photon interference experiment to illustrate their potential for quantum information applications.

Output state in multiple entanglement swapping

The technique of quantum repeaters is a promising candidate for sending quantum states over long distances through a lossy channel. The usual discussion of this technique deals with only a finite-dimensional Hilbert space. However the qubits, with which one implements this procedure, will ``ride'' on continuous degrees of freedom of the carrier particles. Here we analyze the action of quantum repeaters using a model based on pulsed parametric down-conversion entanglement swapping. Our model contains the basic traits of a real experiment. We show that the state created, after the use of any number of parametric down-converters in a series of entanglement swappings, is always an entangled (actually distillable) state, although of a different form than the one that is usually assumed. Furthermore, the output state always violates a Bell inequality.

Experimental realization of entanglement concentration and a quantum repeater.

We report an experimental realization of entanglement concentration using two polarization-entangled photon pairs produced by pulsed parametric down-conversion. In the meantime, our setup also provides a proof-in-principle demonstration of a quantum repeater. The quality of our procedure is verified by observing a violation of Bell's inequality by more than 5 standard deviations. The high experimental accuracy achieved in the experiment implies that the requirement of tolerable error rate in multistage realization of quantum repeaters can be fulfilled, hence providing a useful toolbox for quantum communication over large distances.

Observation of Four-Photon Interference with a Beam Splitter by Pulsed Parametric Down-Conversion

When four photons arrive at a beam splitter, two from each side, a four-photon, six-path interference effect occurs to yield a sixfold enhancement of the probability for all four photons to exit together from the beam splitter. We produce the four-photon state by using the stimulated emission process in a pulsed parametric down-conversion and measure the probability for all four photons to exit from one side of the beam splitter. The observed enhancement factor is in good agreement with a multimode treatment of pulsed down-conversion.

Propagation of transient quantum coherence

We perform a time-resolved quantum interference experiment based on the effect of induced coherence without induced emission using two pulsed parametric down-conversion sources. We periodically vary the transmission of the first source's idler beam and measure the time-dependent visibility of fringes in the interference between the two signal beams. The visibility is found to be correlated with the transmissivity with a time delay corresponding to a combined path of the first idler and the second signal beam. The experiment is discussed in the context of the delayed choice experiments.

Photon statistics of pulsed parametric light

We calculate and measure the photon statistics of weak light pulses emitted in the process of pulsed parametric down-conversion. We find photon bunching as expected from a chaotic light source when viewed through a narrow-band filter.

Quantum interference: experiments and applications

We describe the simplest single–photon encoding schemes and introduce the concept of a quantum optical gate. These gates can be used to build up arbitrary entangled states of many initially separate single photons. Experimental realization of such a gate involves the development of nonlinear phase shift elements sensitice at the single quantum level. We report our experimental efforts using linear optical elements and non–classical sources where we are able to demonstrate interference and entanglement of initially separate single–photon pulses.