Correlated Photon Pairs Research Articles

Demonstration of spectral correlation control in a source of polarization-entangled photon pairs at telecom wavelength

Spectrally correlated photon pairs can be used to improve the performance of long-range fiber-based quantum communication protocols. We present a source based on spontaneous parametric downconversion, which allows one to control spectral correlations within the entangled photon pair without spectral filtering by changing the pump-pulse duration or the characteristics of the coupled spatial modes. The spectral correlations and polarization entanglement are characterized. We find that the generated photon pairs can feature both positive spectral correlations, decorrelation, or negative correlations at the same time as polarization entanglement with a high fidelity of 0.97 (no background subtraction) with the expected Bell state.

The impact of nonlinear losses in the silicon micro-ring cavities on CW pumping correlated photon pair generation

In this paper, 1.5 μm correlated photon pairs are generated under continuous wave (CW) pumping in a silicon micro-ring cavity with a Q factor of 8.1 × 10(4). The ratio of coincidences to accidental coincidences (CAR) is up to 200 under a coincidence time bin width of 5 ns. The experiment result of single side photon count shows that the generation rate does not increase as the square of the pump level due to the nonlinear losses in the cavity which reduce the Q factor and impact the field enhancement effect in the cavity under high pump level. Theoretical analysis shows that the photon pair generation rate in the cavity is proportional to the seventh power of the Q factor, which agrees well with the experiment result. It provides a way to analyze the performance of CW pumping correlated photon pair generation in silicon micro-ring cavities under high pump levels.

Continuous-wave quasi-phase-matched waveguide correlated photon pair source on a III–V chip

We report on the demonstration of correlated photon pair generation in a quasi-phase-matched superlattice GaAs/AlGaAs waveguide using a continuous-wave pump. Our photon pair source has a low noise level and achieves a high coincidence-to-accidental ratio greater than 100, which is the highest value reported in III–V chips so far. This correlated photon pair source has the potential to be monolithically integrated with on-chip pump laser sources fabricated on the same superlattice wafer structure, enabling direct correlated/entangled photon pair production from a compact electrically powered chip.

Generation of correlated photon pairs in micro/nano-fibers

We study the generation of correlated photon pairs via spontaneous four-wave mixing (SFWM) in a 15 cm long micro/nano-fiber (MNF). The MNF is properly fabricated to satisfy the phase-matching condition for generating the signal and idler photon pairs at wavelengths of about 1310 and 851 nm, respectively. Photon-counting measurements yield a coincidence-to-accidental ratio of 530 for a photon production rate of about 0.002 (0.0005) per pulse in the signal (idler) band. We also analyze the spectral information of the signal photons originating from SFWM and Raman scattering (RS). In addition to discovering some unique features of RS, we find the bandwidth of the individual signal photons is much greater than the calculated value for the MNF with homogeneous structure. Our investigations indicate the MNF is a promising candidate for developing the sources of nonclassical light and the spectral property of photon pairs can be used to noninvasively test the diameter and homogeneity of the MNF.

Photon pair generation in a silicon micro-ring resonator with reverse bias enhancement

Photon sources are fundamental components for any quantum photonic technology. The ability to generate high count-rate and low-noise correlated photon pairs via spontaneous parametric down-conversion using bulk crystals has been the cornerstone of modern quantum optics. However, future practical quantum technologies will require a scalable integration approach, and waveguide-based photon sources with high-count rate and low-noise characteristics will be an essential part of chip-based quantum technologies. Here, we demonstrate photon pair generation through spontaneous four-wave mixing in a silicon micro-ring resonator, reporting separately a maximum coincidence-to-accidental (CAR) ratio of 602 ± 37 (for a generation rate of 827kHz), and a maximum photon pair generation rate of 123 MHz ± 11 kHz (with a CAR value of 37). To overcome free-carrier related performance degradations we have investigated reverse biased p-i-n structures, demonstrating an improvement in the pair generation rate by a factor of up to 2 with negligible impact on CAR.

Integrated spatial multiplexing of heralded single-photon sources.

The non-deterministic nature of photon sources is a key limitation for single-photon quantum processors. Spatial multiplexing overcomes this by enhancing the heralded single-photon yield without enhancing the output noise. Here the intrinsic statistical limit of an individual source is surpassed by spatially multiplexing two monolithic silicon-based correlated photon pair sources in the telecommunications band, demonstrating a 62.4% increase in the heralded single-photon output without an increase in unwanted multipair generation. We further demonstrate the scalability of this scheme by multiplexing photons generated in two waveguides pumped via an integrated coupler with a 63.1% increase in the heralded photon rate. This demonstration paves the way for a scalable architecture for multiplexing many photon sources in a compact integrated platform and achieving efficient two-photon interference, required at the core of optical quantum computing and quantum communication protocols.

Two-Dimensional Fluorescence Lifetime Correlation Spectroscopy. 1. Principle

Fluorescence correlation spectroscopy (FCS) is a unique tool for investigating microsecond molecular dynamics of complex molecules in equilibrium. However, application of FCS in the study of molecular dynamics has been limited, owing to the complexity in the extraction of physically meaningful information. In this work, we develop a new method that combines FCS and time-correlated single photon counting (TCPSC) to extract unambiguous information about equilibrium dynamics of complex molecular systems. In this method, which we name two-dimensional fluorescence lifetime correlation spectroscopy (2D FLCS), we analyze the correlation of the fluorescence photon pairs, referring to the fluorescence lifetime. We first obtain the correlations of the photon pairs with respect to the excitation-emission delay times in the form of a two-dimensional (2D) map. Then, the 2D map is converted to the correlations between different species that have distinct fluorescence lifetimes using inverse Laplace transformation. This 2D FLCS is capable of visualizing the equilibration dynamics of complex molecules with microsecond time resolution at the single-molecule level. We performed a kinetic Monte Carlo simulation of a TCPSC-FCS experiment as a proof-of-principle example. The result clearly shows the validity of the proposed method and its high potential in analyzing the photon data of dynamic systems.

Optimizing the use of detector arrays for measuring intensity correlations of photon pairs

Intensity correlation measurements form the basis of many experiments based on spontaneous parametric down-conversion. In the most common situation, two single-photon avalanche diodes and coincidence electronics are used in the detection of the photon pairs, and the coincidence count distributions are measured by making use of some scanning procedure. Here we analyse the measurement of intensity correlations using multi-element detector arrays. By considering the detector parameters such as the detection and noise probabilities, we found that the mean number of detected photons that maximises the visibility of the two photon correlations is approximately equal to the mean number of noise events in the detector array. We provide expressions predicting the strength of the measured intensity correlations as a function of the detector parameters and on the mean number of detected photons. We experimentally test our predictions by measuring far-field intensity correlations of spontaneous parametric down-conversion with an electron multiplying CCD camera, finding excellent agreement with the theoretical analysis.

End-Output Coupling Efficiency Measurement of Silicon Wire Waveguides Based on Correlated Photon Pair Generation

A method to measure the end-output coupling efficiencies between silicon wire waveguides (SWWs) and optical fibers is proposed and demonstrated. The end-output coupling efficiency is calculated through four measured counting rates based on the correlated photon pair generation in the SWW. It provides a way to investigate and optimize the coupling processes at the input and output end of the optical waveguides separately.

Modification of Fluorescence Properties in Single Colloidal Quantum Dots by Coupling to Plasmonic Gap Modes

Efficient Auger relaxation in colloidal quantum dots (QDs) plays a crucial role in determining single-QD fluorescence behavior, for example, leading to fluorescence blinking and pure single-photon emission. Here, we demonstrate that when single-QD emission is coupled to a plasmonic gap mode, the fluorescence behavior is no longer dictated by nonradiative Auger relaxation, instead by radiative decay processes. In this case, annoying blinking can be largely suppressed accompanied by fluorescence enhancement. Moreover, the radiative decay rates for both exciton (X) and biexciton (BX) can be enhanced simultaneously owing to the broadband nature of a plasmonic gap mode with large Purcell effect. As a result, the BX quantum yields can be enhanced as high as that of single X emission which is derived from photon correlation measurement. This demonstration paves the way toward generating correlated photon pairs through BX–X cascade emission at room temperature based on cost-effective colloidal QDs.

Slow light enhanced correlated photon pair generation in photonic-crystal coupled-resonator optical waveguides

We demonstrate the generation of quantum-correlated photon pairs from a Si photonic-crystal coupled-resonator optical waveguide. A slow-light supermode realized by the collective resonance of high-Q and small-mode-volume photonic-crystal cavities successfully enhanced the efficiency of the spontaneous four-wave mixing process. The generation rate of photon pairs was improved by two orders of magnitude compared with that of a photonic-crystal line defect waveguide without a slow-light effect.

Toward a downconversion source of positively spectrally correlated and decorrelated telecom photon pairs

Frequency correlation (or decorrelation) of photon pairs is of great importance in long-range quantum communications and photonic quantum computing. We experimentally characterize a spontaneous parametric downconversion source, based on a β-barium borate crystal cut for type-II phase matching at 1550 nm, which has the capability to emit photons with positive or no spectral correlations. Our system employs a carefully designed detection method exploiting two InGaAs detectors.

Influence of pump chirp on the purity of an all fiber source of correlated photon pairs

The all fiber sources of photon pairs in the 1550 nm telecom band have the advantages of low cost and compatibility with the fiber network. Further improving the purity will improve the practicality of the quantum light sources. For the pulsed pump field propagating through the optical fiber, a certain amount of chirp will be inevitably induced due to the effects of chromatic of dispersion and Kerr nonlinearity, but its influence on the purity has not been studied yet. In this paper, using photon pairs produced by the spontaneous four wave mixing in dispersion shifted fiber, we study the influence of pump chirp on the purity of photon pairs by varying the chirp of the pulsed pump. For the pump with a given bandwidth, our results demonstrate that the purity of photon pairs having a certain production rate, decreases with the increase of the absolute quantity of pump chirp. Therefore, the purity of the photon pairs with relatively small detuning can be improved by using the transform limited pump pulses due to the suppressed Raman scattering.

Minimizing the frequency correlation of photon pairs in photonic crystal fibers

Based on the dispersion property of a given photonic crystal fiber (PCF), we study how to directly generate photon pairs with minimized frequency correlation via pulse-pumped spontaneous four-wave mixing. After illustrating why the intensity correlation function g(2) of individual signal (idler) photons can be used to reliably characterize the frequency correlation of photon pairs, we numerically investigate the dependence of g(2) under various kinds of experimental conditions. The results show that to minimize the frequency correlation, the experimental parameters should be properly optimized by balancing the influences of the high-order dispersion and the intrinsic sinc oscillation of phase matching function, apart from the satisfaction of the specified phase matching condition and the usage of transform-limited pump pulses. To verify the calculated results, we conduct two series of experiments by regulating the pump to respectively satisfy the asymmetric and symmetric group velocity matching conditions in our 0.6 m-long PCF. In both cases, the measured values of g(2) are less than the calculated results due to the inhomogeneity of the PCF; however, the experimental results qualitatively agree with the numerical simulations. Our investigation is very useful in fiber-based quantum state engineering.

Tunable modulational instability sidebands via parametric resonance in periodically tapered optical fibers

We analyze the modulation instability induced by periodic variations of group velocity dispersion and nonlinearity in optical fibers, which may be interpreted as an analogue of the well-known parametric resonance in mechanics. We derive accurate analytical estimates of resonant detuning, maximum gain and instability margins, significantly improving on previous literature on the subject. We also design a periodically tapered photonic crystal fiber, in order to achieve narrow instability sidebands at a detuning of 35 THz, above the Raman maximum gain peak of fused silica. The wide tunability of the resonant peaks by variations of the tapering period and depth will allow to implement sources of correlated photon pairs which are far-detuned from the input pump wavelength, with important applications in quantum optics.

Ultra-low power generation of twin photons in a compact silicon ring resonator

We demonstrate efficient generation of correlated photon pairs by spontaneous four wave mixing in a 5 μm radius silicon ring resonator in the telecom band around 1550 nm. By optically pumping our device with a 200 μW continuous wave laser, we obtain a pair generation rate of 0.2 MHz and demonstrate photon time correlations with a coincidence-to-accidental ratio as high as 250. The results are in good agreement with theoretical predictions and show the potential of silicon micro-ring resonators as room temperature sources for integrated quantum optics applications.

Quantum-correlated photon pair generation in tellurite microstructured optical fibers

We theoretically investigated the generation of quantum-correlated photon pair through spontaneous four-wave mixing in tellurite microstructured optical fiber (MOF). We evaluated the performance of photon pair generation in tellurite fibers based on Raman gain coefficient spectra. It was shown that the TBSN16P6W tellurite fiber provided a low Raman noise on correlation photon generation over a wide pump-idler detuning range. We can choose proper tellurite composition to obtain a low Raman gain window over wide range for correlated photon pair generation. We also designed the tellurite MOF structure to obtain a small dispersion value with high nonlinear coefficient at telecommunication wavelengths, thus realize efficient quantum-correlated photon pair generation.

Time-Resolved, Confocal Single-Molecule Tracking of Individual Organic Dyes and Fluorescent Proteins in Three Dimensions

We demonstrate following individual fluorescent protein constructs and individual organic dyes as they diffuse in 3-D in solution at rates up to 1 μm(2)/s over distances of several micrometers in X, Y, and Z. Our 3-D tracking method is essentially a stage scanning confocal microscope that uses a unique spatial filter geometry and active feedback 200 times/s to follow fast 3-D motion. Here we detail simulations used to find optimal feedback parameters for following individual fluorescent proteins in 3-D and show that a wide range of parameters are capable of following individual proteins diffusing at 1 μm(2)/s rates. In addition, we experimentally show that through 3-D single-molecule tracking of a protein oligomer series (monomer, dimer, and tetramer) of the fluorescent protein Azami Green one can determine the protein oligomerization state. We also perform time-resolved spectroscopy (photon pair correlation measurements) during the measured 3-D trajectories. The photon pair correlation measurements show clear fluorescence photon antibunching, demonstrating that the trajectories are of single fluorescent molecules. We note that the rates of single-molecule diffusive motion we follow (approximately 1 μm(2)/s) are comparable to or faster than many intracellular transport processes.

Generation of correlated photon pairs in different frequency ranges

The feasibility to generate correlated photon pairs at variable frequencies is investigated. For this purpose, we consider the interaction of an off-resonant laser field with a two-level system possessing broken inversion symmetry. We show that the system generates non-classical photon pairs exhibiting strong intensity-intensity correlations. The intensity of the applied laser tunes the degree of correlation while the detuning controls the frequency of one of the photons which can be in the THz-domain. Furthermore, we observe the violation of a Cauchy-Schwarz inequality characterizing these photons.

Generation of non-classical correlated photon pairs via a ladder-type atomic configuration: theory and experiment

We experimentally generate a non-classical correlated two-color photon pair at 780 and 1529.4 nm in a ladder-type configuration using a hot 85Rb atomic vapor with the production rate of ~10(7)/s. The non-classical correlation between these two photons is demonstrated by strong violation of Cauchy-Schwarz inequality by the factor R = 48 ± 12. Besides, we experimentally investigate the relations between the correlation and some important experimental parameters such as the single-photon detuning, the powers of pumps. We also make a theoretical analysis in detail and the theoretical predictions are in reasonable agreement with our experimental results.