Biphoton Correlation Research Articles

Tunable delay control of entangled photons based on dispersion cancellation.

We propose and demonstrate a novel approach for controlling the temporal position of the biphoton correlation function using pump frequency tuning and dispersion cancellation; precise waveguide engineering enables biphoton generation at different pump frequencies while the idea of nonlocal dispersion cancellation is used to create the relative signal-idler delay and simultaneously prevents broadening of their correlation. Experimental results for delay shifts up to ±15 times the correlation width are shown along with discussions of the performance metrics of this approach.

Bi-photon spectral correlation measurements from a silicon nanowire in the quantum and classical regimes.

The growing requirement for photon pairs with specific spectral correlations in quantum optics experiments has created a demand for fast, high resolution and accurate source characterisation. A promising tool for such characterisation uses classical stimulated processes, in which an additional seed laser stimulates photon generation yielding much higher count rates, as recently demonstrated for a χ(2) integrated source in A. Eckstein et al. Laser Photon. Rev. 8, L76 (2014). In this work we extend these results to χ(3) integrated sources, directly measuring for the first time the relation between spectral correlation measurements via stimulated and spontaneous four wave mixing in an integrated optical waveguide, a silicon nanowire. We directly confirm the speed-up due to higher count rates and demonstrate that this allows additional resolution to be gained when compared to traditional coincidence measurements without any increase in measurement time. As the pump pulse duration can influence the degree of spectral correlation, all of our measurements are taken for two different pump pulse widths. This allows us to confirm that the classical stimulated process correctly captures the degree of spectral correlation regardless of pump pulse duration, and cements its place as an essential characterisation method for the development of future quantum integrated devices.

Temporal compression and shaping of chirped biphotons using Fresnel-inspired binary phase shaping

Chirped biphotons generated via spontaneous parametric down-conversion in chirped quasi-phase-matched nonlinear crystals have ultrabroadband frequency spectra, while their correlation times are not ultranarrow, due to the nonlinear frequency dependence of the phase factor. Based on the idea of Fresnel zone lenses, we propose a Fresnel-inspired binary phase-shaping scheme for tailoring broadband biphoton spectra. By applying binary phase masks to the biphoton spectrum, a Fresnel zone lens was produced in the frequency domain, compressing the broadband temporal wave packets of the chirped biphotons to the Fourier-transform limit due to the effective removal of the quadratic phase. Furthermore, biphoton correlation trains can also be generated by properly shaping the biphoton spectrum with this scheme. This provides a method for biphoton compression and shaping, which avoids the length dependence and the impact of higher-order dispersion in a dispersive medium by using the phase compensation method. Our results could prove useful in quantum metrology, lithography, and coding of entangled photons.

Classical-to-quantum transition with broadband four-wave mixing.

A key question of quantum optics is how nonclassical biphoton correlations at low power evolve into classical coherence at high power. Direct observation of the crossover from quantum to classical behavior is desirable, but difficult due to the lack of adequate experimental techniques that cover the ultrawide dynamic range in photon flux from the single photon regime to the classical level. We investigate biphoton correlations within the spectrum of light generated by broadband four-wave mixing over a large dynamic range of ∼80 dB in photon flux across the classical-to-quantum transition using a two-photon interference effect that distinguishes between classical and quantum behavior. We explore the quantum-classical nature of the light by observing the interference contrast dependence on internal loss and demonstrate quantum collapse and revival of the interference when the four-wave mixing gain in the fiber becomes imaginary.

On the geometry of spatial biphoton correlation in spontaneous parametric down conversion

Analytical expressions are derived for the distribution rates of spatial coincidences in the counting of photons produced by spontaneous parametric down conversion. Gaussian profiles are assumed for the wave function of the idler and signal light created in type‐I spontaneous parametric down conversion. The distribution rates describe ellipses on the detection planes that are oriented at different angles according to the photon coincidences in either horizontal–horizontal, vertical–vertical, horizontal–vertical, or vertical–horizontal position variables. The predictions are in agreement with the experimental data obtained with a type‐I beta‐barium borate crystal that is illuminated by a 100‐mW violet pump laser as well as with the results obtained from the geometry defined by the phase‐matching conditions. Copyright © 2014 John Wiley & Sons, Ltd.

Engineering frequency-time quantum correlation of narrow-band biphotons from cold atoms.

The nonclassical photon pair, generated via a parametric process, is naturally endowed with a specific form of frequency-time quantum correlations. Here, we report complete control of frequency-time quantum correlations of narrow-band biphotons generated via spontaneous four-wave mixing in a cold atomic ensemble. We have experimentally confirmed the generation of frequency-anticorrelated, frequency-correlated, and frequency-uncorrelated narrow-band biphoton states, as well as verifying the strong nonclassicality of the correlations. Our work opens up new possibilities for engineering narrow-band entangled photons for various quantum optical and quantum information applications.

Generation of biphoton correlation trains through spectral filtering.

We demonstrate the generation of two-photon correlation trains based on spectral filtering of broadband biphotons. Programmable amplitude filtering is employed to create biphoton frequency combs, which when coupled with optical dispersion allows us to experimentally verify the temporal Talbot effect for entangled photons. Additionally, an alternative spectral phase-filtering approach is shown to significantly improve the overall efficiency of the generation process when a comb-like spectrum is not required. Our technique is ideal for the creation of tunable and high-repetition-rate biphoton states.

Propagation characteristics of biphotons in cold atomic vapor

We investigate the propagation characteristics of the narrowband Stokes/anti-Stokes photons in cold atomic vapor. The four-wave mixing process results from parametric amplification of the anti-Stokes photons. We find that the process of parametric amplification is very similar to the light pulse propagating through an anomalous dispersion gain medium. Finally, we obtain the general solutions of the Glauber biphoton correlation functions, which are in good agreement with the experiment results.

Biphoton generation in quadratic waveguide arrays: a classical optical simulation.

Quantum entanglement became essential in understanding the non-locality of quantum mechanics. In optics, this non-locality can be demonstrated on impressively large length scales, as photons travel with the speed of light and interact only weakly with their environment. Spontaneous parametric down-conversion (SPDC) in nonlinear crystals provides an efficient source for entangled photon pairs, so-called biphotons. However, SPDC can also be implemented in nonlinear arrays of evanescently coupled waveguides which allows the generation and the investigation of correlated quantum walks of such biphotons in an integrated device. Here, we analytically and experimentally demonstrate that the biphoton degrees of freedom are entailed in an additional dimension, therefore the SPDC and the subsequent quantum random walk in one-dimensional arrays can be simulated through classical optical beam propagation in a two-dimensional photonic lattice. Thereby, the output intensity images directly represent the biphoton correlations and exhibit a clear violation of a Bell-like inequality.

A miniature ultrabright source of temporally long, narrowband biphotons

We demonstrate a miniature source of long biphotons utilizing the cluster effect and double-pass pumping in a monolithic doubly resonant parametric down-converter. We obtain a biphoton correlation time of 17.1 ns with a generation rate of 1.10×105 biphotons/(s mW) and an estimated linewidth of 8.3 MHz.

Classical characterization of biphoton correlation in waveguide lattices

We propose a classical experimental scheme for simulating quantum correlations of path-entangled biphotons, propagating in arbitrary one- or two-dimensional arrays of evanescently coupled waveguides. Our approach requires only intensity and phase measurements and can be used to characterize the evolution of the biphoton wave packet. The precision of the method and its robustness to phase fluctuations are investigated by a statistical analysis.

Spatiotemporal structure of biphoton entanglement in type-II parametric down-conversion

We investigate the spatiotemporal structure of the biphoton correlation in type-II parametric down-conversion (PDC). As in type-I PDC [Phys. Rev. Lett. 102, 223601 (2009)], we find that the correlation is nonfactorizable in space and time. Differently from type I, the type-II correlation in the spontaneous regime displays an asymmetric V-shaped structure in any cross section including time and one transverse dimension. This asymmetry along the temporal coordinate originates from the signal-idler group velocity mismatch and tends to disappear as the parametric gain is raised. We observe a progressive transition toward a symmetric X-shaped geometry similar to that found in type I when stimulated PDC becomes dominant. We also give quantitative evaluations of the localization in space and in time of the correlation, analyze its behavior for different crystal tuning angles, and underline qualitative differences with respect to type-I PDC.

Tailoring the spatiotemporal structure of biphoton entanglement in type-I parametric down-conversion

We investigate the spatiotemporal structure of the biphoton entangled state produced by parametric down-conversion (PDC) at the output face of the nonlinear crystal. We analyze the geometry of biphoton correlation for different gain regimes (from ultralow to high), different crystal lengths, and different tuning angles of the crystal. While for collinear or quasicollinear phase matching a X-shaped geometry, nonfactorizable in space and time, dominates, in the highly noncollinear conditions we observe a remarkable transition to a factorizable geometry. We show that the geometry of spatiotemporal correlation is a consequence of the angle-frequency relationship imposed by phase matching and that the fully spatiotemporal analysis provides a key to control the spatiotemporal properties of the PDC entangled state and in particular to access a biphoton localization in time and space in the femtosecond and micrometer range, respectively.

Two schemes of logic gates for one-way quantum computing

Two schemes of measurement-based gates are considered. The first scheme uses atomic cluster states generated by using the proposed cooperative process involving classical fields. The second scheme is based on the quantum correlation of biphotons and allows encoding classical information by the states of a quantum system.

Four-wave mixing in three-level systems: Interference and entanglement

The interference of degenerate four-wave mixing (FWM) in three-level systems is examined in both the classical and quantum regimes with a backward configuration. In classical FWM, we find that the phase difference between two indistinguishable FWM transition paths can be varied by different driving laser parameters, and leads to interference in the amplitude and polarization of the generated conjugate field. In the paired-photon generation case, the interference in the nonlinearity disappears because of the time ordering in biphoton generation. However, because of the slow group velocity at the degenerate frequency and polarization, the biphoton-amplitude interference between two Feynman paths can erase the time-ordering information at the detectors. For small group delay, the biphoton correlation, determined by the third-order nonlinearity, shows antibunching and damped Rabi oscillations. For large group delay, where the biphoton bandwidth is determined by phase matching, we show that the biphoton interference leads to a bunching effect. The feasibility of generating polarization entanglement is also discussed.