Biphoton Wave Packets Research Articles

Observation of highly correlated ultrabright biphotons through increased atomic ensemble density in spontaneous four-wave mixing

The pairing ratio, a crucial metric assessing a biphoton source's ability to generate correlated photon pairs, remains underexplored despite theoretical predictions. This study presents experimental findings on the pairing ratio, utilizing a double-Λ spontaneous four-wave mixing biphoton source in cold atoms. At an optical depth (OD) of 20, we achieved an ultrahigh biphoton generation rate of up to 1.3×107 per second, with a successful pairing ratio of 61%. Increasing the OD to 120 significantly improved the pairing ratio to 89%, while maintaining a consistent biphoton generation rate. This achievement, marked by high generation rates and robust biphoton pairing, holds great promise for advancing efficiency in quantum communication and information processing. Additionally, in a scenario with a lower biphoton generation rate of 5.0×104 per second, we attained an impressive signal-to-background ratio of 241 for the biphoton wavepacket, surpassing the Cauchy-Schwarz criterion by approximately 1.5×104 times. Published by the American Physical Society 2024

Two-photon spectral modulation via temporal manipulation: Quantum optical synthesis of spectral modes from temporal square waves

Precise manipulation of the time–frequency modes of entangled photons is crucial for future quantum science and technologies. Recently, the frequency-domain-quantum-optical-synthesis (FD-QOS) method was demonstrated by creating a superposition of different joint temporal amplitudes: those temporal distributions can be controlled by manipulating the joint spectral amplitude in 2D frequency space via a Fourier optical relation. This FD-QOS method provides an efficient, flexible, and easy-to-control way to precisely modulate the temporal distributions of the entangled photon in an ultrafast region. However, manipulation of only the temporal modes is not sufficient for various applications in quantum information, since spectral modulations are also needed on many occasions. Here, we present a proof-of-concept experiment of two-photon spectral modulation via temporal manipulation of a biphoton wave packet. This protocol, called time-domain-quantum-optical-synthesis (TD-QOS), is achieved by adjusting the relative phases between two joint temporal distributions. In addition, the two-photon joint spectral distributions are characterized by measuring the joint spectral intensities and Hong–Ou–Mandel interferences. The combination of FD-QOS and TD-QOS enables complete control over the biphoton states. Our work would further develop quantum technologies that rely on the time–frequency modes of entangled photons.

Temporally ultralong biphotons with a linewidth of 50 kHz

We report the generation of biphotons, with a temporal full width at the half maximum (FWHM) of 13.4 ± 0.3 µs and a spectral FWHM of 50 ± 1 kHz, via the process of spontaneous four-wave mixing with laser-cooled atoms. The temporal width is the longest, and the spectral linewidth is the narrowest to date. This is also the first biphoton result that obtains a linewidth below 100 kHz, reaching a new milestone. The very long biphoton wave packet has a signal-to-background ratio of 3.4, which violates the Cauchy–Schwarz inequality for classical light by 4.8 folds. Furthermore, we demonstrated a highly tunable-linewidth biphoton source and showed that while the biphoton source’s temporal and spectral width were controllably varied by about 24 folds, its generation rate only changed by less than 15%. A spectral brightness or generation rate per pump power per linewidth of 1.2× 106 pairs/(s mW MHz) was achieved at the temporal width of 13.4 µs. The above results were made possible by the low decoherence rate and high optical depth of the experimental system, as well as a novel scheme of classical fields’ and biphotons’ propagation directions in the experiment. This work has demonstrated a high-efficiency ultranarrow-linewidth biphoton source and has made substantial advancements in quantum technology utilizing heralded single photons.

Temporal profile of biphotons generated from a hot atomic vapor and the spectrum of electromagnetically induced transparency

We systematically studied the temporal profile of biphotons, i.e., pairs of time-correlated single photons, generated from a hot atomic vapor via the spontaneous four-wave mixing process. The measured temporal width of biphoton wave packet or two-photon correlation function against the coupling power was varied from about 70 to 580 ns. We derived an analytical expression of the biphoton's spectral profile in the Doppler-broadened medium. The analytical expression reveals that the spectral profile is mainly determined by the effect of electromagnetically induced transparency (EIT), and behaves like a Lorentzian function with a linewidth approximately equal to the EIT linewidth. Consequently, the biphoton's temporal profile influenced by the Doppler broadening is an exponential-decay function, which was consistent with the experimental data. Employing a weak input probe field of classical light, we further measured the EIT spectra under the same experimental conditions as those in the biphoton measurements. The theoretical predictions of the biphoton wave packets calculated with the parameters determined by the classical-light EIT spectra are consistent with the experimental data. The consistency demonstrates that in the Doppler-broadened medium, the classical-light EIT spectrum is a good indicator for the biphoton's temporal profile. Besides, the measured biphoton's temporal widths well approximated to the predictions of the analytical formula based on the biphoton's EIT effect. This study provides an analytical way to quantitatively understand the biphoton's spectral and temporal profiles in the Doppler-broadened medium.

Randomly aperiodically poled LiNbO3 crystal design by Monte Carlo–Metropolis with simulated annealing optimization for ultrabroadband photon pair generation

We report a scheme for generating ultrabroadband two-photon states by spontaneous parametric downconversion (SPDC) using randomly aperiodically poled crystals designed with an optimization algorithm based on the Monte Carlo-Metropolis method with simulated annealing. A particular SPDC source is discussed, showing results of the spectral and temporal properties of the emitted two-photon states, obtaining almost transform-limited SPDC biphoton wave packets. We also analyze the effect of fabrication errors on the SPDC.

Quantum optical synthesis in 2D time–frequency space

Conventional optical synthesis, the manipulation of the phase and amplitude of spectral components to produce an optical pulse in different temporal modes, is revolutionizing ultrafast optical science and metrology. These technologies rely on the Fourier transform of light fields between time and frequency domains in one-dimensional space. However, within this treatment it is impossible to incorporate the quantum correlation among photons. Here we expand the Fourier synthesis into high dimensional space to deal with the quantum correlation, and carry out an experimental demonstration by manipulating the two-photon probability distribution of a biphoton in two-dimensional time and frequency space. As a potential application, we show manipulation of a heralded single-photon wave packet, which is never explained by the conventional one-dimensional Fourier optics. Our approach opens up a new pathway to tailor the temporal characteristics of a biphoton wave packet with high dimensional quantum-mechanical treatment. We anticipate such high dimensional treatment of light in time and frequency domains could bridge the research fields between quantum optics and ultrafast optical measurements.

Temporally Correlated Narrowband Biphoton Interference Based on Mach–Zehnder Interferometer

AbstractSince early 1990s, Mach–Zehnder interferometer has been used to investigate the interference of biphoton wave packets. Due to subpicosecond time coherence of biphoton generated by spontaneous parametric downconversion process, some physical processes are ignored in the interferometer, most likely the biphoton time‐domain interference. Here, the two‐photon interference phenomenon based on the Mach–Zehnder interferometer is theoretically studied, where the correlated photon pairs are produced by the four‐wave mixing in atomic system. In particular, the quantum interference effect to effectively control the coherent time of two‐photon by adjusting the input delay is used. In the damped Rabi oscillation regime, two‐photon bunching and antibunching effects are observed. In addition, in the group‐delay regime, the interference between biphoton precursor, slow‐light wave packets and also in between the precursor and the slow‐light wave packets is observed, which had never been reported before. These results may have potential applications in the fields of biphoton shaping and quantum information processing.

Stochastic numerical simulations of a fully spatiotemporal Hong-Ou-Mandel dip

We develop a fully spatio-temporal numerical model, based on stochastic simulations, simulating the generation of spatio-temporal multimode spontaneous parametric down conversion, the propagation of the signal and idler beams through a Hong-Ou-Mandel interferometer and the detection of the outgoing beams with two separate detectors arrays. Spatial and temporal properties of the two-photon interference are investigated by measuring the spatial distribution of the momenta coincidence counts between the time integrated outgoing far-field images and a full spatio-temporal HOM dip is exhibited. Numerical results also demonstrate that the spatio-temporal coherence properties of bi-photon wave packets can be fully characterized with detectors arrays with no temporal resolution.

Quantum manipulation of biphoton spectral distributions in a 2D frequency space toward arbitrary shaping of a biphoton wave packet.

Arbitrary shaping of optical waveform is fundamental interest from basic science to advanced optical technologies. However, it is still challenging task for shaping a biphoton wave packet. Here we experimentally manipulate the spectrum and phase of a biphoton wave packet in a two-dimensional frequency space. The spectrum is shaped by adjusting the temperature of the crystal, and the phase is controlled by tilting the dispersive glass plate. The manipulating effects are confirmed by measuring the two-photon spectral intensity (TSI) and the Hong-Ou-Mandel (HOM) interference patterns. The technique in this work paves the way for arbitrary shaping of a multi-photon wave packet in a quantum manner.

Anisotropic narrowing of biphoton wave packet distribution in spontaneous parametric down-conversion with biaxial crystal

We investigate theoretically the strong anisotropy in the wave packet distribution of biphoton pairs generated via spontaneous parametric down-conversion (SPDC) process with biaxial crystals. From the angle relationship in biaxial crystals, we deduce the wave function of type I SPDC by considering the longitudinal detuning under paraxial approximation. Taking BiB3O6 (BIBO) as an example, we numerically simulate the single-particle and coincidence distributions in two observation planes under the optimum phase-matching scheme. In contrast to uniaxial crystals, a stronger anisotropy effect and a higher entanglement of the photon pair states are obtained, which are attributed to the complex structure of biaxial crystals. These results are important for the generation of highly entangled biphoton pairs, especially for multi-photon pairs generation.

Quantum interference of biphotons with a Doppler frequency shift

We report a theoretical study of transformation of a biphoton state of light under Bragg diffraction on a travelling sound wave in an acousto-optic modulator (AOM). It is shown that the diffraction of AOM biphotons emitted during the collinear parametric scattering of light leads to a shift of the carrier frequency of a biphoton wave packet, which exceeds twice the Doppler frequency shift for the classical field. A method is proposed for measuring the Doppler frequency shift of a biphoton, which is based on interference between independent biphotons.

Orthogonal spectral coding of entangled photons.

We extend orthogonal optical coding, previously applied to multiuser classical communication networks, to entangled photons. Using a pulse shaper and sum-frequency generation for ultrafast coincidence detection, we demonstrate encoding and decoding of biphoton wave packets. Applying one code to the signal photon spreads the wave packet in time and creates a null at zero delay; filtering the idler with the matched code recovers a narrow correlation peak, whereas applying any other code leaves the wave packet spread. Our results could prove useful in the development of code-based quantum communication networks.

Generation of nonclassical narrowband photon pairs from a cold rubidium cloud

We report the generation of nonclassical narrowband photon pairs via the spontaneous four-wave mixing process in a cold Rubidium (87Rb) cloud prepared in a magneto-optical trap. The generated photon pairs exhibit strong violation of the Cauchy-Schwarz inequality by a factor of 625 ± 45, confirming nonclassical correlation between the photons. The biphoton wave packets, measured for several different experimental conditions, are in good agreement with the theoretical results.

An optical device for controlling dispersion in quantum communication

We theoretically demonstrate a new scheme for cancelling the dispersive effects in quantum communication with two entangled photons propagating in optical fibres. In the case of two entangled photons generated by spontaneous parametric down-conversion and propagating in two optical fibres, it is demonstrated that the dispersive effect in the biphoton wavepacket is cancelled by inserting only one Fabry–Perot cavity into one of the fibre's lines. No special dispersive material or interference of the transmitted photons in a beam splitter (or interferometer) is necessary. In the scheme, where one of the photons is reflected by the Fabry–Perot cavity before being transmitted by the fibre, a full dispersion cancellation is shown with no decrease in the number of photons at the optical fibres exit.

Chirped quasi-phase-matching with Gauss sums for production of biphotons

We study the theory of linearly chirped biphoton wave packets produced in two basic quasi-phase-matching configurations: chirped photonic-like crystals and chirped aperiodically poled crystals. The novelty is that these structures are considered as definite assembles of nonlinear layers that leads to a detailed description of spontaneous parametric down-conversion processes through the discrete Gauss sums. In this approach, the spectra of biphoton wave packets for each of the two presented quasi-phase-matching configurations display new important properties that were not obtained and explained in the standard phenomenological approach.

The physics of ghost imaging: nonlocal interference or local intensity fluctuation correlation?

In this special Volume on Quantum Imaging, Shapiro and Boyd present a review on The Physics of Ghost Imaging. Here I argue that their Review is incomplete, as it overlooks a form of ghost imaging that requires quantum mechanical nonlocal effects for their understanding. From the very beginning, Klyshko brought attention to the nonlocal nature of biphoton interference and biphoton imaging by introducing his “advanced wave” theory [1]. His “advanced wave” propagates backward in time from a photodetector to the source and then propagates forward in time from the source to another distant photodetector. The “advanced wave” connects the object-arm and the image-arm, as well as two independent photodetection events, in his biphoton interference and imaging model. Strekalov et al. and Pittman et al. realized nonlocal biphoton interference [2] and biphoton imaging [3] experimentally. The physics community immediately named these two experiments “ghost interference” and “ghost imaging”. Simultaneously with the experimental realization, we found that Klyshko’s advanced wave theory can be replaced by a standard quantum theory of second-order coherence in which a nonlocal biphoton wavepacket is introduced to connect the two arms and two photodetection events [4]. Soon after that, we realized that nonlocal interference and imaging are not restricted to entangled biphoton systems: it also happens in thermal radiation. Ghost imaging can be observed from thermal light in the Fresnel near-field zone in a lensless configuration [5–7]. Although nonlocality is a feature of quantum mechanics [8], this concept is not part of classical physics. In the classical theory of light, ghost imaging, especially thermal light ghost imaging, is a result of locally measured intensity fluctuation correlations. So this brings us to ask, is ghost imaging a nonlocal interference phenomenon (and hence quantum) or is it a result of local intensity fluctuation

Temporal ghost imaging of a time object, dispersion cancelation, and nonlocal time lens with bi-photon state

We present a theory of temporal diffraction, temporal imaging of a bi-photon state, and temporal ghost imaging of a time object. By applying factional Fourier transform to the bi-photon wave packet propagating in space, we could obtain a theory that shows the physical origin of dispersion cancelation, temporal imaging, nonlocal effects of time lenses, and temporal ghost imaging. We introduce the temporal diffraction distance for bi-photon wave packet and show that the bi-photon wave packet behaves like a single wave packet whose temporal diffraction distance is determined by the coherent sum of the temporal diffraction distances for the signal and the idler beams. This property yields the well known dispersion cancelation, the recovery of original bi-photon wave packet in temporal imaging, and the nonlocal combination of two time lenses placed in different arms. We also propose a method for ghost imaging of an arbitrary time object.

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.

Engineering biphoton wave packets with an electromagnetically induced grating

We propose to shape biphoton wave packets with an electromagnetically induced grating in a four-level double-$\Lambda$ cold atomic system. We show that the induced hybrid grating plays an essential role in directing the new fields into different angular positions, especially to the zeroth-order diffraction. A number of interesting features appear in the shaped two-photon waveforms. For example, broadening or narrowing the spectrum would be possible in the proposed scheme even without the use of a cavity.

Chirped Biphotons and their Compression in Optical Fibers

We show that broadband biphoton wave packets produced via spontaneous parametric down-conversion in crystals with linearly aperiodic poling can be easily compressed in time using the effect of group-velocity dispersion in optical fibers. This result could foster important developments in quantum metrology and lithography.