Energy-time Entangled Photon Pairs Research Articles

Generation of entangled photon pairs from a silicon bichromatic photonic crystal cavity

Integrated quantum photonics leverages the on-chip generation of nonclassical states of light to realize key functionalities of quantum devices. Typically, the generation of such nonclassical states relies on whispering gallery mode resonators, such as integrated optical micro-rings, which enhance the efficiency of the underlying spontaneous nonlinear processes. While these kinds of resonators excel in maximizing either the temporal confinement or the spatial overlap between different resonant modes, they are usually associated with large mode volumes, imposing an intrinsic limitation on the efficiency and footprint of the device. Here, we engineer a source of time-energy entangled photon pairs based on a silicon photonic crystal cavity, implemented in a fully CMOS-compatible platform. In this device, resonantly enhanced spontaneous four-wave mixing converts pump photon pairs into signal/idler photon pairs under the energy-conserving condition in the telecommunication C-band. The design of the resonator is based on an effective bichromatic confinement potential, allowing it to achieve up to nine close-to-equally spaced modes in frequency, while preserving small mode volumes, and the whole chip, including grating couplers and access waveguides, is fabricated in a single run on a silicon-on-insulator platform. Besides demonstrating efficient photon pair generation, we also implement a Franson-type interference experiment, demonstrating entanglement between signal and idler photons with a Bell inequality violation exceeding five standard deviations. The high generation efficiency combined with the small device footprint in a CMOS-compatible integrated structure opens a pathway toward the implementation of compact quantum light sources in all-silicon photonic platforms.

High-dimensional quantum key distribution using energy-time entanglement over 242 km partially deployed fiber

Entanglement-based quantum key distribution (QKD) is an essential ingredient in quantum communication, owing to the property of source-independent security and the potential on constructing large-scale quantum communication networks. However, implementation of entanglement-based QKD over long-distance optical fiber links is still challenging, especially over deployed fibers. In this work, we report an experimental QKD using energy-time entangled photon pairs that transmit over optical fibers of 242 km (including a section of 19 km deployed fibers). The QKD is realized through the protocol of dispersive-optics QKD (DO-QKD) with high-dimensional encoding to utilize coincidence counts more efficiently. A reliable, high-accuracy time synchronization technology for long-distance entanglement-based QKD is developed based on the distribution of optical pulses in quantum channels. Our system operates continuously for more than 7 d without active polarization or phase calibration. We ultimately generate secure keys with secure key rates of 0.22 bps and 0.06 bps in the asymptotic and finite-size regimes, respectively. It shows that entanglement-based DO-QKD is reliable for long-distance realization in the field if its high requirement on time synchronization is satisfied.

Photoelectron spectroscopy with entangled photons; enhanced spectrotemporal resolution

In this theoretical study, we show how photoelectron signals generated by time-energy entangled photon pairs can monitor ultrafast excited state dynamics of molecules with high joint spectral and temporal resolutions, not limited by the Fourier uncertainty of classical light. This technique scales linearly, rather than quadratically, with the pump intensity, allowing the study of fragile biological samples with low photon fluxes. Since the spectral resolution is achieved by electron detection and the temporal resolution by a variable phase delay, this technique does not require scanning the pump frequency and the entanglement times, which significantly simplifies the experimental setup, making it feasible with current instrumentation. Application is made to the photodissociation dynamics of pyrrole calculated by exact nonadiabatic wave packet simulations in a reduced two nuclear coordinate space. This study demonstrates the unique advantages of ultrafast quantum light spectroscopy.

Realization of a source-device-independent quantum random number generator secured by nonlocal dispersion cancellation

Quantum random number generators (QRNGs) can provide genuine randomness by exploiting the intrinsic probabilistic nature of quantum mechanics, which play important roles in many applications. However, the true randomness acquisition could be subjected to attacks from untrusted devices involved or their deviations from the theoretical modeling in real-life implementation. We propose and experimentally demonstrate a source-device-independent QRNG, which enables one to access true random bits with an untrusted source device. The random bits are generated by measuring the arrival time of either photon of the time–energy entangled photon pairs produced from spontaneous parametric downconversion, where the entanglement is testified through the observation of nonlocal dispersion cancellation. In experiment, we extract a generation rate of 4 Mbps by a modified entropic uncertainty relation, which can be improved to gigabits per second by using advanced single-photon detectors. Our approach provides a promising candidate for QRNGs with no characterization or error-prone source devices in practice.

Ultra-efficient generation of time-energy entangled photon pairs in an InGaP photonic crystal cavity

The typical approaches to generate heralded single photons rely on parametric processes, with the advantage of generating highly entangled states at the price of a random pair emission. To overcome this limit, degenerate spontaneous Four-Wave-Mixing is a reliable technique which combines two pump photons into a pair of signal and idler photons via Kerr nonlinear optical effect. By exploiting the intrinsic small confinement volume and thermally tuning the resonances of a 20 μm-long Photonic Crystal cavity, we efficiently generate time-energy entangled photon pairs and heralded single photons at a large maximum on-chip rate of 22 MHz, using 36 μW of pump power. We measure time-energy entanglement with net visibility up to 96.6 % using 1 second integration time constant. Our measurements demonstrate the viability of Photonic Crystal cavities to act as an alternative and efficient photon pair source for quantum photonics.

Widely flexible and finely adjustable nonlocal dispersion cancellation with wavelength tuning.

In fiber-based quantum information processing with energy-time entangled photon pairs, optimized dispersion compensation is vital to preserve the strong temporal correlation of the photon pairs. We propose and experimentally verify that, by simply tuning the wavelength of the entangled photon pairs, nonlocal dispersion cancellation (NDC) can provide a widely flexible and finely adjustable solution for optimizing the dispersion compensation, which cannot be reached with the traditional local dispersion cancellation (LDC) instead. By way of example, when a 50 km-long single-mode fiber (SMF) is dispersion compensated by a 6.2-km-long commercial dispersion compensating fiber (DCF) based on the LDC configuration, it will lead to an almost invariant over-compensation in the wavelength range of 1500-1600 nm which restricts the observed temporal coincidence width of the self-developed energy-time entangled photon-pairs source to a minimum of ∼110 ps. While in the NDC configuration, the dispersion compensation can be readily optimized by tuning the signal wavelength to 1565.7 nm and a minimum coincidence width of 86.1 ± 0.7 ps is observed, which is mainly limited by the jitter of the single-photon detection system. Furthermore, such optimized dispersion compensation can also be achieved as the fiber length varies from 48 km to 60 km demonstrating the wide flexibility of NDC. Thanks to these capabilities, elaborate dispersion compensation modules are no longer required, which makes NDC a more versatile tool in fiber-based quantum information and metrology applications.

Synthetic five-wave mixing in an integrated microcavity for visible-telecom entanglement generation

Nonlinear optics processes lie at the heart of photonics and quantum optics for their indispensable role in light sources and information processing. During the past decades, the three- and four-wave mixing (χ(2) and χ(3)) effects have been extensively studied, especially in the micro-/nano-structures by which the photon-photon interaction strength is greatly enhanced. So far, the high-order nonlinearity beyond the χ(3) has rarely been studied in dielectric materials due to their weak intrinsic nonlinear susceptibility, even in high-quality microcavities. Here, an effective five-wave mixing process (χ(4)) is synthesized by incorporating χ(2) and χ(3) processes in a single microcavity. The coherence of the synthetic χ(4) is verified by generating time-energy entangled visible-telecom photon pairs, which requires only one drive laser at the telecom waveband. The photon-pair generation rate from the synthetic process shows an estimated enhancement factor over 500 times upon intrinsic five-wave mixing. Our work demonstrates a universal approach of nonlinear synthesis via photonic structure engineering at the mesoscopic scale rather than material engineering, and thus opens a new avenue for realizing high-order optical nonlinearities and exploring functional photonic devices.

Single atom photon pair source

Sources of entangled photon pairs are a crucial ingredient for many applications in quantum information and communication. Of particular interest are narrow-band sources with bandwidths that are compatible with solid state systems such as atomic media for storage and manipulation of the photons. Here, we experimentally realize a source of energy-time entangled photon pairs where the photons pairs are generated by scattering light from a single two-level atom and separated from the coherently scattered light via a narrow-band filter. We verify the performance of our pair-source by measuring the second order correlation function of the atomic fluorescence and we observe that one can continuously tune the photon statistics of the atomic fluorescence from perfect photon anti-bunching to strong photon bunching expected for a photon pair source. Our experiment demonstrates a novel way to realize a photon pair source for photons with spectral bandwidths and resonance frequencies that are inherently compatible with atomic media.

Impact of fiber dispersion on the performance of entanglement-based dispersive optics quantum key distribution

Dispersive optics quantum key distribution (DO-QKD) based on energy-time entangled photon pairs is an important QKD scheme. In DO-QKD, the arrival time of photons is used in key generation and security analysis, which would be greatly affected by fiber dispersion. In this work, we established a theoretical model of the entanglement-based DO-QKD system, considering the protocol, physical processes (such as fiber transmission and single-photon detection), and the analysis of security tests. Based on this theoretical model, we investigate the influence of chromatic dispersion introduced by transmission fibers on the performance of DO-QKD. By analyzing the benefits and costs of dispersion compensation, the system performance under G.652 and G.655 optical fibers are shown, respectively. The results show that dispersion compensation is unnecessary for DO-QKD systems in campus networks and even metro networks. Whereas, it is still required in DO-QKD systems with longer fiber transmission distances.

Second-order interference of true thermal light from a warm atomic ensemble in two independent unbalanced interferometers

We report the demonstration of a second-order interference experiment by use of thermal light emitted from a warm atomic ensemble in two spatially separated unbalanced Michelson interferometers (UMIs). This novel multipath correlation interference with thermal light has been theoretically proposed by Tamma [New J. Phys. 18, 032002 (2016)NJOPFM1367-263010.1088/1367-2630/18/3/032002]. In our experiment, the bright thermal light used for second-order interference is superradiantly emitted via collective two-photon coherence in Doppler-broadened cascade-type Rb 87 atoms. Owing to the long coherence time of the thermal light from the atomic ensemble, we observe its second-order interference in the two independent UMIs by means of time-resolved coincidence detection. The temporal waveforms of the interfering thermal light in the two spatially separated UMIs exhibit similarities with the temporal two-photon waveform of time–energy entangled photon pairs in Franson interferometry. Our results can contribute toward a better understanding of the relation between first- and second-order interferences that are at the heart of photonics-based quantum information science.

Self-calibrating optical low-coherence reflectometry with energy-time entangled photons for absolute distance measurements

Optical low-coherence reflectometry is capable of unambiguously measuring positions of stacked, partially reflective layers in a sample object. It relies on the low coherence of the light source and the absolute distances are obtained from the position reading of a mechanical motor stage. We show how to exploit the simultaneous high and low coherence properties of energy-time entangled photon pairs to directly calibrate the position scale of an optical low coherence reflectometry scan with a reference laser wavelength. In the experiment, a precision of 1.6 nm and good linearity is demonstrated.

Electrically driven source of time-energy entangled photons based on a self-pumped silicon microring resonator.

Time-energy entangled photon pairs are fundamental resources for quantum communication protocols since they are robust against environmental fluctuations in optical fiber networks. Pair sources based on spontaneous four-wave mixing in silicon microring resonators usually employ expensive external tunable lasers to compensate for ambient fluctuations; adopting self-pumped configurations, instead, lifts the need for such external source. Here we demonstrate the emission of time-energy entangled photon pairs at telecom wavelengths from a silicon self-pumped ring, obtaining a Franson interference fringe with 93.9%±0.9% visibility.

Nonlocal two-photon interference of energy-time entangled photon pairs generated in Doppler-broadened ladder-type Rb87 atoms

We experimentally demonstrate nonlocal two-photon interference of energy-time entangled photon pairs via spontaneous four-wave mixing (SFWM) process in Doppler-broadened ladder-type $^{87}\mathrm{Rb}$ atoms. The Doppler-broadened ladder-type atomic system offers collective two-photon coherence which results in strong temporal correlation between the entangled photons. The spacelike separated photon pairs clearly show nonlocal two-photon interference with high visibility of $97.1\ifmmode\pm\else\textpm\fi{}2.5%$, which violates the Bell-CHSH (Clauser-Horne-Shimony-Holt) inequality by 10.5 standard deviations, in which the travel time of a single-photon towards the interferometer is much longer than the coincidence window. Our work will provide an atom-based entangled photon source which is applicable for long distance atom-memory-based quantum communication.

High-visibility Franson interference of time-energy entangled photon pairs from warm atomic ensemble.

We experimentally demonstrate Franson interference of a time-energy entangled photon pair generated via collective two-photon coherence in the 5S1/2-5P3/2-5D5/2 transition of warm Rb87 atoms. The two unbalanced Michelson interferometers used in our setup are spatially separated in order to understand entanglement as a nonlocal property of the photon pairs from the warm atomic ensemble. We observe a Franson interference fringe with a high visibility of 99.1±1.3% with continuous-wave-mode photon pairs from the cascade-type atomic ensemble. To the best of our knowledge, this work is the first demonstration of the nonlocal two-photon interference experiment in separated photon channels by use of two-photon pairs emitted from a cascade-type atomic system as originally proposed by Franson [Phys. Rev. Lett.62, 2205 (1989)PRLTAO0031-900710.1103/PhysRevLett.62.2205].

Energy-time entanglement-based dispersive optics quantum key distribution over optical fibers of 20 km

An energy-time entanglement-based dispersive optics quantum key distribution (DO-QKD) is demonstrated experimentally over optical fibers of 20 km. In the experiment, the telecom band energy-time entangled photon pairs are generated through spontaneous four-wave mixing in a silicon waveguide. The arrival time of photons is registered for key generation and security test. High-dimensional encoding in the arrival time of photons is used to increase the information per coincidence of photon pairs. The bin sifting process is optimized by a three-level structure, which significantly reduces the raw quantum bit error rate (QBER) due to timing jitters of detectors and electronics. A raw key generation rate of 151 kbps with a QBER of 4.95% is achieved, in a time bin encoding format with 4 bits per coincidence. This experiment shows that the entanglement-based DO-QKD can be implemented in an efficient and convenient way, which has great potential for quantum secure communication networks in the future.

Polarization diversity phase modulator for measuring frequency-bin entanglement of a biphoton frequency comb in a depolarized channel.

Phase modulation has emerged as a technique to create and manipulate high-dimensional frequency-bin entanglement. A necessary step to extending this technique to depolarized channels, such as those in a quantum networking environment, is the ability to perform phase modulation independent of photon polarization. This is also necessary to harness hyperentanglement in the polarization and frequency degrees of freedom for operations such as Bell state discrimination. However, practical phase modulators are generally sensitive to the polarization of light, and this makes them unsuited to such applications. We overcome this limitation by implementing a polarization diversity scheme to measure frequency-bin entanglement for arbitrary orientations of co- and cross-polarized time-energy entangled photon pairs.

Time-Energy Entangled Photon Pairs from Doppler-Broadened Atomic Ensemble via Collective Two-Photon Coherence.

We experimentally demonstrate two-photon interference of a time-energy entangled photon pair generated via collective two-photon coherence in Doppler-broadened cascade-type ^{87}Rb atoms. The two photons originally proposed by J. D. Franson are realized as a photon pair due to collective effects, which are generated from the cascade atomic system with a relatively long lifetime of the initial state and a considerably shorter lifetime of the intermediate state. The achievement of two-photon interference with photon-pair sources generated from inhomogeneous atomic ensembles constitutes an important result for time-energy entanglement based on an atom-photon interaction.

Direct Characterization of Ultrafast Energy-Time Entangled Photon Pairs

Energy-time entangled photons are critical in many quantum optical phenomena and have emerged as important elements in quantum information protocols. Entanglement in this degree of freedom often manifests itself on ultrafast time scales, making it very difficult to detect, whether one employs direct or interferometric techniques, as photon-counting detectors have insufficient time resolution. Here, we implement ultrafast photon counters based on nonlinear interactions and strong femtosecond laser pulses to probe energy-time entanglement in this important regime. Using this technique and single-photon spectrometers, we characterize all the spectral and temporal correlations of two entangled photons with femtosecond resolution. This enables the witnessing of energy-time entanglement using uncertainty relations and the direct observation of nonlocal dispersion cancellation on ultrafast time scales. These techniques are essential to understand and control the energy-time degree of freedom of light for ultrafast quantum optics.

Quantum enhancement of accuracy and precision in optical interferometry

White-light interferometry is one of today’s most precise tools for determining the properties of optical materials. Its achievable precision and accuracy are typically limited by systematic errors due to a high number of interdependent data-fitting parameters. Here, we introduce spectrally resolved quantum white-light interferometry as a novel tool for optical property measurements, notably, chromatic dispersion in optical fibres. By exploiting both spectral and photon-number correlations of energy-time entangled photon pairs, the number of fitting parameters is significantly reduced, which eliminates systematic errors and leads to an absolute determination of the material parameter. By comparing the quantum method to state-of-the-art approaches, we demonstrate the quantum advantage of 2.4 times better measurement precision, despite requiring 62 times fewer photons. The improved results are due to conceptual advantages enabled by quantum optics, which are likely to define new standards in experimental methods for characterising optical materials.

Bipartite Bell inequalities with three ternary-outcome measurements—from theory to experiments

We explore quantum nonlocality in one of the simplest bipartite scenarios. Several new facet-defining Bell inequalities for the scenario are obtained with their quantum violations analyzed in details. Surprisingly, all these inequalities involving only genuine ternary-outcome measurements can be violated maximally by some two-qubit entangled states, such as the maximally entangled two-qubit state. This gives further evidence that in analyzing the quantum violation of Bell inequalities, or in the application of the latter to device-independent quantum information processing tasks, the commonly held wisdom of equating the local Hilbert space dimension of the optimal state with the number of measurement outcomes is not necessarily justifiable. In addition, when restricted to the minimal qubit subspace, it can be shown that one of these Bell inequalities requires non-projective measurements to attain maximal quantum violation, thereby giving the first example of a facet-defining Bell inequality where a genuine positive-operator-valued measure is relevant. We experimentally demonstrate the quantum violation of this and two other Bell inequalities for this scenario using energy–time entangled photon pairs. Using the obtained measurement statistics, we demonstrate how characterization of the underlying resource in the spirit of device-independence, but supplemented with auxiliary assumptions, can be achieved. In particular, we discuss how one may get around the fact that, due to finite-size effects, raw measurement statistics typically violate the non-signaling condition.