Characterizing mid-infrared micro-ring resonator with frequency conversion.

Due to the high cost, low-performance lasers and detectors in the mid-infrared (MIR) band, the development of MIR-integrated devices is very slow. Here, we demonstrate an effective method to characterize the parameters of MIR devices by using frequency conversion technology. We designed and fabricated rib waveguides and the micro-ring resonators (MRRs) on a silicon-on-sapphire platform. The MIR laser for the test is generated by difference frequency generation, and the transmission spectrum of the MIR-MRRs is detected by sum frequency generation. The experimental results show that the waveguide transmission loss is 4.5 dB/cm and the quality factor of the micro-ring reaches 38000, which is in good agreement with the numerical simulations. This work provides a useful method to characterize MIR integrated devices based on the frequency conversion technique, which can boost the development of MIR integrated optics in the future.

Telecom-wavelength conversion in a high optical depth cold atomic system.

We experimentally investigate the frequency down-conversion through the four-wave mixing (FWM) process in a cold 85Rb atomic ensemble, with a diamond-level configuration. An atomic cloud with a high optical depth (OD) of 190 is prepared to achieve a high efficiency frequency conversion. Here, we convert a signal pulse field (795 nm) attenuated to a single-photon level, into a telecom light at 1529.3 nm within near C-band range and the frequency-conversion efficiency can reach up to 32%. We find that the OD is an essential factor affecting conversion efficiency and the efficiency may exceed 32% with an improvement in the OD. Moreover, we note the signal-to-noise ratio of the detected telecom field is higher than 10 while the mean signal count is larger than 0.2. Our work may be combined with quantum memories based on cold 85Rb ensemble at 795 nm and serve for long-distance quantum networks.

Observing multiple processes of backward-phase-matching spontaneous parametric downconversion in a single-periodic crystal.

A nonlinear process based on backward quasi-phase matching (BQPM) can be used to realize mirrorless optical parametric oscillation, the generation of paired photons with a separable joint spectral amplitude and narrow wavelength bandwidth, and the preparation of counterpropagating polarization-entangled photons, which shows distinct advantages over some applications based on forward quasi-phase matching. In this work, three types of BQPM in a bulk periodically poled potassium titanyl phosphate crystal with a single period are theoretically analyzed. Experimentally, the harmonic wave generated by second-harmonic generation in type 0 and type I exhibits a narrow bandwidth of 15.5 GHz. Furthermore, photon pairs generated by spontaneous parametric downconversion in all types of BQPM (type 0, type I, and type II) at 7th order are observed and characterized. Their coincidence-to-accidental ratios are all greater than 5 × 103 in the pump power range from 10 mW to 500 mW. This research lays the foundation for further applications of BQPM in nonlinear optics, quantum optics, and quantum information processing.

Optical memory for arbitrary perfect Poincaré states in an atomic ensemble.

Inherent spin angular momentum (SAM) and orbital angular momentum (OAM), which manifest as polarization and spatial degrees of freedom (DOFs) of photons, hold a promise of large capability for applications in classical and quantum information processing. To enable these photonic spin and orbital dynamic properties strongly coupled with each other, Poincaré states have been proposed and offer advantages in data multiplexing, information encryption, precision metrology, and quantum memory. However, since the transverse size of Laguerre-Gaussian beams strongly depends on their topological charge numbers | l |, it is difficult to store asymmetric Poincaré states due to the significantly different light-matter interaction for distinct spatial modes. Here, we experimentally realize the storage of perfect Poincaré states with arbitrary OAM quanta using the perfect optical vortex, in which 121 arbitrarily selected perfect Poincaré states have been stored with high fidelity. The reported work has great prospects in optical communication and quantum networks for dramatically increased encoding flexibility of information.

Observation of Anomalous Orbital Angular Momentum Transfer in Parametric Nonlinearity.

Orbital angular momentum (OAM) conservation plays an important role in shaping and controlling structured light with nonlinear optics. The OAM of a beam originating from three-wave mixing should be the sum or difference of the other two inputs because no light-matter OAM exchange occurs in parametric nonlinear interactions. Here, we report anomalous OAM transfer in parametric upconversion, in which a Hermite-Gauss mode signal interacts with a specially engineered pump capable of astigmatic transformation, resulting in Laguerre-Gaussian mode sum-frequency generation (SFG). The anomaly here refers to the fact that the pump and signal both carry no net OAM, while their SFG does. We reveal experimentally that there is also an OAM inflow to the residual pump, having the same amount of that to the SFG but with the opposite sign, and thus holds system OAM conservation. This unexpected OAM selection rule improves our understanding of OAM transfer among interacting waves and may inspire new ideas for controlling OAM states via nonlinear optics.

Highly Efficient Storage of 25-Dimensional Photonic Qudit in a Cold-Atom-Based Quantum Memory.

Building an efficient quantum memory in high-dimensional Hilbert spaces is one of the fundamental requirements for establishing high-dimensional quantum repeaters, where it offers many advantages over two-dimensional quantum systems, such as a larger information capacity and enhanced noise resilience. To date, it remains a challenge to develop an efficient high-dimensional quantum memory. Here, we experimentally realize a quantum memory that is operational in Hilbert spaces of up to 25 dimensions with a storage efficiency of close to 60% and a fidelity of 84.2±0.6%. The proposed approach exploits the spatial-mode-independent interaction between atoms and photons which are encoded in transverse-size-invariant vortex modes. In particular, our memory features uniform storage efficiency and low crosstalk disturbance for 25 individual spatial modes of photons, thus allowing the storing of qudit states programmed from 25 eigenstates within the high-dimensional Hilbert spaces. These results have great prospects for the implementation of long-distance high-dimensional quantum networks and quantum information processing.

Measuring the tuning curve of spontaneous parameter downconversion using a comet-tail-like pattern.

Comet-tail-like interference patterns are observed using photons from the spontaneous parametric downconversion (SPDC) process. The patterns are caused by the angular-spectrum-dependent interference and the diffraction of a blazed grating. We present a theoretical explanation and simulation results for these patterns, which are in good agreement with the experimental results. The most significant feature of the patterns is the bright parabolic contour profile, from which one can deduce the parameter of the parabolic tuning curve of the SPDC process. This method could be helpful when designing experiments based on SPDC.

Long-Lived Memory for Orbital Angular Momentum Quantum States.

Quantum memories that are capable of storing multiple spatial modes offer advantages in speed and robustness when incorporated into quantum networks. When it comes to spatial degrees of freedom, orbital angular momentum (OAM) modes have received widespread attention since they enable encoding with inherent infinite number of dimensions. Although the faithful storage of OAM qubits or qutrits has been realized in previous works, the achieved lifetimes are still on the order of a few microseconds as limited by the spatially dependent decoherence. We here demonstrate a long-lived quantum memory for OAM qutrits by suppressing the decoherence in the transverse and longitude direction simultaneously; the achieved fidelity beats the quantum-classical criteria after a storage time of 400 µs, which is 2 orders of magnitude longer than earlier works. The present work is promising for establishing high-dimensional quantum networks.

Interference fringes in a nonlinear Michelson interferometer based on spontaneous parametric down-conversion.

Quantum nonlinear interferometers (QNIs) can measure the infrared physical quantities of a sample by detecting visible photons. A QNI with Michelson geometry based on the spontaneous parametric down-conversion in a second-order nonlinear crystal is studied systematically. A simplified theoretical model of the QNI is presented. The interference visibility, coherence length, equal-inclination interference, and equal-thickness interference for the QNI are demonstrated theoretically and experimentally. As an application example of the QNI, the refractive index and the angle between two surfaces of a BBO crystal are measured using equal-inclination interference and equal-thickness interference.

Fourth-harmonic generation of orbital angular momentum light with cascaded quasi-phase matching crystals.

Orbital angular momentum (OAM) light, combined with the nonlinear process to expand the frequency range, has drawn increasing research interest in recent years. Here, we implement the first, to the best of our knowledge, experimental fourth-harmonic generation of OAM light with two cascaded quasi-phase-matching crystals. A Laguerre-Gaussian beam was transmitted through a duplet crystals system and frequency-doubled twice by two separate second-harmonic generation processes, which transduced the frequency of the OAM beam from telecom band to visible band and then to ultraviolet (UV) band. The topological charge of the OAM beam was increased substantially in the cascaded frequency conversion processes. In this experiment, we verify the OAM conservation by utilizing a specially designed interferometer, and the results correspond well with the numerical simulation. This work provides an effective method for the generation of UV OAM beams with high topological charges.

Real-time quantum edge enhanced imaging.

With the development of optical information processing technology, image edge enhancement technology has rapidly received extensive attention, especially in the field of quantum imaging. However, quantum edge enhanced imaging faces challenges in terms of time-consuming acquisition processes and the complexity of the devices used, which limits practical applications in real-time usage scenarios. Here we introduce and experimentally demonstrate a real-time (0.5 Hz) quantum edge enhanced imaging method that combines the spiral phase contrast technique with heralded single-photon imaging. The edge enhancement results show high quality and background free from raw data. Compared with direct imaging, our configuration can improve the signal-to-noise ratio significantly using the tight time correlations between photon pairs. The method also offers competitive advantages over ghost imaging, including higher brightness and a compact optical fiber delay rather than a free space delay. Additionally, we explore curved edge enhancement for specific feature recognition and the oriented shadow effect. Overall, this efficient and versatile platform paves an alternative path toward real-time quantum edge detection in applications including nondestructive bio-imaging, night vision and covert monitoring.

Entangled qutrits generated in four-wave mixing without post-selection.

High-dimensional entangled states and quantum repeaters are important elements in efficient long-range quantum communications. The high-dimensional property associated with the orbital angular momentum (OAM) of each photon improves the bandwidth of the quantum communication network. However, the generation of high-dimensional entangled states by the concentration method reduces the brightness of the entangled light source, making extensions to these higher dimensions difficult. To overcome this difficulty, we propose to generate entangled qutrits in the OAM space by loading the pump light with OAM. Compared with the concentration method, our experimental results show that the rate of generation of photon pairs improves significantly with an observed 5.5-fold increase. The increased generation rate provides the system with the ability to resist the noise and improve the fidelity of the state. The S value of the Clauser-Horne-Shimony-Holt inequality increases from 2.48 ± 0.07 to 2.69 ± 0.04 under the same background noise, and the fidelity of the reconstructed density matrix improves from 57.8 ± 0.14% to 70 ± 0.17%. These achievements exhibit the enormous advantages of high-dimensional entanglement generation.

High-quality versatile photonic sources for multiple quantum optical experiments.

Entangled sources are important components for quantum information science and technology (QIST). The ability to generate high-quality entangled sources will determine the extent of progress in this field. Unlike previous schemes, a thin quasi-phase matching nonlinear crystal and a dense-wave-division-multiplexing device are used here to build high-quality versatile photonic sources with a simple configuration that can be used to perform Hong-Ou-Mandel interference, time-energy entanglement and multi-channel polarization entanglement experiments. The measurement results from various quantum optical experiments show the high quality of these photonic sources. These multi-functional photonic sources will be very useful in a variety of QIST applications.

Experimental realization of optical storage of vector beams of light in warm atomic vapor.

Vector beams have drawn considerable interest recently because of their unique properties in the transverse plane. Here we experimentally realize optical storage of a vector beam of light in a warm cell. The vector beam is tailored using a Sagnac interferometer containing an internal vortex phase plate, and the light pulse is stored in warm rubidium vapor. The preservation of both the spatial structure and the phase information is verified after retrieval. The implementation of vector beam storage in a room-temperature memory has potential for use in the fabrication of versatile vortex-based quantum networks.

Nonlinear frequency conversion and manipulation of vector beams in a Sagnac loop.

Vector beams (VBs) are widely investigated for their special intensities and polarization distributions, which are useful in optical micromanipulation, optical microfabrication, optical communication, and single molecule imaging. To date, nonlinear frequency conversion (NFC) and manipulation of VBs remain challenging because of the polarization sensitivity of most nonlinear processes. Here we report an experimental realization of NFC and manipulation of VBs that can be used to expand the available frequency band. The main idea of our scheme is the introduction of a Sagnac loop to solve the polarization dependence problem of NFC in nonlinear crystals. Additionally, we find that a linearly polarized VB should be transformed into a hybrid-polarized VB in exponential form before performing NFC. The experimental results agree well with those of our theoretical model. The proposed method is also applicable to other wavebands and second-order nonlinear processes, and may be generalized to the quantum regime for single photons.

On-chip generation of time-and wavelength-division multiplexed multiple time-bin entanglement.

Optical quantum states based on entangled photons are the key resource in quantum-information science. The realization of multiplexed multiple entanglement are necessary for developing high-capacity quantum information process. Silicon-on-insulator (SOI) has recently become a leading platform for generating and processing of non-classical optical states. In this work, by combining the wavelength- and time-division multiplexing technologies, we demonstrate a multiplexing time-bin entangled photon pair source based on a silicon nanowire waveguide and distribute entangled photons into 3(time) × 14(wavelength) channels independently. The indistinguishability of photon pairs in each time channel is confirmed by a fourfold Hong-Ou-Mandal quantum interference. Our work paves a new and promising way to achieve a high capacity quantum communication and to generate a multiple-photon non-classical state.

Quantum frequency conversion for multiplexed entangled states generated from micro-ring silicon chip.

Silicon-on-chip photonic circuits are among some very promising platforms for generating nonclassical photonic quantum state, because of its low loss, small footprint, and compatibility with complementary metal-oxide-semiconductor (CMOS) and telecommunications techniques. Dense wavelength division multiplexing (DWDM) is a leading technique for enhancing the transmission capacity of both classical and quantum communications. To bridge the frequency gap between silicon-chip and other quantum systems, such as quantum memories, a quantum interface is indispensable. Here, we demonstrate a quantum interface for multiplexed energy-time entanglement states, which are generated on a silicon micro-ring cavity that is based on frequency up-conversion. By switching the pump wavelength, energy-time entanglement from any channel can be selected at will after being up-converted. The high visibilities of two-photon interference over three channels after frequency up-conversion clearly prove that the entanglement is fully preserved during the quantum frequency conversion (QFC) process. Our work provides new perspectives regarding channel capacity enhancement in quantum communications and for quantum resources being transferred between two different quantum systems.

Revealing the Behavior of Photons in a Birefringent Interferometer.

The interferometer is one of the most important devices for revealing the nature of light and for precision optical metrology. Although many experiments were performed for probing photon behavior in various configurations, a complete study of photon behavior in a birefringent interferometer has not been performed, to our knowledge. By using an environmental turbulence immune Mach-Zehnder interferometer, we observe tunable photonic beatings by rotating a birefringent crystal versus the temperature of the crystal for both the single photon and two photons. Furthermore, the two-photon interference fringes beat 2 times faster than the single-photon interference fringes. This beating effect is used to determine the thermal dispersion coefficients of the two principal refractive axes with a single measurement: the two-photon interference shows superresolution and high sensitivity. Obvious differences between two-photon and single-photon interference are also revealed in unbalanced situations. In addition, the influence of the photon bandwidth on the beating behaviors that come from polarization-dependent decoherence is also investigated. Our findings will be important for better understanding the behavior of two-photon interference in a birefringent interferometer and for precision optical metrology with quantum enhancement.

Coherent frequency bridge between visible and telecommunications band for vortex light.

In quantum communications, vortex photons can encode higher-dimensional quantum states and build high-dimensional communication networks (HDCNs). The interfaces that connect different wavelengths are significant in HDCNs. We construct a coherent orbital angular momentum (OAM) frequency bridge via difference frequency conversion in a nonlinear bulk crystal for HDCNs. Using a single resonant cavity, maximum quantum conversion efficiencies from visible to infrared are 36%, 15%, and 7.8% for topological charges of 0,1, and 2, respectively. The average fidelity obtained using quantum state tomography for the down-converted infrared OAM-state of topological charge 1 is 96.51%. We also prove that the OAM is conserved in this process by measuring visible and infrared interference patterns. This coherent OAM frequency-down conversion bridge represents a basis for an interface between two high-dimensional quantum systems operating with different spectra.

Two-color hyper-entangled photon pairs generation in a cold 85Rb atomic ensemble.

Hyper-entangled photon pairs are very promising in the quantum information field for enhancing the channel capacity in communication and improving compatibility for networks. Here we report on the experimental generation of a hyper-entangled photon pair at a wavelength of 795 nm and 1475 nm via the spontaneous four-wave mixing process in a cold 85Rb atomic ensemble. The photons in each pair generated are entangled in both the time-frequency and polarization degrees of freedom. Such hyper-entangled photon pairs with special wavelength have potential applications in high-dimensional quantum communication and quantum physics.