Early Warning Signals of the Tipping Point in Strongly Interacting Rydberg Atoms.

The identification of tipping points is essential for the prediction of collapses or other sudden changes in complex systems. Applications include studies of ecology, thermodynamics, climatology, and epidemiology. However, detecting early signs of proximity to a tipping is made challenging by complexity and nonlinearity.

Higher-order and fractional discrete time crystals in Floquet-driven Rydberg atoms.

Higher-order and fractional discrete time crystals (DTCs) are exotic phases of matter where the discrete time translation symmetry is broken into higher-order and non-integer category. Generation of these unique DTCs has been widely studied theoretically in different systems. However, no current experimental methods can probe these higher-order and fractional DTCs in any quantum many-body systems.

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.

Quantum entanglement and interference at 3 μm.

Given the important advantages of the mid-infrared optical range (2.5 to 25 μm) for biomedical sensing, optical communications, and molecular spectroscopy, extending quantum information technology to this region is highly attractive. However, the development of mid-infrared quantum information technology is still in its infancy.

Ergodicity breaking from Rydberg clusters in a driven-dissipative many-body system.

It is challenging to probe ergodicity breaking trends of a quantum many-body system when dissipation inevitably damages quantum coherence originated from coherent coupling and dispersive two-body interactions. Rydberg atoms provide a test bed to detect emergent exotic many-body phases and nonergodic dynamics where the strong Rydberg atom interaction competes with and overtakes dissipative effects even at room temperature. Here, we report experimental evidence of a transition from ergodic toward ergodic breaking dynamics in driven-dissipative Rydberg atomic gases.

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.

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.

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).

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 Rb 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.

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.

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.

Harmonics-assisted optical phase amplifier.

The change in the relative phase between two light fields serves as a basic principle for the measurement of the physical quantity that guides this change. It would therefore be highly advantageous if the relative phase could be amplified to enhance the measurement resolution. One well-known method for phase amplification involves the use of the multi-photon number and path-entangled state known as the NOON state; however, a high-number NOON state is very difficult to prepare and is highly sensitive to optical losses.

Deep learning enhanced Rydberg multifrequency microwave recognition.

Recognition of multifrequency microwave (MW) electric fields is challenging because of the complex interference of multifrequency fields in practical applications. Rydberg atom-based measurements for multifrequency MW electric fields is promising in MW radar and MW communications. However, Rydberg atoms are sensitive not only to the MW signal but also to noise from atomic collisions and the environment, meaning that solution of the governing Lindblad master equation of light-atom interactions is complicated by the inclusion of noise and high-order terms.

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.

Angular-spectrum-dependent interference.

Optical interference is not only a fundamental phenomenon that has enabled new theories of light to be derived but it has also been used in interferometry for the measurement of small displacements, refractive index changes, and surface irregularities. In a two-beam interferometer, variations in the interference fringes are used as a diagnostic for anything that causes the optical path difference (OPD) to change; therefore, for a specified OPD, greater variation in the fringes indicates better measurement sensitivity. Here, we introduce and experimentally validate an interesting optical interference phenomenon that uses photons with a structured frequency-angular spectrum, which are generated from a spontaneous parametric down-conversion process in a nonlinear crystal.

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.

All-optical reversible single-photon isolation at room temperature.

Nonreciprocal devices operating at the single-photon level are fundamental elements for quantum technologies. Because magneto-optical nonreciprocal devices are incompatible for magnetic-sensitive or on-chip quantum information processing, all-optical nonreciprocal isolation is highly desired, but its realization at the quantum level is yet to be accomplished at room temperature. Here, we propose and experimentally demonstrate two regimes, using electromagnetically induced transparency (EIT) or a Raman transition, for all-optical isolation with warm atoms.

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.

Metasurface enabled quantum edge detection.

Metasurfaces consisting of engineered dielectric or metallic structures provide unique solutions to realize exotic phenomena including negative refraction, achromatic focusing, electromagnetic cloaking, and so on. The intersection of metasurface and quantum optics may lead to new opportunities but is much less explored. Here, we propose and experimentally demonstrate that a polarization-entangled photon source can be used to switch ON or OFF the optical edge detection mode in an imaging system based on a high-efficiency dielectric metasurface.

Experimental demonstration of Einstein-Podolsky-Rosen entanglement in rotating coordinate space.

Einstein-Podolsky-Rosen (EPR) entanglement involving a pair of particles entangled in their positions and momenta is of special interest in the field of quantum information. Previously, EPR entanglement has been studied in different physical systems but in fixed coordinate spaces. Here, we demonstrate an experiment of ghost imaging and ghost interference in rotated position-momentum spaces by using position-momentum entangled photons generated from a hot atomic ensemble.

Classical simulation of high-dimensional entanglement by non-separable angular-radial modes.

An analogous model system for high-dimensional quantum entanglement is proposed, based on the angular and radial degrees of freedom of the improved Laguerre Gaussian mode. Experimentally, we observed strong violations of the Bell-CGLMP inequality for maximally non-separable states of dimension 2 through 10. The results for violations in classical non-separable state are in very good agreement with quantum instance, which illustrates that our scheme can be a useful platform to simulate high-dimensional non-local entanglement.

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.