Widely tunable single-photon source with high spectral-purity from telecom wavelength to mid-infrared wavelength based on MgO:PPLN

By utilizing the extended phase-matching(EPM)method,we investigate the generation of single photons with high spectral-purity in a magnesium-doped periodically-poled lithium niobate(MgO:PPLN)crystal via the spontaneous parametric down-conversion(SPDC)process.By adjusting the temperature and pump wavelength,the wavelength of the single photons can be tuned from telecom to mid-infrared(MIR)wavelengths,for which the spectral-purity can be above 0.95 with high transmission filters.In experiments,we engineer a MgO:PPLN with poling period of 20.35µm which emits the EPM photon pair centered at 1496.6 nm and 1644.0 nm and carry out the joint spectral intensity(JSI)and Glauber’s second-order self-correlation measurements to characterize the spectral purity.The results are in good agreement with the numerical simulations.Our work may provide a valuable approach for the generation of spectrally pure single photons at a wide range of wavelengths which is competent for various photonic quantum technologies.

Degenerate polarization entangled photon source based on a single Ti-diffusion lithium niobate waveguide in a polarization Sagnac interferometer

Combining a Ti-diffusion periodically poled lithium niobate(PPLN)waveguide with a Sagnac interferometer,two opposite directions type-II spontaneous parametric down conversions(SPDC)occur coherently and yield a high brightness,high stability polarization entanglement source.The source produces degenerate photon pairs at 1540.4 nm with a brightness of B=(1.36±0.03)×10^(6) pairs/(s·nm·m W).We perform quantum state tomography to reconstruct the density matrix of the output state and obtain a fidelity of F=0.983±0.001.The high brightness and phase stability of our waveguide source enable a wide range of quantum information experiments operating at a low pump power as well as hold the advantage in mass production which can promote the practical applications of quantum technologies.

Generation of broadband polarization-orthogonal photon pairs via the dispersion-engineered thin-film lithium niobate waveguide

Broadband photon pairs are highly desirable for quantum metrology,quantum sensing,and quantum communication.Such sources are usually designed through type-0 phase-matching spontaneous parametric down-conversion(SPDC)that makes the photon pairs hard to separate in the frequency-degenerate case and thus limits their applications.In this paper,we design a broadband frequency-degenerate telecom-band photon pair source via the type-II SPDC in a dispersion-engineered thin-film lithium niobate waveguide,where the polarization modes of photon pairs are orthogonal and thus are easily separated deterministically.With a 5-mm-long waveguide,our design can achieve a bandwidth of 5.56 THz(44.8 nm),which is 8.6 times larger than that of the bulk lithium niobate,and the central wavelength can be flexibly adjusted.Our design is a promising approach towards high-quality integrated photon sources and may have wide applications in photonic quantum technologies.

Effect of dimension variation for second-harmonic generation in lithium niobate on insulator waveguide[Invited]

We study the effect of dimension variation for second-harmonic generation(SHG) in lithium niobate on insulator(LNOI)waveguides. Non-trivial SHG profiles in both type-0 and type-I quasi-phase matching are observed during the wavelength tuning of the fundamental light. Theoretical modeling shows that the SHG profile and efficiency can be greatly affected by the waveguide cross-section dimension variations, especially the thickness variations. In particular, our analysis shows that a thickness variation of tens of nanometers is in good agreement with the experimental results. Such investigations could be used to evaluate fabrication performance of LNOI-based nonlinear optical devices.

Narrowband photonic quantum entanglement with counterpropagating domain engineering

Narrowband photonic entanglement is a crucial resource for long-distance quantum communication and quantum information processing,including quantum memories.We demonstrate the first polarization entanglement with 7.1 GHz inherent bandwidth by counterpropagating domain engineering,which is also confirmed by Hong–Ou–Mandel interference with 155-ps base-to-base dip width and(97.1±0.59)%high visibility.The entanglement is harnessed with 18.5-standard-deviations Bell inequality violation,and further characterized with state tomography of(95.71±0.61)%fidelity.Such narrowband entanglement sets a cornerstone for practical quantum information applications.

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.