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Physical Review A

Mode-locked biphoton generation by concurrent quasi-phase-matching

Y. F. Bai, Ping Xu, Zhenda Xie, Yan-Xiao Gong, Shining N. Zhu

Published May 7, 2012• 7 citations
Publisher

Abstract

We report a scheme for mode-locked biphoton generation by engineering quasi-phase-matching materials. An aperiodically poled lithium tantalate is designed to supply multiple reciprocal vectors for two-photon frequency comb generation. Temporally, such two-photon state exhibits mode-locked pulse train. The two-photon pulse duration reaches 4.2 fs when the frequency comb is centered at 800 nm and covers 442 nm. The HOM interference of the mode-locked biphoton approaches a 1.26-$\ensuremath{\mu}$m dip and exhibits a reappearance of dip and peak, which can be taken as a new scheme for engineering quantum light source. Furthermore, we also design another aperiodically poled crystal to generate two sets of frequency combs. The HOM interference will appear as a revival of spatial beatings. The mode-locked biphotons suggest applications in precise time synchronization, quantum metrology, quantum optical coherence tomography, and quantum computing.

DOI
10.1103/physreva.85.053807

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We report a scheme for mode-locked biphoton generation by engineering quasi-phase-matching materials. An aperiodically poled lithium tantalate is designed to supply multiple reciprocal vectors for two-photon frequency comb generation. Temporally, such two-photon state exhibits mode-locked pulse train. The two-photon pulse duration reaches 4.2 fs when the frequency comb is centered at 800 nm and covers 442 nm. The HOM interference of the mode-locked biphoton approaches a 1.26-\ensuremathμ\ensuremath{\mu}\ensuremathμm dip and exhibits a reappearance of dip and peak, which can be taken as a new scheme for engineering quantum light source. Furthermore, we also design another aperiodically poled crystal to generate two sets of frequency combs. The HOM interference will appear as a revival of spatial beatings. The mode-locked biphotons suggest applications in precise time synchronization, quantum metrology, quantum optical coherence tomography, and quantum computing.
Entanglement is the hallmark of quantum mechanics. Quantum entanglement -- putting two or more identical particles into a non-factorable state -- has been leveraged for applications ranging from quantum computation and encryption to high-precision metrology. Entanglement is a practical engineering resource and a tool for sidestepping certain limitations of classical measurement and communication. Engineered nonlinear optical waveguides are an enabling technology for generating entangled photon pairs and manipulating the state of single photons. This dissertation reports on: i) frequency conversion of single photons from the mid-infrared to 843nm as a tool for incorporating quantum memories in quantum networks, ii) the design, fabrication, and test of a prototype broadband source of polarization and frequency entangled photons; and iii) a roadmap for further investigations of this source, including applications in quantum interferometry and high-precision optical metrology. \n \nThe devices presented herein are quasi-phase-matched lithium niobate waveguides. Lithium niobate is a second-order nonlinear optical material and can mediate optical energy conversion to different wavelengths. This nonlinear effect is the basis of both quantum frequency conversion and entangled photon generation, and is enhanced by i) confining light in waveguides to increase conversion efficiency, and ii) quasi-phase matching, a technique for engineering the second-order nonlinear response by locally altering the direction of a material's polarization vector. Waveguides are formed by diffusing titanium into a lithium niobate wafer. Quasi-phase matching is achieved by electric field poling, with multiple stages of process development and optimization to fabricate the delicate structures necessary for broadband entangled photon generation. \n \nThe results presented herein update and optimize past fabrication techniques, demonstrate novel optical devices, and propose future avenues for device development. Quantum frequency conversion from 1848nm to 843nm is demonstrated for the first time, with >75% single-photon conversion efficiency. A new electric field poling methodology is presented, combining elements from multiple historical techniques with a new fast-feedback control system. This poling technique is used to fabricate the first chirped-and-apodized Type-II quasi-phase-matched structures in titanium-diffused lithium niobate waveguides, culminating in a measured phasematching spectrum that is predominantly Gaussian (R^2 = 0.80), nearly eight times broader than the unchirped spectrum, and agrees well with simulations.
Ultra-broadband frequency entangled biphotons have exceptionally short temporal duration and can achieve narrow Hong–Ou–Mandel (HOM) interference patterns, making them vital for quantum metrology applications. However, the bandwidth of previously demonstrated biphotons used for HOM interference has been limited to less than 400 nm, which is not wide enough to achieve ultra-high precision in quantum metrology. In order to push the precision to its limit, we have theoretically designed a 20-mm-long chirped periodically poled lithium niobate (CPPLN) crystal with type-II phase matching. Pumped by a 405 nm laser, the down-converted signal and idler photons can cover the wavelength range from 420 to 5200 nm, approaching the transparent upper bound of the LN crystal. In the simulated Hong–Ou–Mandel (HOM) interference, the full-width at half-maximum of the interference pattern is 965 attoseconds, with a visibility of 99.44%. Following a similar approach, we also designed a type-0 CPPLN crystal to achieve higher brightness. These ultra-broadband biphoton sources can generate ultra-narrow quantum interference patterns, which have the potential for attosecond-level high-precision quantum metrology.
Summary form only given. Here, we demonstrate the first scheme to generate pulsed quantum frequency combs without the need for an external laser. We constructed a laser cavity (Fig. 1a) around a 4-port integrated micro-ring resonator, featuring narrow bandpass filters to select a single resonance, an optical gain component (erbium-doped fiber amplifier), and a mode-locking element (an intensity modulator). By setting the intensity modulation frequency to the fundamental repetition rate of the cavity (determined by the external cavity length), we achieved mode-locked pulsed operation (Fig. 1b), exciting a pulsed two-photon comb. Driving the intensity modulator at multiples of the fundamental repetition rate (upwards of 4x) also resulted in stable mode-locking (Fig. 1c), significant as this enables an increase of the photon pair generation rate without sacrificing its coincidence-to-accidental ratio (CAR). We characterized the photon statistics as a function of the average ring input power (see Fig. 1d), achieving a maximum CAR of 110 that decreases with increasing power and increasing coincidence rates. Autocorrelation measurements confirmed the purity of the generated state (pure single-frequency modes, even at multiples of the repetition rate), confirming that the resonance was excited over its entire bandwidth, and obtained a heralded anti-bunching dip of 0.245<;0.5 as expected of a source operating in the single-photon regime.
We report a simple scheme for direct generation of frequency-bin entangled photon pairs via spontaneous parametric downconversion. Our fabricated nonlinear optical crystal with two different poling periods can simultaneously satisfy two different, spectrally symmetric nondegenerate quasi-phase-matching conditions, enabling the direct generation of entanglement in two discrete frequency-bin modes. Our produced photon pairs exhibited Hong-Ou-Mandel interference with high-visibility beating oscillations- a signature of two-mode frequency-bin entanglement. Moreover, we demonstrate deterministic entanglement-mode conversion from frequency-bin to polarization modes, with which our source can be more versatile for various quantum applications. Our scheme can be extended to direct generation of high-dimensional frequency-bin entanglement, and thus will be a key technology for frequency-multiplexed optical quantum information processing.
Counter-propagating parametric conversion processes in non-linear bulk crystals have been shown to feature unique properties for efficient narrowband frequency conversion. In quantum optics, the generation of photon pairs with a counter-propagating parametric down-conversion process (PDC) in a waveguide, where signal and idler photons propagate in opposite directions, offers unique material-independent engineering capabilities. However, realizing counter-propagating PDC necessitates quasi-phase-matching (QPM) with extremely short poling periods. Here, we report on the generation of counter-propagating single-photon pairs in a self-made periodically poled lithium niobate waveguide with a poling period on the same order of magnitude as the generated wavelength. The single photons of the biphoton state bridge GHz and THz bandwidths with a separable joint temporal-spectral behavior. Furthermore, they allow the direct observation of the temporal envelope of heralded single photons with state-of-the art photon counters.