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APL Quantum

Time-reversible and fully time-resolved ultra-narrowband biphoton frequency combs

Kai-Chi Chang, Xiang Cheng, Murat Can Sarihan, Chee Wei Wong

Published Feb 23, 2024• 14 citations
Publisher

Abstract

Time-reversibility, which is inherent in many physical systems, is crucial in tailoring temporal waveforms for optimum light–matter interactions. Among the time-reversible atomic systems, narrowband biphoton sources are essential for efficient quantum storage. In this work, we demonstrate time-reversed and fully time-resolved ultra-narrowband single-sided biphoton frequency combs with an average free-spectral range (FSR) of 42.66 MHz and an average linewidth of 4.60 MHz in the telecommunication band. We experimentally observe the fully time-resolved and reversible temporal oscillations by second-order cross correlation and joint temporal intensity measurements. The potential benefits of the time-reversed and fully time-resolved temporal oscillations from our source include enhancing the efficiency of quantum storage in atomic memories and maximizing the utilization of temporal information in multimode biphoton frequency combs. We further verify the heralded single-photon state generation from the multimode biphoton frequency combs by using Hanbury Brown and Twiss interference measurements. To the best of our knowledge, this 42.66 MHz FSR of our photon-pair source represents the narrowest among all of the different configurated biphoton sources reported to date. This ultra-narrow FSR and its 4.60 MHz linewidth provide the highest frequency mode number of 5786 and the longest coherence time among all the singly configurated biphoton sources so far. Our time-reversed and fully time-resolved massive-mode biphoton source could be useful for high-dimensional quantum information processing and efficient time–frequency multiplexed quantum storage toward long-distance and large-scale quantum networks.

DOI
10.1063/5.0180543

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Time-reversibility, which is inherent in many physical systems, is crucial in tailoring temporal waveforms for optimum light–matter interactions. Among the time-reversible atomic systems, narrowband biphoton sources are essential for efficient quantum storage. In this work, we demonstrate time-reversed and fully time-resolved ultra-narrowband single-sided biphoton frequency combs with an average free-spectral range (FSR) of 42.66 MHz and an average linewidth of 4.60 MHz in the telecommunication band. We experimentally observe the fully time-resolved and reversible temporal oscillations by second-order cross correlation and joint temporal intensity measurements. The potential benefits of the time-reversed and fully time-resolved temporal oscillations from our source include enhancing the efficiency of quantum storage in atomic memories and maximizing the utilization of temporal information in multimode biphoton frequency combs. We further verify the heralded single-photon state generation from the multimode biphoton frequency combs by using Hanbury Brown and Twiss interference measurements. To the best of our knowledge, this 42.66 MHz FSR of our photon-pair source represents the narrowest among all of the different configurated biphoton sources reported to date. This ultra-narrow FSR and its 4.60 MHz linewidth provide the highest frequency mode number of 5786 and the longest coherence time among all the singly configurated biphoton sources so far. Our time-reversed and fully time-resolved massive-mode biphoton source could be useful for high-dimensional quantum information processing and efficient time–frequency multiplexed quantum storage toward long-distance and large-scale quantum networks.
Abstract Utilizing the double-Λ spontaneous four-wave mixing (SFWM) process, the biphoton source generates narrow-linewidth pairs of signal and probe photons. In a medium, the signal photon propagates at nearly the speed of light in a vacuum, while the probe photon propagates as slow light. Typically, signal photons arrive at the detector first and are used as the heralding photons in conventional biphoton sources. In this work, we propose using probe photons as the heralding photons to enhance the heralding probability, an approach that has been overlooked previously. We also investigate a time-reversed double-Λ SFWM biphoton source using heated atomic vapor. Compared with the conventional biphoton source under the same experimental conditions, the time-reversed one exhibits a time-reversed temporal profile with a similar full-width-at-half-maximum linewidth of 3.4 MHz, increased the heralding efficiency by a factor of 5.3, and enhanced the detection rate by 1.3 times. With the time-reversed source, we achieved a heralding probability of 82±6% and a generation rate of (1.8 ± 0.2)× 10 6 pairs/s, referring to biphotons collected within polarization-maintained single-mode optical fibers. Furthermore, the time-reversed temporal profile is more suitable for quantum memory. Simulation results show that, at an optical depth of 150 (or 50), the storage efficiency of a quantum memory using the time-reversed source can reach 91% (or 81%), compared with 81% (or 67%) using the conventional source. This study demonstrates the significance of using the slow-light photon in biphoton pairs as the heralding photon for quantum operations. We have achieved a biphoton source with high heralding probability, high generation rate, and narrow linewidth in a room-temperature or hot medium.
We report the generation of biphotons, with a temporal full width at the half maximum (FWHM) of 13.4 ± 0.3 µs and a spectral FWHM of 50 ± 1 kHz, via the process of spontaneous four-wave mixing with laser-cooled atoms. The temporal width is the longest, and the spectral linewidth is the narrowest to date. This is also the first biphoton result that obtains a linewidth below 100 kHz, reaching a new milestone. The very long biphoton wave packet has a signal-to-background ratio of 3.4, which violates the Cauchy–Schwarz inequality for classical light by 4.8 folds. Furthermore, we demonstrated a highly tunable-linewidth biphoton source and showed that while the biphoton source’s temporal and spectral width were controllably varied by about 24 folds, its generation rate only changed by less than 15%. A spectral brightness or generation rate per pump power per linewidth of 1.2× 106 pairs/(s mW MHz) was achieved at the temporal width of 13.4 µs. The above results were made possible by the low decoherence rate and high optical depth of the experimental system, as well as a novel scheme of classical fields’ and biphotons’ propagation directions in the experiment. This work has demonstrated a high-efficiency ultranarrow-linewidth biphoton source and has made substantial advancements in quantum technology utilizing heralded single photons.
Biphoton frequency combs (BFCs) are promising quantum sources for large-scale and high-dimensional quantum information and networking systems. In this context, the spectral purity of individual frequency bins will be critical for realizing quantum networking protocols like teleportation and entanglement swapping. Measurement of the temporal autocorrelation function of the unheralded signal or idler photons comprising the BFC is a key tool for characterizing their spectral purity and in turn verifying the utility of the biphoton state for networking protocols. Yet the experimentally obtainable precision for measuring BFC correlation functions is often severely limited by detector jitter. The fine temporal features in the correlation function---not only of practical value in quantum information, but also of fundamental interest in the study of quantum optics---are lost as a result. We propose a scheme to circumvent this challenge through electro-optic phase modulation, experimentally demonstrating time-resolved Hanbury Brown--Twiss characterization of BFCs generated from an integrated 40.5-GHz Si3N4{\mathrm{Si}}_{3}{\mathrm{N}}_{4}Si3​N4​ microring, up to a $3\ifmmode\times\else\texttimes\fi{}3$-dimensional two-qutrit Hilbert space. Through slight detuning of the electro-optic drive frequency from the comb's free spectral range, our approach leverages Vernier principles to magnify temporal features, which would otherwise be averaged out by detector jitter. We demonstrate our approach under both continuous-wave and pulsed-pumping regimes, finding excellent agreement with theory. Our method reveals not only the collective statistics of the contributing frequency bins but also their temporal shapes---features lost in standard fully integrated autocorrelation measurements.
Narrow-band time-frequency entangled biphotons are generated from spontaneous four-wave mixing in cold atom clouds. The coherence time of the entangled biphotons can be extended to sub-microseconds by the slow light effect. The temporal wavefunction of the biphotons can be manipulated through modulating the spectral or spatial mode of the controlling laser beams. Concerning a pair of entangled biphoton and the resulting heralded single photon, it was commonly believed that, time-frequency entanglement damages the temporal purity of the single photon. However, the case is totally different for biphotons with long coherence time which is far beyond the time resolution of single-photondetectors. We demonstrate that, the single photon heralded from these time-frequency entangled biphotons is in a pure temporal state. Therefore, single photons are able to be shaped through the time-frequency entanglement with their partner photons, while the single photons could be found to be still in a pure state and thus useful for quantum information processing and communication technology.
The nonclassical light sources, such as frequency-time entangled photons, are anticipated to offer significant benefits for emerging quantum optical sensing or spectroscopic measurements and manifest on ultrafast time scales (sub-ps to fs). However, the constrained time resolution (ns to ps) of photon-counting detectors poses challenges in comprehensively characterizing their detailed properties on ultrafast time scales. Therefore, we present a novel asynchronous optical sampling (ASOPS) technique utilizing two-color optical frequency combs to demonstrate highly precise and sensitive ultrafast time-resolved cross-correlation measurements at the single-photon level. By employing photon counting statistics, this method successfully reconstructed the picosecond pulse width cross-correlation waveforms at extremely low power level (<1 photon per pulse), while effectively suppressing the residual temporal jitter between the two combs via optically triggered averaging using asynchronous optical sampling of combs. The use of repetition frequency stabilized distinct-wavelength pulses allowed for the effective suppression of strong background light from the pump through spectral filtering, achieving single-photon sensitivity. Subsequently, we parametrically down converted the frequency doubled light from the Er comb in the nonlinear ppKTP waveguide to generate quantum entangled photons at telecom band. A 9.04% Klyshko efficiency with a photon pair generation rate of 0.98 MHz/mW was obtained using heralding detection. Employing the established ASOPS technique to the generated photon pairs enabled the realization of ultrafast time-resolved and quantum mechanical correlation measurements. This paves the way for a versatile and comprehensive manipulation of quantum-entangled photon pairs in the time-domain, with potential applications in ultrafast optical quantum technology and ultrashort fluorescence measurements.