Temporal Purity and Quantum Interference of Single Photons from Two Independent Cold Atomic Ensembles

Abstract

The temporal purity of single photons is crucial to the indistinguishability of independent photon sources for the fundamental study of the quantum nature of light and the development of photonic technologies. Currently, the technique for single photons heralded from time-frequency entangled biphotons created in nonlinear crystals does not guarantee the temporal-quantum purity, except using spectral filtering. Nevertheless, an entirely different situation is anticipated for narrow-band biphotons with a coherence time far longer than the time resolution of a single-photon detector. Here we demonstrate temporally pure single photons with a coherence time of 100 ns, directly heralded from the time-frequency entangled biphotons generated by spontaneous four-wave mixing in cold atomic ensembles, without any supplemented filters or cavities. A near-perfect purity and indistinguishability are both verified through Hong-Ou-Mandel quantum interference using single photons from two independent cold atomic ensembles. The time-frequency entanglement provides a route to manipulate the pure temporal state of the single-photon source. DOI: 10.1103/PhysRevLett.117.013602 The purity of the quantum state of a single photon is a prerequisite of its indistinguishability with single photons from other independent sources, and the latter is an essential basis for the realization of a scalable quantum network with distant and independent nodes [1][2][3][4][5]. Furthermore, the temporal purity of a single photon is crucial to the development of photonic technologies for quantum information science [6,7]. The traditional method to produce indistinguishable single photons is heralding time-frequency entangled biphotons generated from spontaneous parametric down-conversion (SPDC) in a nonlinear crystal which is pumped by ultrashort pulses [4,5,8]. In recent decades many new physical systems have been developed [9-13] to obtain pure single photons without time-frequency entanglement built in.In the community of quantum communication, SPDC in χ ð2Þ nonlinear media is still the preferable way to produce entangled biphotons because of its simplicity in the operation and the potential for on-chip integration and scaling up [14][15][16]. However, the intrinsic feasible phase matching condition of SPDC crystal allows an extremely broad range of temporal modes. Therefore, the typical temporal coherence time of the photon source is of femtosecond scale. Compared to the time response of most commercial single-photon detectors, which is about 1 ns, this temporal coherence time is so short that the trigger photon of the heralded single photon is measured with a large time uncertainty. This time uncertainty damages the temporal quantum purity of the single-photon source. To circumvent the time uncertainty problem due to the slowness of the detectors, a common practice is to use external spectral filtering including passive filtering with narrowband filters [4,5,17] and active filtering with an optical cavity [18]. In this case, the temporal state o...

Efficient quantum memory for single-photon polarization qubits

“…We focus the pump beam using a plano-convex lens to the center of the atomic cloud to generate the Gaussian-shaped biphoton waveform. As compared to other waveform shaping techniques using time-domain modulations 31,32 , this method gives a high heralding efficiency and maintains the frequency entan- Under detection of a Stokes photon, its paired frequency anticorrelated anti-Stokes photon is projected to a pure singlephoton state with well defined temporal waveform 33,34 :…”

Spectral–temporal biphoton waveform of photon pairs from cascade-type warm atoms

“…One of the advantages of photon pairs generated from atomic ensembles is the indistinguishability of the pairs [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] . The properties of the photon source are important for generating interfering photons from a series independent photon pairs 17,18 . Photon-pair sources generated from cold and warm atomic ensembles have been experimentally demonstrated via the spontaneous four-wave mixing (SpFWM) process [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] .…”

“…The properties of the photon source are important for generating interfering photons from a series independent photon pairs 17,18 . Photon-pair sources generated from cold and warm atomic ensembles have been experimentally demonstrated via the spontaneous four-wave mixing (SpFWM) process [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] . Recently, highly bright, entangled SpFWM photon-pair sources from a warm atomic ensemble have been demonstrated by exploiting the collective two-photon coherence in a cascade-type atomic system 16 .…”

Generation and Manipulation of Nonclassical Photon Sources in Nonlinear Processes

“…QPM crystals have the advantages of high generation rate and narrow bandwidth, which are frequently used in photon pair generation in modern quantum optics experiments [12][13][14][15][16][17][18][19]. For SFWM, the commonly used materials are hot or cold atomic ensembles [20][21][22][23][24] and guided-wave materials such as dispersionshifted fibers (DSF) [25][26][27][28][29], photonic crystal fibers (PCF), [30][31][32], and siliconon-insulator (SOI) waveguide [33][34][35][36][37][38][39][40][41]. To look for new kinds of nonlinear materials for generating high-quality photon sources is still a very hot topic in QIST.…”