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Advanced Quantum Technologies

Efficient Conversion from Atomic Spin Wave to Single Photon with Controllable Temporal Length

Jiajin Lu, Minjie Wang, Haole Jiao, Mengqi Xi, Zhifang Yang, S. G. Li

Published Feb 12, 2025• 1 citation

Abstract

Abstract The single photons with controllable temporal length are essential to connect various heterogeneous nodes in quantum networks. The spontaneous Raman scattering (SRS) process in atomic ensembles is a candidate for generating heralded single photon sources. However, efficient conversion from atomic spin waves to single photons with controllable temporal length has not yet been demonstrated experimentally. Here SRS is used in a cold atomic ensemble to generate Stokes photons, which herald the generation of the correlated atomic spin waves. By applying read pulses with adjustable duration to the atoms, the spin waves are converted to heralded single photons (anti‐Stokes photons) with controllable temporal length. The conversion efficiency from atomic spin waves to anti‐Stokes photons is improved by utilizing cavity enhancement. The heralded single photons are proved to be nonclassical and to be in a pure state when the photon duration ranges from 100 ns to 3 µs. The conversion efficiency of photons from atomic spin waves is () for the anti‐Stokes photon duration of 3 µs (100 ns). The research offers an experimental foundation for the interconnection of heterogeneous quantum nodes.

DOI: 10.1002/qute.202400480

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Abstract The single photons with controllable temporal length are essential to connect various heterogeneous nodes in quantum networks. The spontaneous Raman scattering (SRS) process in atomic ensembles is a candidate for generating heralded single photon sources. However, efficient conversion from atomic spin waves to single photons with controllable temporal length has not yet been demonstrated experimentally. Here SRS is used in a cold atomic ensemble to generate Stokes photons, which herald the generation of the correlated atomic spin waves. By applying read pulses with adjustable duration to the atoms, the spin waves are converted to heralded single photons (anti‐Stokes photons) with controllable temporal length. The conversion efficiency from atomic spin waves to anti‐Stokes photons is improved by utilizing cavity enhancement. The heralded single photons are proved to be nonclassical and to be in a pure state when the photon duration ranges from 100 ns to 3 µs. The conversion efficiency of photons from atomic spin waves is () for the anti‐Stokes photon duration of 3 µs (100 ns). The research offers an experimental foundation for the interconnection of heterogeneous quantum nodes.

We propose a scheme of heralded storages of single photons via spontaneous Raman scattering (SRS) enhanced by a chiral standing-wave cavity, which creates a non-classically correlated pair of a spin wave and a Stokes photon (heralded photon) when a single-photon “write” pulse is applied to an atomic ensemble. In this scheme, to decrease the power of the write pulse for SRS, the write laser and the Stokes fields are aligned to propagate near-collinearly through the atoms. The short storage lifetime remains a challenge in standing-wave cavity-enhanced SRS. To prolong the storage lifetime, we achieve a long-wavelength spin-wave memory by removing the backward-scattering Stokes field from the cavity, which is realized by using non-reciprocal couplings between the Stokes fields and the atoms. In the experiment, the memory lifetime is demonstrated up to ∼100 μs, which is beyond the limitation (∼350 ns) of the storage in the atomic ensembles coupled to standing-wave cavities. We achieve a spin-wave memory for the write pulse containing a mean photon number of ∼4. The quantum correlation of the Stokes-spin-wave pairs is beyond 2, which gives strong evidence of non-classical behavior.

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The generation of ultra-narrowband, pure and storable single photons with widely tunable wave shape is an enabling step toward hybrid quantum networks requiring interconnection of remote disparate quantum systems. It allows interaction of quantum light with several material systems, including photonic quantum memories, single trapped ions and opto-mechanical systems. Previous approaches have offered a limited tuning range of the photon duration of at most one order of magnitude. Here we report on a heralded single photon source with controllable emission time based on a cold atomic ensemble, which can generate photons with temporal durations varying over three orders of magnitude up to 10 μs without a significant change of the readout efficiency. We prove the nonclassicality of the emitted photons, show that they are emitted in a pure state, and demonstrate that ultra-long photons with nonstandard wave shape can be generated, which are ideally suited for several quantum information tasks.

Controls of waveforms (pulse durations) of single photons are important tasks for effectively interconnecting disparate atomic memories in hybrid quantum networks. So far, the waveform control of a single photon that is entangled with an atomic memory remains unexplored. Here, we demonstrated control of waveform length of the photon that is entangled with an atomic spin-wave memory by varying light-atom interaction time in cold atoms. The Bell parameter S as a function of the duration of photon pulse is measured, which shows that violations of Bell inequality can be achieved for the photon pulse in the duration range from 40 ns to 50 µs, where, S = 2.64 ± 0.02 and S = 2.26 ± 0.05 for the 40-ns and 50-µs durations, respectively. The measured results show that S parameter decreases with the increase in the pulse duration. We confirm that the increase in photon noise probability per pulse with the pulse-duration is responsible for the S decrease.

In spontaneous Raman scattering in an atomic ensemble, a collective atomic spin wave is created in correlation with the Stokes field. When the Stokes photons from two or more such atomic ensembles are made indistinguishable, a 'which-way' collective atomic spin excitation is generated among the independent atomic ensembles. We demonstrate this phenomenon experimentally by reading out the atomic spin excitations and observing interference between the read-out beams. When a single-photon projective measurement is made on the indistinguishable Stokes photons, this simple scheme can be used to entangle independent atomic ensembles. Compared to other currently used methods, this scheme can be easily scaled up and has greater efficiency.