Shanghai Jiaotong University increases photon generation rate by 10 times using room temperature quantum memory

Photons with high generation rate are one of the essential resources for quantum communication, quantum computing and quantum metrology. Due to the inherent storage properties, memory-based photonic sources are a promising way to realize large-scale quantum information processing. However, such photonic sources are mostly implemented in extremely low-temperature systems or isolated systems, limiting their physical scalability.

To this end, Hyun-Min Kim's team at Shanghai Jiao Tong University has implemented a broadband room-temperature single-photon source based on a long-range nonresonant Duan-Lukin-Cirac-Zoller (FORD) quantum memory. The researchers obtained a significant increase in photon generation rate of up to ten times by using high-speed feedback control and repetition until a successful writing process. This storage-enhanced single-photon source based on a broadband room-temperature quantum memory provides a promising approach for building large-scale quantum storage networks under ambient conditions.

The related research was published in Scientific Reports on December 19 [1].

01Room temperature quantum memory improves photon generation rate

In the past decades, the development of memory-based single-photon sources has focused on suppressing decoherence and noise during photon generation to achieve high performance, which has driven numerous advances in ultra-low-temperature and well-isolated systems.

However, as the number of controlled quantum units increases, the complexity and size of the physical system grows rapidly, which may limit its further application in large-scale quantum networks. Room temperature systems allow for operational simplicity and better scalability, but losses and decoherence caused by the thermal motion of atoms hinder the long lifetime and high recovery efficiency of the stored collective excitations. At the same time noise from fluorescence and atomic collisions reduces the fidelity of the obtained photons to unacceptable levels.

Therefore, how to maintain the collective excitation well in the presence of severe decoherence and to extract the signal photons from the strong noise has been a long-term challenge to make room-temperature memory-based photonic sources work in a quantum regime. Recently, efforts have been made to realize room-temperature single-photon sources with built-in memory to further enhance photon generation, especially when the broadband characteristics allow operation at high data rates, which would make them more attractive for real-world applications.

Figure 1 Enhanced photon generation using broadband room temperature quantum memory

The generation of single photons is shown in Figure 1. Initially all atoms are prepared by pumping light in the ground state|g⟩. Then non-resonant writing pulses are incident on the atomic system, which aims to induce Stokes photons by spontaneous Raman scattering (SRS), heralding the generation of interatomic correlated collective excitations. There are three main steps.

In the first step, the writing process. In the three-energy λ-type configuration, the ground state |g⟩ and |s⟩ represent the F=3 and F=4 hyperfine states of the 6S1/2 state, respectively, while the 6P3/2 state is labeled as |e⟩. The gray area indicates the wide virtual state addressed by the strong write pulse, and the emitted Stokes photons are colored green.

In the second step, the readout process. The generated anti-Stokes photons are colored blue.

In the third step, a single write pulse is expanded into a sequence of write pulses and Stokes photons are continuously induced until one is successfully detected, thus increasing the efficiency of generating spin waves.

To increase the generation rate of anti-Stokes photons, a repetitive writing process is combined into the FORD quantum memory. As shown in Figure 1c, instead of making one write attempt in each cycle, the team programmed M bursts to be incident into the atomic system.

Since the memory is the central element of the entire generation process, its performance, especially the noise level and lifetime, determines the single photon purity and the achievable enhanced generation rate.

Under ideal conditions of constant recovery efficiency, the enhanced photon generation rate increases approximately linearly with the number of writing pulses when the number of writing pulses is small. As shown in the red dots in Figure 2, the measured increase in enhancement is close to a linear trend, indicating that although the average recovery efficiency decreases with the number of write pulses, however, the increase in the probability of generating collective excitation is now the main determinant of the generation rate and that further expansion of the write pulse sequence helps to efficiently obtain higher generation rates.

Figure 2 Increase in generation rate as a function of the number of pulses written. The red dotted data corresponds to the results with feedback control, and the blue diamond data was obtained from the experiment without feedback control.

Reference link：

[1]https://www.nature.com/articles/s41598-022-25060-1