A fundamental element of quantum information processing with photonic qubits is the nonclassical quantum interference between two photons when they bunch together via the Hong-Ou-Mandel (HOM) effect. Ultimately, many such photons must be processed in complex interferometric networks. For this purpose, it is essential to synchronize the arrival times of the flying photons and to keep their purities high. On the basis of the recent experimental success of single-photon storage with high purity, we demonstrate for the first time the HOM interference of two heralded, nearly pure optical photons synchronized through two independent quantum memories. Controlled storage times of up to 1.8 μs for about 90 events per second were achieved with purities that were sufficiently high for a negative Wigner function confirmed with homodyne measurements.
Quantum memories are a key technology for distributing entanglement in large-scale quantum networks. They allow for coherent control and temporal reordering of single photons which is a fundamental prerequisite for quantum repeaters, local synchronization and buffering of quantum information. These applications require bright and pure sources of indistinguishable single photons, as well as long-lived, efficient, and high-bandwidth quantum memories with minimal noise. The demonstration of robust, scalable, multi-mode photonic interfaces between different types of quantum memories and single-photon sources remains an open experimental challenge. This work contributes to this endeavor by investigating a heterogeneous interface between a semiconductor quantum dot single-photon source and a room-temperature atomic vapor memory. The first part of this work presents the first room-temperature, ladder-type atomic vapor memory that operates on the Cs D1 line at 895\,nm. The performance of the memory is characterized using attenuated laser pulses at the single-photon level. The memory achieves an internal efficiency of
ηint=15(1)% at a storage time of
τs=17.4(2)\,ns. We derive a quantitative benchmark for the memory compatibility with quantum dot single-photon sources. The memory's high signal-to-noise ratio of 830(80), and its spectral acceptance window of
W=560\,MHz promise compatibility with single photons from state-of-the-art InGaAs quantum dot devices. Furthermore, we present such a nano-integrated quantum dot device and characterize its optical properties. We measure a radiative lifetime of 1.3(1)\,ns and an inhomogeneously broadened linewidth of 5.1(1)\,GHz. The temperature tuning of the emission wavelength to the Cs D1 line allows coupling of the quantum dot to the memory in a joint experiment. The storage of single photons from a quantum dot source in a room-temperature atomic memory is demonstrated with on-demand retrieval. The variable storage time of up to
τs=19.8\,ns and the memory's fractional delay of
Df=14(1) present a significant advancement in the research field. We compare our results with other types of interfaces between single-photon sources and quantum memories and discuss the performance in the context of quantum network applications. This work marks an important step towards local buffering and synchronization of quantum information in heterogeneous quantum network nodes.
In the last two decades, many quantum optics experiments have demonstrated small-scale quantum information processing applications with several photons. Beyond such proof-of-principle demonstrations, efficient preparation of large, but definite, numbers of photons is of great importance for further scaling up and speeding up photonic quantum information processing. Typical single-photon generation techniques based on nonlinear parametric processes face challenges of probabilistic generation. Here we demonstrate efficient synchronization of photons from multiple nonlinear parametric heralded single-photon sources (HSPSs), using quantum memories (QMs). Our low-loss optical memories greatly enhance (~30x) the generation rate of coincidence photons from two independent HSPSs, while maintaining high indistinguishability (95.7%) of the synchronized photons. As an application, we perform the first demonstration of HSPS-based measurement-device-independent quantum key distribution (MDI-QKD). The synchronized HSPSs demonstrated here will pave the way toward efficient quantum communication and larger scale optical quantum computing.
To realize a large-scale quantum network, both quantum memory and the interference of retrieved indistinguishable photons are essentially required to perform multi-photon synchronization and quantum-interference-mediated entanglement swapping. Significant progress has been achieved in low-temperature and well-isolated systems. However, linking independent quantum memories at room temperature remain challenging. Here, we present an experimental demonstration of Hong–Ou–Mandel interference between single photons from two independent room-temperature quantum memories. We manage to simultaneously operate two such quantum memories and individually obtain a memory-built-in quantum correlation of Stokes and anti-Stokes photons by a far-off-resonance Duan–Lukin–Cirac–Zoller protocol. We also successfully enhance the Hong–Ou–Mandel interference rate up to about 15 times by increasing each photon rate, which is achieved by coordinating two quantum memories with a repeat-until-success fashion. We observe the visibility of quantum interference up to 75.0% without reduction of any background noise, well exceeding the classical limit of 50%. Our results, together with its straightforward, broadband, and room-temperature features, open up a promising way towards realizing large-scale quantum networks at ambient conditions.
First generation quantum repeater networks require independent quantum memories capable of storing and retrieving indistinguishable photons to perform quantum-interference-mediated high-repetition entanglement swapping operations. The ability to perform these coherent operations at room temperature is of prime importance in order to realize large scalable quantum networks. Here we address these significant challenges by observing Hong-Ou-Mandel (HOM) interference between indistinguishable photons carrying polarization qubits retrieved from two independent room-temperature quantum memories. Our elementary quantum network configuration includes: (i) two independent sources generating polarization-encoded qubits; (ii) two atomic-vapor dual-rail quantum memories; and (iii) a HOM interference node. We obtained interference visibilities after quantum memory retrieval of
V=(41.9±2.0)% for few-photon level inputs and
V=(25.9±2.5)% for single-photon level inputs. Our prototype network lays the groundwork for future large-scale memory-assisted quantum cryptography and distributed quantum networks.