Memory-assisted quantum networks require photon sources with controllable temporal and correlation properties. The Duan-Lukin-Cirac-Zoller (DLCZ) protocol provides a platform based on spontaneous Raman scattering in atomic ensembles, but a unified predictive theory connecting control parameters to correlations under realistic propagation and noise conditions remains lacking. Here we present a propagation-inclusive open-system quantum theory that retains write-induced population redistribution while combining Heisenberg-Langevin dynamics with Maxwell-Schrödinger propagation. We experimentally validate its key predictions. The theory predicts time-dependent Stokes generation, spin-wave evolution, retrieved anti-Stokes wavepackets, and time-resolved cross-correlations. Experiments confirm robust correlations under retrieval tuning and enhanced correlations for shorter write pulses, consistent with the different scaling of correlated coincidences and accidental backgrounds with the mean spin-wave excitation number. Classically controlled retrieval enables temporal gating and slicing of the anti-Stokes wavepacket, establishing a quantitative framework for correlation engineering in memory-compatible DLCZ photon sources.
Memory-assisted quantum networks require photon sources with controllable temporal and correlation properties. The Duan-Lukin-Cirac-Zoller (DLCZ) protocol provides a platform based on spontaneous Raman scattering in atomic ensembles, but a unified predictive theory connecting control parameters to correlations under realistic propagation and noise conditions remains lacking. Here we present a propagation-inclusive open-system quantum theory that retains write-induced population redistribution while combining Heisenberg-Langevin dynamics with Maxwell-Schrödinger propagation. We experimentally validate its key predictions. The theory predicts time-dependent Stokes generation, spin-wave evolution, retrieved anti-Stokes wavepackets, and time-resolved cross-correlations. Experiments confirm robust correlations under retrieval tuning and enhanced correlations for shorter write pulses, consistent with the different scaling of correlated coincidences and accidental backgrounds with the mean spin-wave excitation number. Classically controlled retrieval enables temporal gating and slicing of the anti-Stokes wavepacket, establishing a quantitative framework for correlation engineering in memory-compatible DLCZ photon sources.
With the accelerating developments in the field of quantum communication, there will be a need for sources of highly quantum single-photons. In addition to communication applications, the purity, brightness, and fidelity of a given single-photon source are intrinsically linked to the success rate and scalability of a range of photonic technologies in quantum computing, metrology, memory, and cryptography. Further, quantum repeater architectures for maintaining quantum information over long distances will become crucial. In these schemes, end-to-end entanglement is generated by establishing entanglement between the ends of several elementary segments, followed by successive entanglement-swapping operations to increase the entanglement distance until it spans the transmission channel. The most widely studied proposal for the practical implementation of a quantum repeater is the Duan-Lukin-Cirac-Zoller (DLCZ) protocol. This consists of off-resonant Raman excitation by a write pulse, which induces a spin-wave excitation in an atomic ensemble and emits a Stokes photon. At a later time, this spin-wave may be read with a high-power resonant pulse, resulting in the emission of an anti-Stokes photon into a well-defined spatial mode. The Stokes photon may be used for entanglement with another ensemble, while the anti-Stokes may be read out for an entanglement-swapping operation. Notably, in the DLCZ protocol, the atomic ensemble serves as both the quantum memory and a heralded single-photon source. Using atomic ensembles for single-photon sources enables the production of highly pure, indistinguishable photons with minimal spectral broadening; however, the generation rate remains a challenge. In the case of DLCZ, the Raman-scattering step is highly probabilistic, reducing the anti-Stokes generation rate considerably. Furthermore, DLCZ requires operation in the low-excitation-probability regime to avoid multiphoton noise contributions. Additionally, the profile of the induced spin wave is suboptimal for readout. To compensate for these drawbacks, in this thesis we report the experimental realization of a variant of DLCZ that leverages collectively enhanced superradiant (SR) emission. Superradiant emission is characterized by intense bursts of photons emitted in a shorter amount of time than independent atomic state decay. By switching to a resonant excitation and optimizing the write profile, the induced spin-wave is primed for readout, thus increasing retrieval efficiency and the level of quantum correlation. We report data for both the DLCZ and SR schemes under similar optical conditions and compare the two. At high write power, the SR protocol exhibits conditional readout probabilities as high as $\sim$1.2\%, compared to $\sim$0.6\% for the DLCZ protocol under the same conditions. We also analyze the temporal distribution of anti-Stokes emission and find a degree of maintenance of the spin-wave profile, even in the anti-Stokes direction.
The Duan-Lukin-Cirac-Zoller (DLCZ) process in the atomic ensemble is an important means to generate quantum correlation and entanglement between photons and atoms (quantum interface). When a write pulse acts on atoms, the DLCZ quantum memory process will be generated, which has been extensively studied. In the process a spontaneous Raman scattering (SRS) of a Stokes photon is generated, and a spin-wave excitation stored in the atomic ensemble is created at the same time. The higher probability of the generation of Stokes photons will cause more noise and reduce entanglement. On the contrary, the low generation probability of Stokes photons affects the success probability of entanglement distribution on a quantum repeater. How to increase generation probability of Stokes photons without causing more noise is an urgent problem to be resolved. In this work, a 87Rb atomic ensemble is placed in a standing wave cavity which resonates with the Stokes photon. This cavity has a trip length of 0.6 m and a free spectral range (FSR) of 256 MHz. The optical loss of all the optical elements in this cavity is 9%, of which 4% loss originates from the other optical elements and 5% loss from the vacuum chamber of the magneto-optical trap (MOT). The fineness of the cavity with the cold atoms is measured to be ~19.1. By calculating the total probability of Stokes photon emission out of the cavity, we derive the enhancement factor of this standing wave cavity when the cavity loss is l. When this cavity is locked with PDH frequency locking technique, we observe that the production probability of the Stokes photons is 8.7 times higher than that without cavity due to the optical cavity enhancement effect. Under this condition, the relationship between the generation probability of Stokes photons and the power of write beam is studied. The write excitation probability changes linearly with the power of write beam. This work provides an experimental solution to reducing the noise caused by time multimode operation in DLCZ scheme.
We present the quantum theory for creation of collective atomic memory and generation of nonclassically correlated photon pairs from an ensemble via the protocol of Duan et al. [Nature (London) 414, 413 (2001)]. The temporal evolution of photon numbers, photon statistics, and cross-correlation between the Stokes and anti-Stokes fields is found by solving the equation of motion for atomic spin-wave excitations. We consider a low-finesse cavity model with collectively enhanced signal-to-noise ratio, which remains still considerably large in the free-space limit. Our results describe analytically the dependence of quantum correlations on spin decoherence time and time delay between the write and read lasers and reproduce the observed data very well including the generated pulse shapes, strong violation of Cauchy-Schwarz inequality and conditional generation of anti-Stokes single-photon pulse. The approach we developed may be used also for quantum description of storage and retrieval of quantum information, especially when the statistical properties of nonclassical pulses are studied.
A source that can generate atom-photon quantum correlations or entanglement based on a quantum memory is a basic building block of quantum repeaters (QRs). To achieve high entanglement generation rates in ensemble-based QRs, spatial-, temporal-, and spectral-multimode memories are needed. Previous temporal-multimode memories are based on rephasing mechanisms in inhomogeneously broadened media. Here, by applying a train of write pulses in time, with each pulse coming from a different direction, to a homogeneously broadened atomic ensemble to induce Duan-Lukin-Cirac-Zoller-like Raman processes, we prepare up to 19 pairs of modes, namely, one spin-wave mode and one photonic time bin. Spin-wave-photon (i.e., atom-photon) entanglement is probabilistically produced in these mode pairs. Based on the stored spin-wave modes together with feedforward-controlled readout, we build a temporally multiplexed source and then demonstrate an 18.8-fold increase in the probability of the generation of spin-wave-photon entanglement compared to the sources that use individual modes. The measured Bell parameter for the multiplexed source is $2.30\ifmmode\pm\else\textpm\fi{}0.02$, and the memory lifetime is 30 \ensuremath{\mu}s.
The realization of quantum networks and quantum repeaters remains an outstanding challenge in quantum communication. These rely on the entanglement of remote matter systems, which in turn requires the creation of quantum correlations between a single photon and a matter system. A practical way to establish such correlations is via spontaneous Raman scattering in atomic ensembles, known as the Duan-Lukin-Cirac-Zoller (DLCZ) scheme. However, time multiplexing is inherently difficult using this method, which leads to low communication rates even in theory. Moreover, it is desirable to find solid-state ensembles where such matter-photon correlations could be generated. Here we demonstrate quantum correlations between a single photon and a spin excitation in up to 12 temporal modes, in a ^{151}Eu^{3+}-doped Y_{2}SiO_{5} crystal, using a novel DLCZ approach that is inherently multimode. After a storage time of 1 ms, the spin excitation is converted into a second photon. The quantum correlation of the generated photon pair is verified by violating a Cauchy-Schwarz inequality. Our results show that solid-state rare-earth-ion-doped crystals could be used to generate remote multimode entanglement, an important resource for future quantum networks.
Quantum memory is considered to be one of the key elements in the fields of quantum computing and quantum communication. Warm atomic vapor cells for quantum memory in DLCZ (for Duan, Lukin, Cirac, and Zoller) protocol are appealing due to the perceived reduction in experimental complexity and commercial availability. However, reported results on quantum memory using warm vapor cells were done under widely different experimental conditions and produced ambiguous results. In order for the memory to exhibit non-classical behavior, to a high degree of certainty, the cross-correlation value between the Stokes and anti-Stokes photons needs to be greater than two. In this work we demonstrate quantum memory with cross-correlation value between the Stokes and anti-Stokes photons greater than two lasting for 4 μs using warm Rb vapor with buffer gas for nearly co-propagating write and read beams.