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.
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.
Long-distance entanglement distribution is an important task for quantum communication, but difficult to achieve due to the loss of photons in optical fiber transmission. Quantum repeater is a scheme to solve this problem. In this scheme, the long distance of entanglement distribution is divided into several small parts, the entanglement is established first at both ends of each part, then, the entanglement distance is extended through the entanglement exchange of adjacent interval parts, in order to achieve the long distance entanglement distribution. Of them, the Duan-Lukin-Cirac-Zoller (DLCZ) protocol based on the cold atom ensemble and the linear optics which can generate and store entanglement, is regarded as one of the most potential schemes. In the process of DLCZ, retrieval efficiency is an important index of the quantum repeater, because it will influence each entanglement exchange operation between adjacent quantum repeater nodes. Generally, the retrieval efficiency is improved by optimizing the reading pulse, increasing the optical depth (OD) of the atomic ensemble and the cavity enhancement. The ring cavity constrains the light field to increase the intensity of the interaction between light and atoms, and effectively improve the retrieval efficiency of the quantum memory. In this work, atomic ensembles are placed in a ring cavity. The cavity length is 3.3 m and the fineness is 13.5. The optical loss of all ring cavity is 21%, mainly including 15% loss of other optical elements and 6% loss of the cell. In order to increase the retrieval efficiency, we need to ensure the mode resonance of read-out photon, write-out photon and locking. The cavity needs two input beams of light: one comes from the path of read-out photon and the other from the path of write-out photon in the reverse direction. The two beams are locked at the same frequency as the write-out photon and the read-out photon respectively. The cavity length is adjusted by moving the cavity mirrors’ positions through translating the frame, to make two light modes resonate. The acousto-optic modulator (AOM) is inserted into the path of the locking to control the frequency of the locking. By adjusting the AOM to change the frequency of the locking, the locking can be coincident with the write-out and read-out cavity modes. Then, the three-mode resonance can be achieved When the cavity mode resonates with the atomic line, it will lead the atomic formants to split. thereby affecting the enhancement effect of retrieval efficiency. In the experiment, the detuning of the read light will affect the frequency of the read-out photon, and further affect the detuning of the cavity mode with the resonance line of the atom. Thus, by increasing the detuning between the reading light and the atomic transition line, the frequency splitting between the two modes can be reduced, then enhance the retrieval efficiency. We study the relation between the enhancement factor of the retrieval efficiency and the detuning amount of the reading light relative to the atomic resonance line. The results show that when the detuning amount of reading light is 80 MHz, the intrinsic readout efficiency is 45%, and the readout efficiency is enhanced by 1.68 times.
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.
Practical realization of quantum repeaters requires quantum memories with high retrieval efficiency, multi-mode storage capacities, and long lifetimes. Here, we report a high-retrieval-efficiency and temporally multiplexed atom-photon entanglement source. A train of 12 write pulses in time is applied to a cold atomic ensemble along different directions, which generates temporally multiplexed pairs of Stokes photons and spin waves via Duan-Lukin-Cirac-Zoller processes. The two arms of a polarization interferometer are used to encode photonic qubits of 12 Stokes temporal modes. The multiplexed spin-wave qubits, each of which is entangled with one Stokes qubit, are stored in a "clock" coherence. A ring cavity that resonates simultaneously with the two arms of the interferometer is used to enhance retrieval from the spin-wave qubits, with the intrinsic retrieval efficiency reaching 70.4%. The multiplexed source gives rise to a ∼12.1-fold increase in atom-photon entanglement-generation probability compared to the single-mode source. The measured Bell parameter for the multiplexed atom-photon entanglement is 2.21(2), along with a memory lifetime of up to ∼125 µs.
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.
Abstract Entanglement between a spin-wave qubit (memory qubit) and a photonic qubit is a basic building block for quantum repeaters. Duan-Lukin-Cirac-Zoller (DLCZ) scheme, which generates spin waves via spontaneous Raman scattering (SRS) of Stokes photons in atomic ensemble, provides a promising way to generate such entanglement. In a recent work [arXiv: 2006.05631, accepted by communications physics], DLCZ-like quantum memory that generates long-lived atom-photon entanglement has been experimentally demonstrated, where magnetic-field-insensitive (MFI) coherence is used to store spin waves. For realizing such MFI spin-wave storage, the atoms have to be initially prepared in a specific Zeeman sublevel, which is achieved by applying optical pumping lasers. Here, we demonstrate the memory lifetimes for the cases that the atoms are perfectly and imperfectly prepared in the specific Zeeman level, respectively. The experimental results show that the spin waves associated with magnetic-field-sensitive (MFS) and MFI coherences will be simultaneously created for the case that the atoms are imperfectly prepared in the Zeeman sublevel. Thus, the read outs will experience decay oscillations due to interferences between the two spin waves and the memory lifetime will be shorten due to dephasing of MFS coherence. A detailed theoretical analysis has been developed for explaining the experimental results. The present work will help one to understand decoherence of spin waves (SWs) and then enable one to obtain optimal lifetime of the entanglement storage in the cold atoms.
Entanglement swapping (ES) between memory repeater links is critical for establishing quantum networks via quantum repeaters. So far, ES with atomic-ensemble-based memories has not been achieved. Here, we experimentally demonstrated ES between two entangled pairs of spin-wave memories via Duan-Lukin-Cirac-Zoller scheme. With a cloud of cold atoms inserted in a cavity, we produce non-classically-correlated spin-wave-photon pairs in 12 spatial modes and then prepare two entangled pairs of spin-wave memories via a multiplexed scheme. Via single-photon Bell measurement on retrieved fields from two memories, we project the two remaining memories never entangled previously into an entangled state with the measured concurrence of C = 0.0124(0.003). The successful probability of ES in our scheme is increased by three times, compared with that in non-multiplexed scheme. Our presented work shows that the generation of entanglement (C>0) between the remaining memory ensembles requires the average cross-correlation function of the spin-wave-photon pairs to be >30 .