Generation of Narrow-Band Polarization-Entangled Photon Pairs for Atomic Quantum Memories

Abstract

We report an experimental realization of a narrow-band polarization-entangled photon source with a linewidth of 9.6 MHz through cavity-enhanced spontaneous parametric down-conversion. This linewidth is comparable to the typical linewidth of atomic ensemble based quantum memories. Single-mode output is realized by setting a reasonable cavity length difference between different polarizations, using of temperature controlled etalons and actively stabilizing the cavity. The entangled property is characterized with quantum state tomography, giving a fidelity of 94% between our state and a maximally entangled state. The coherence length is directly measured to be 32 m through two-photon interference.PACS numbers: 03.67. Bg, 42.65.Lm The storage of photonic entanglement with quantum memories plays an essential role in linear optical quantum computation (LOQC) [1] to efficiently generate large cluster states [2], and in long-distance quantum communication (LDQC) to make efficient entanglement connections between different segments in a quantum repeater [3]. For the atomic ensemble based quantum memories [4,5,6], typical spectrum linewidth required for photons is on the order of several MHz. While spontaneous parametric down-conversion (SPDC) is the main method to generate entangled photons [7], the linewidth determined by the phase-matching condition is usually on the order of several THz which is about 10 6 times larger, making it unfeasible to be stored. Moreover, interference of independent broad-band SPDC sources requires a synchronization precision of several hundred fs [8]. While in LDQC, for the distance on the order of several hundred km, it becomes extremely challenging for the current synchronization technology [9,10]. But for a narrowband continuous-wave source at MHz level, due to the long coherence time, synchronization technique will be unnecessary, while coincidence measurements with time resolution of several ns with current commercial singlephoton detectors will be enough to interfere independent sources.Passive filtering with optical etalons is a direct way to get MHz level narrow-band entangled photons from the broad-band SPDC source, but it will inevitably result in a rather low count rate. In contrast, cavity-enhanced SPDC [11,12] provides a good solution for this problem. By putting the nonlinear crystal inside a cavity, the generation probability for the down-converted photons whose frequency matches the cavity mode will be enhanced greatly. The cavity acts as an active filter. The frequency of the generated photons lies within the cavity mode, which can be easily set to match the required atomic linewidth. Experimentally, Ou et al.[11] has realized a type-I source, in which the two photons generated have the same polarization, making it very difficult to generate entanglement. Wang et al.[13] made a further step by putting two type-I nonlinear crystals within a ring cavity to generate polarization entanglement, but unfortunately the output is multi-mode which does not fit the requirement of an...

Spectral Engineering of Slow Light, Cavity Line Narrowing, and Pulse Compression

“…It is often desirable to generate entangled photons with a very narrow bandwidth, for example to match the bandwidth of an optical quantum memory or the bandwidth of an optical transition [32,33]. Photons with narrow bandwidth can be generated by cavity-enhanced spontaneous down conversion, where a periodically-poled non-linear crystal is enclosed inside a cavity [34]. A rare-earth-ion-doped slow light cavity could be combined with a periodically-poled non-linear crystal.…”

Narrowband Biphotons: Generation, Manipulation, and Applications

“…To reduce their bandwidth, optical cavity may be used for active filtering [18,19,20,21]. However, multiple cavity modes are resonated simultaneously due to the broad gain linewidth of the forward-wave parametric interaction and the biphotons are generated in multiple longitudinal modes.…”

Generation of Narrow-Band Polarization-Entangled Photon Pairs at a Rubidium D1 Line

“…For the case that each output port has one photon, the photon pair will be in polarization entangled state in the post-selected manner. In contrast to the past experiment [25] H , in which, and V modes of the photon pairs are separated on a polarizing beam splitter (PBS), and filtered by two different etalons, respectively, then combined on another PBS for generating entangled photon pairs, in our experiment, the H and V modes of the photon pairs are filtered by a set of etalons, and then sent to an 50/50 NPBS to generate entangled two polarization qubits. So our experimental setup is relative simple，which will benefit the practical application of entanglement source in quantum information process.…”

“…By putting the nonlinear crystal inside a cavity, the linewidth of generated photon pairs is limited by the cavity linewidth and the generation probability for the down-converted photons whose frequency matches the cavity mode will be enhanced greatly. Significant progress has been made in this regard [19][20][21][22][23][24][25] . Recently, the generation of single-mode narrow-band polarization entangled source has been reported through cavity-enhanced SPDC [25] , and the storage of such entangled photons has been demonstrated in a cold atomic ensemble [26] In this paper, we also demonstrate a narrow-band polarization-entangled photon pairs at Rb D .…”