Subnatural-Linewidth Polarization-Entangled Photon Pairs with Controllable Temporal Length

Abstract

We demonstrate an efficient experimental scheme for producing polarization-entangled photon pairs from spontaneous four-wave mixing (SFWM) in a laser-cooled 85 Rb atomic ensemble, with a bandwidth (as low as 0.8 MHz) much narrower than the rubidium atomic natural linewidth. By stabilizing the relative phase between the two SFWM paths in a Mach-Zehnder interferometer configuration, we are able to produce all four Bell states. These subnatural-linewidth photon pairs with polarization entanglement are ideal quantum information carriers for connecting remote atomic quantum nodes via efficient light-matter interaction in a photon-atom quantum network. DOI: 10.1103/PhysRevLett.112.243602 PACS numbers: 42.50.Dv, 03.67.Bg, 42.65.Lm The connectivity of a long-distance photon-atom quantum network strongly depends on efficient interactions between flying photonic quantum bits and local long-lived atomic matter nodes [1,2]. Such efficient quantum interfaces, which convert quantum states (such as time-frequency waveform and polarizations) between photons and atoms, require the photons to have a bandwidth sufficiently narrower than the natural linewidth of related atomic transitions (such as 6 MHz for rubidium D1 and D2 lines). As a standard method for producing entangled photons, spontaneous parametric down-conversion (SPDC) in a nonlinear crystal usually has a wide bandwidth (larger than terahertz) and very short coherence time (less than picosecond). Many efforts have been investigated in the past more than one decade ago to narrow down the SPDC photon bandwidth by using optical cavities [3][4][5][6][7][8]. However, the bandwidth of SPDC polarization-entangled photon pairs is still wider than most atomic transitions and leads to a very low efficiency of storing these polarization states in a quantum memory [5,9].Our motivation was stimulated by the recent progress in generating subnatural-linewidth biphotons by using continuous-wave spontaneous four-wave mixing (SFWM) in a laser-cooled atomic ensemble with electromagnetically induced transparency (EIT) [10,11]. Photons produced from this method not only have narrow bandwidth but also automatically match the atomic transitions. The applications of these narrow-band photons include the demonstration of a single-photon memory with a storage efficiency of about 50% [12], a single-photon precursor [13], and coherent control of single-photon absorption and reemission [14]. However, while this method provides a natural entanglement mechanism in the time-frequency domain, it is extremely difficult to produce polarization entanglement because of the polarization selectivity of EIT in a nonpolarized atomic medium [15]. It is possible to generate the polarization entanglement by scarifying the EIT effect, but the photon generation efficiency is low and the bandwidth is not narrower than the atomic natural linewidth [16]. The "writing-reading" technique with optical pumping provides a solution to polarization entanglement but results in reducing time-frequency entanglement [17].In...

Measuring the Biphoton Temporal Wave Function with Polarization-Dependent and Time-Resolved Two-Photon Interference

“…The key is to have the Stokes and anti-Stokes photons sharing the same spatial modes. For example, in the case where the Stokes and anti-Stokes photons are generated with a small angle to the pumpcoupling direction [21][22][23], we can follow the modified configuration shown in the Supplemental Material [34], where the phase-matched photon pairs are produced into two symmetric paths and counteroverlapped by two mirrors. This method can also be applied to narrow-band biphotons generated from cavity-enhanced spontaneous parametric down-conversion [17].…”

“…Using spontaneous four-wave mixing (SFWM) in cold atoms [20], the sub-MHz biphoton generation with a coherence time on the order of microseconds has been demonstrated [21][22][23]. Such a long coherence time of single photons allows manipulating their temporal waveforms [24][25][26] and their interaction with atoms in the time domain [27][28][29].…”

Narrowband Biphotons: Generation, Manipulation, and Applications

“…By properly choosing the driving laser polarizations and the phase between the two SFWM paths, all four polarization-entangled Bell states can be realized for subnatural-linewidth biphotons [57]. Fig.…”

“…In addition, compared with SPDC using a single pump laser, there are more freedoms to manipulate the the biphoton generation in SFWM with two driving lasers. We will show in this section how to engineer the biphoton quantum states by controlling the temporal [55,56] and spatial patterns [25,35] as well as the polarizations [48,57] of the classical driving fields. It is well know that time-frequency entangled photon pairs can be used to efficiently produce heralded single photons with well defined relative time origin.…”

“…As a result, the system can only work at a low OD in the Rabi oscillation regime [48]. Figure 17 shows a more robust scheme for generating polarization entanglement for narrowband biphotons in the group delay regime [57]. The pump laser beam is equally split into two beams after the first polarization beam splitter (PBS1).…”