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Scientific Reports

Coherence time limit of the biphotons generated in a dense cold atomcloud

Zhiguang Han, Peng Qian, Lu Zhou, J. F. Chen, Weiping Zhang

Published Mar 17, 2015• 37 citations
Publisher

Abstract

Biphotons with narrow bandwidth and long coherence time can enhance light-atom interaction, which leads to strong coupling between photonic and atomic qubits. Such strong coupling is desirable in quantum information processing, quantum storage and communication. In particular, paired photons with a long coherence time over submicroseconds facilitate the direct manipulation of biphoton wavefunction. In this paper, we report the narrow-band biphotons with a coherence time of 2.34 μs generated from spontaneous four-wave mixing (SFWM) in a dense cold atom cloud, in which the anti-Stokes photons go through a narrow electromagnetically-induced transparency (EIT) window. In our knowledge, this is the best record of coherence time for paired photons achieved so far. A number of factors limiting the coherence time are analyzed in detail. We find the EIT coherence plays an essential role in determining the coherence time for paired photons. The EIT dephasing rate is the ultimate limit to the coherence time, and an ultra-long coherence time above ten microseconds is possible by further improvement of the dephasing rate below 100 kHz.

DOI
10.1038/srep09126

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Biphotons with narrow bandwidth and long coherence time can enhance light-atom interaction, which leads to strong coupling between photonic and atomic qubits. Such strong coupling is desirable in quantum information processing, quantum storage and communication. In particular, paired photons with a long coherence time over submicroseconds facilitate the direct manipulation of biphoton wavefunction. In this paper, we report the narrow-band biphotons with a coherence time of 2.34 μs generated from spontaneous four-wave mixing (SFWM) in a dense cold atom cloud, in which the anti-Stokes photons go through a narrow electromagnetically-induced transparency (EIT) window. In our knowledge, this is the best record of coherence time for paired photons achieved so far. A number of factors limiting the coherence time are analyzed in detail. We find the EIT coherence plays an essential role in determining the coherence time for paired photons. The EIT dephasing rate is the ultimate limit to the coherence time, and an ultra-long coherence time above ten microseconds is possible by further improvement of the dephasing rate below 100 kHz.
Driven by the desire of narrow-band photon pairs source, in this Mphil thesis work, we demonstrate the generation of photon pairs with controllable coherence time up to 1.72 microseconds, using spontaneous four-wave mixing with electromagnetically induced transparency in a laser-cooled atomic ensemble with an optical depth (OD) as high as 130. At such a high OD, we find that the pump laser intensity profile has a significant effect on the time-frequency entangled two-photon waveform. We also confirm the quantum particle nature of heralded narrow-band single photons generated from this source.
We explore a mechanism for producing time-frequency entangled photon pairs (termed a biphoton) from an ensemble of atom-like solid-state quantum emitters. Four distinct energy levels of the solid-state system render four spin-conserving optical transitions as observed in color centers. This feature opens up the possibility to generate a four-wave mixing biphoton based on an electromagnetic induced transparency (EIT) for long-coherence quantum communication as demonstrated in cold atomic systems. We propose a narrow EIT window below a lifetime-limited linewidth of a SiV− in diamond, assuming a few hundred MHz. Consequently, the EIT-induced narrowband guarantees biphoton coherence time to be at least a few tens of a nanosecond without a cavity. Assessing the criteria of solid-state parameters applicable to the existing biphoton model from cold atoms will accelerate solid-state biphoton source research. This study shows that a realization of negligible ground state dephasing of a solid-state sample will be a crucial step toward a solid-state biphoton generation for more than a 100 ns time scale with a subnatural atomic linewidth of a few MHz.
Biphotons are entangled photon pairs that cannot be described as a product state of two individual photons. In this talk we summarize our recent progress in generating narrowband biphotons from spontaneous four-wave mixing (SFWM) in cold atoms with electromagnetically induced transparency (EIT). Here we are particularly interested in the group delay regime, where the biphoton time-frequency entanglement and joint wave function can be manipulated by controlling the group-delay-induced phase matching condition of the paired photons. The biphoton coherence time, thus its bandwidth of the joint spectrum, is determined by the relative group delay between the paired photons. The longest biphoton coherence time on record is more than 3 µs. We find that for a fixed optical depth, the twophoton waveform is independent of the atomic density distribution. However, the spatial profiles of the two driving laser beams have significant effects on the biphoton temporal wavefunction. We demonstrate a technique of shaping the biphoton temporal wavefunction by directly mapping the spatial profile of the pump laser beam onto the biphoton temporal waveform. This spatial-to-temporal mapping, or series-toparallel conversion, allows for coding the spatial information of the pump spatial profile to the sequential temporal modes of the biphtoon quantum state. Using a computer-controlled spatial light modulator, we produced biphotons with temporal waveforms of Gaussian, exponential, and double-peak shapes. As compared to temporally modulating the driving laser fields that reduces the frequency entanglement, this spatial-to-temporal biphoton shaping does not scarify the time-frequency entanglement and can be used to produce heralded narrowband single photons with controllable waveforms.
Narrow-band time-frequency entangled biphotons are generated from spontaneous four-wave mixing in cold atom clouds. The coherence time of the entangled biphotons can be extended to sub-microseconds by the slow light effect. The temporal wavefunction of the biphotons can be manipulated through modulating the spectral or spatial mode of the controlling laser beams. Concerning a pair of entangled biphoton and the resulting heralded single photon, it was commonly believed that, time-frequency entanglement damages the temporal purity of the single photon. However, the case is totally different for biphotons with long coherence time which is far beyond the time resolution of single-photondetectors. We demonstrate that, the single photon heralded from these time-frequency entangled biphotons is in a pure temporal state. Therefore, single photons are able to be shaped through the time-frequency entanglement with their partner photons, while the single photons could be found to be still in a pure state and thus useful for quantum information processing and communication technology.
The generation of nonclassical photon pairs with a long coherence time is key for applications that range from fundamental to quantum communication and metrology. Spontaneous four-wave mixing with electromagnetically induced transparency has been demonstrated as one of the most efficient methods; however, narrowing the bandwidth and producing photon pairs with a temporal length beyond 1 μs remains a technical challenge due to noise considerations and the need for cold atoms with a high optical depth (OD). In this work, we demonstrate the generation of narrowband photon pairs with a controllable coherence time up to 1.72 μs in a laser-cooled atomic ensemble with an OD as high as 130. At such a high OD, we find that the pump laser field spatial profile has a significant effect on the time–frequency entangled two-photon waveform. We also confirm the quantum particle nature of heralded narrowband single photons generated from this source.