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Register a new userSpectral compression and entanglement reduction in the cascaded biphoton state with cavities
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The cascaded biphoton state generated from a cold atomic ensemble presents one of the strongly correlated resources that can preserve and relay quantum information. Under the four-wave mixing condition, the emitted signal and idler photons from the upper and lower excited states become highly correlated in their traveling directions and entangled in continuous frequency spaces. In this system, we theoretically study the spectral compression of the biphoton source using an external cavity and show the reduction in its frequency entanglement entropy. This indicates, respectively, an improved light absorption efficiency for the idler photon as well as an almost pure biphoton source which is useful in optical quantum networks. We further investigate the limit of the spectral compression that can be achieved by using multiple cavities. Our results show the capability and potential of the biphoton source with external cavities, where the performance of atom-based quantum memory can be enhanced and the entanglement property can be manipulated by tailoring the spectral compression.
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We propose to shape biphoton wave packets with an electromagnetically induced grating in a four-level double-$Lambda$ cold atomic system. We show that the induced hybrid grating plays an essential role in directing the new fields into different angular positions, especially to the zeroth-order diffraction. A number of interesting features appear in the shaped two-photon waveforms. For example, broadening or narrowing the spectrum would be possible in the proposed scheme even without the use of a cavity.
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We propose an experiment to generate deterministic entanglement between separate nitrogen vacancy (NV) centers mediated by the mode of a photonic crystal cavity. Using numerical simulations the applicability and robustness of the entanglement operation to parameter regimes achievable with present technology is investigated. We find that even with moderate cavity Q-factors of $10^{4}$ a concurrence of $c>0.6$ can be achieved within a time of $t_{max}approx150$~ns, while Q-factors of $10^{5}$ promise $c>0.8$. Most importantly, the investigated scheme is relative insensitive to spectral diffusion and differences between the optical transitions frequencies of the used NV centers.
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