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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Narrow-band filter for quantum light

Alessandro Ceré, Florian Wolfgramm, M. Napolitano, Morgan W. Mitchell

Published Jun 1, 2009• 1 citation
Publisher

Abstract

Atom-photon interaction promises to be a fundamental resource for applications like quantum computations. For an efficient interaction between light and atoms, the bandwidth of the light should match the natural linewidth of the atom transition to address, 6 MHz in the case of the Di line of rubidium. Cavity-enhanced down-conversion (CEDC) produces bright narrow-band quantum light with suitable linewidth but multimode in frequency. This requires the use of one or more filter cavities. This paper investigates a narrow-band filter which can be used for filtering the spectral mode of interest with a high extinction ratio for the other modes. Combined with a CEDC, this filter provides light for optimal interaction with rubidium atoms.

DOI
10.1109/cleoe-eqec.2009.5191592

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Atom-photon interaction promises to be a fundamental resource for applications like quantum computations. For an efficient interaction between light and atoms, the bandwidth of the light should match the natural linewidth of the atom transition to address, 6 MHz in the case of the Di line of rubidium. Cavity-enhanced down-conversion (CEDC) produces bright narrow-band quantum light with suitable linewidth but multimode in frequency. This requires the use of one or more filter cavities. This paper investigates a narrow-band filter which can be used for filtering the spectral mode of interest with a high extinction ratio for the other modes. Combined with a CEDC, this filter provides light for optimal interaction with rubidium atoms.
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Cavity-enhanced spontaneous parametric down-conversion is capable of efficient generation of single photons with suitable spectral properties for interfacing with the atoms. However, beside the remarkable progress of this technique, multimode longitudinal emission remains a major drawback. Here, we demonstrate a bright source of single photons that overcomes this limitation by a novel mode-selection technique based on the introduction of an additional birefringent element to the cavity. This enables us to tune the double resonance condition independent of the phase matching and thus to achieve single-mode operation without mode filters. Our source emits single-frequency-mode photons at 852 nm, which is compatible to the Cs D2 line, with a bandwidth of 10.9 MHz and a detected photon-pair rate of 2.5 kHz at 10 mW of pump power, while maintaining g(2)(0) = 0.13. The efficiency of our source is further underlined by detecting a four-photon rate of 0.28 Hz at 20 mW of pump power. These detected rates correspond to a photon-pair generation rate of 47.5 Hz, and a four-photon generation rate of 37 Hz. Such photon generation rates open up a variety of new applications reaching from hybrid light-matter interactions to optical quantum information tasks based on long temporal coherence.
There has been increasing interest in the generation of such photon pairs using a spontaneous parametric down-conversion (SPDC) source inside an optical cavity to enhance the down-conversion into modes resonant to the cavity, each of the modes having a MHz spectrum. These optical parametric oscillators (OPO) operated far below threshold also provide a well defined spatial mode. Our setup consists of a bow-tie cavity with a periodically poled KTP crystal as nonlinear medium, phase-matched for parametric down-conversion. This photon-pair source is designed to provide photons suitable for interaction with atoms. These photons could be directly used in applications towards a quantum memory of light, like in experiments on slow light or the storage of single photons in EIT media. In this way an optical polarization rotation in an atomic sample could be measured achieving super-resolution.
We present a comparison between lens cavity filters and atomic line filters, discussing their relative merits for applications in quantum optics. We describe the design, characterization, and stabilization procedure of a lens cavity filter, which consists of a high-reflection coated commercially available plano-convex lens, and compare it to an ultra-narrow atomic band-pass filter utilizing the D2 absorption line in atomic rubidium vapor. We find that the cavity filter peak transmission frequency and bandwidth can be chosen arbitrarily but the transmission frequency is subject to thermal drift and the cavity needs stabilization to better than a few mK, while the atomic filter is intrinsically stable and tied to an atomic resonance frequency such that it can be used in a non-laboratory environment.