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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Polarization-purity spectra of a tapered-fiber-coupled microsphere cavity system at cryogenic temperatures

Masazumi Fujiwara, Akira Tanaka, Kiyota Toubaru, Hong-Quan Zhao, Hideaki Takashima, Shigeki Takeuchi

Published Feb 2, 2011• 0 citations

Abstract

An ultrahigh-Q optical microcavity coupled with a tapered fiber is an ideal system for the cavity quantum electrodynamics (CQED). In particular realizing this system at cryogenic temperature is vitally important and has been recently explored for various CQED applications including solid-state atom-photon strong coupling, vibrational mode cooling, and photonic quantum gates. These cryogenic fiber-coupled microcavity systems, however, suffer from mechanical vibrations due to cooling systems and distortions caused by large temperature change. These factors may cause the degradation in polarization of probe light field in the system. Here we report the analysis of the polarization state in a tapered-fiber-coupled microsphere cavity at cryogenic temperatures. By scanning the wavelength of the probe light at around 637 nm, which can be used for the diamond nitrogen vacancy centers, the spectral analysis of the polarization state was performed at 8-30 K. We have found that the degree of polarization (DOP, classical analogue of purity) at cryogenic temperatures does not show significant change compared to that measured at room temperature. This fact indicates that the system can conserve the polarization at low temperature to the extent comparable to that at room temperature, which is enough for the evaluation of the quantum phase gate.

DOI: 10.1117/12.874587

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We succeeded in measuring phase shift spectra of a microsphere cavity coupled with a tapered fiber using a weak coherent probe light at the single photon level. We utilized a tapered fiber with almost no depolarization and constructed a very stable phase shift measurement scheme based on polarization analysis using photon counting. Using a very weak probe light (n = 0.41), we succeeded in observing the transition in the phase shift spectrum between undercoupling and overcoupling (at gap distances of 500 and 100 nm, respectively). We also used quantum state tomography to obtain a 'purity spectrum'. Even in the overcoupling regime, the average purity was 0.982 ± 0.024 (minimum purity: 0.892), suggesting that the coherence of the fiber-microsphere system was well preserved. Based on these results, we believe this system is applicable to quantum phase gates using single light emitters such as diamond nitrogen vacancy centers.

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We report the direct integration and efficient coupling of nitrogen vacancy (NV) color centers in diamond nanophotonic structures into a fiber-based photonic architecture at cryogenic temperatures. NV centers are embedded in diamond micro-waveguides (μWGs), which are coupled to fiber tapers. Fiber tapers have low-loss connection to single-mode optical fibers and hence enable efficient integration of NV centers into optical fiber networks. We numerically optimize the parameters of the μWG-fiber-taper devices designed particularly for use in cryogenic experiments, resulting in 35.6% coupling efficiency, and experimentally demonstrate cooling of these devices to the liquid helium temperature of 4.2 K without loss of the fiber transmission. We observe sharp zero-phonon lines in the fluorescence of NV centers through the pigtailed fibers at 100 K. The optimized devices with high photon coupling efficiency and the demonstration of cooling to cryogenic temperatures are an important step to realize fiber-based quantum nanophotonic interfaces using diamond spin defect centers.