9 research outputs found
Coupling of ultrathin tapered fibers with high-Q microsphere resonators at cryogenic temperatures and observation of phase-shift transition from undercoupling to overcoupling
We cooled ultrathin tapered fibers to cryogenic temperatures and controllably
coupled them with high-Q microsphere resonators at a wavelength close to the
optical transition of diamond nitrogen vacancy centers. The 310-nm-diameter
tapered fibers were stably nanopositioned close to the microspheres with a
positioning stability of approximately 10 nm over a temperature range of 7-28
K. A cavity-induced phase shift was observed in this temperature range,
demonstrating a discrete transition from undercoupling to overcoupling
Optical transmittance degradation in tapered fibers
We investigated the cause of optical transmittance degradation in tapered
fibers. Degradation commences immediately after fabrication and it eventually
reduces the transmittance to almost zero. It is a major problem that limits
applications of tapered fibers. We systematically investigated the effect of
the dust-particle density and the humidity on the degradation dynamics. The
results clearly show that the degradation is mostly due to dust particles and
that it is not related to the humidity. In a dust free environment it is
possible to preserve the transmittance with a degradation of less than the
noise (+/- ?0.02) over 1 week
Phase shift spectra of a fiber-microsphere system at the single photon level
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 (\bar{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 \pm
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.Comment: 8 pages, 3 figure
Fiber-microsphere system at cryogenic temperatures toward cavity QED using diamond NV centers
The coupling of a microsphere resonator to a tapered fiber was demonstrated at cryogenic temperatures (8 - 13 K) and investigated with a probe laser light whose frequency around the zero phonon line of nitrogen vacancy centers in diamond (638 nm). For this purpose, a liquid-helium-flow cryostat with a large sample chamber is developed and a resonance dip with a Q of 2 × 10^[6] is observed. The resonance frequency and the coupling condition are found to be stable for a period of one hour