Takahiro Kuga

New High-Efficiency Source of Photon Pairs for Engineering Quantum Entanglement

We constructed an efficient source of photon pairs using a waveguide-type nonlinear device and performed a two-photon interference experiment with an unbalanced Michelson interferometer. As the interferometer has two arms of different lengths, photons from the short arm arrive at the detector earlier than those from the long arm. We find that the arrival time difference (Delta L/c) and the time window of the coincidence counter (Delta T) are important parameters which determine the boundary between the classical and quantum regimes. Fringes of high visibility ( 80% +/- 10%) were observed when Delta T < Delta L/c. This result is explained only by quantum theory and is clear evidence for quantum entanglement of the interferometers optical paths.

Mach-Zehnder Bragg interferometer for a Bose-Einstein condensate

We construct a Mach-Zehnder interferometer using Bose-Einstein condensed rubidium atoms and optical Bragg diffraction. In contrast to interferometers based on normal diffraction, where only a small percentage of the atoms contribute to the signal, our Bragg diffraction interferometer uses all the condensate atoms. The condensate coherence properties and high phase-space density result in an interference pattern of nearly 100% contrast. The two arms of the interferometer may be completely separated in space, making it an ideal tool that can be used to detect vortices or other topological condensate phases.

A simplified 461-nm laser system using blue laser diodes and a hollow cathode lamp for laser cooling of Sr.

We develop a simplified light source at 461 nm for laser cooling of Sr without frequency-doubling crystals but with blue laser diodes. An anti-reflection coated blue laser diode in an external cavity (Littrow) configuration provides an output power of 40 mW at 461 nm. Another blue laser diode is used to amplify the laser power up to 110 mW by injection locking. For frequency stabilization, we demonstrate modulation-free polarization spectroscopy of Sr in a hollow cathode lamp. The simplification of the laser system achieved in this work is of great importance for the construction of transportable optical lattice clocks.

Measurement of ultrafast optical pulses with two-photon interference.

We introduce a new two-photon interference scheme for measurement of the coherence time and pulse width of ultrafast pulses. Experiments were performed with the output pulses of a cw mode-locked dye laser, with results in good agreement with the conventional second-harmonic autocorrelation measurement.

Observation and Analysis of Visible Overtone Band Transitions of NH3

A sensitive spectrometer with a ring dye laser is constructed to observe very weak visible transitions of molecules. The sensitivity is estimated to be 3×10-9/cm in the absorption coefficient. About 1000 high overtone and combination band lines with Δv=5 of NH3 are observed in the frequency range 15260–15590 cm1. Among them, 193 lines are assigned and the rotational constants in the highly excited vibrational states are determined.

Carbon nanotube/polymer composite coated tapered fiber for four wave mixing based wavelength conversion.

In this paper, we demonstrate a nonlinear optical device based on a fiber taper coated with a carbon nanotube (CNT)/polymer composite. Using this device, four wave mixing (FWM) based wavelength conversion of 10 Gb/s Non-return-to-zero signal is achieved. In addition, we investigate wavelength tuning, two photon absorption and estimate the effective nonlinear coefficient of the CNTs embedded in the tapered fiber to be 1816.8 W(-1)km(-1).

Saturated absorption spectroscopy of acetylene molecules with an optical nanofiber

We performed saturated absorption spectroscopy of acetylene (C₂H₂) ν₁ + ν₃ band transitions with an optical nanofiber (ONF). Owing to high-intensity light around the ONF, we observed a Lamb dip at relatively low-power laser (~10 mW) without a cavity. Our results showed that the simple ONF spectrometer is advantageous for performing saturation absorption spectroscopy and serves as a practical low-cost wavelength reference in the optical fiber communication band.

3 dB Wideband squeezing in photon number fluctuations from a light emitting diode

We have observed more than 3 dB squeezing in photon number fluctuations from a light emitting diode (LED) at liquid nitrogen temperature. The noise reduction is uniform within our measurement bandwidth of 1MHz, and its amount agrees with the emission efficiency of the LED. To our knowledge, it is the largest squeezing ever observed using a LED.

Generation of weak sub-Poissonian light by a high-efficiency light-emitting diode

Weak sub-Poissonian light was generated with a high-efficiency light-emitting diode (LED). The lowest intensity of sub-Poissonian light generated in our experiment was about thousand times as small as those of previous experiments. As an injection current to LED decreased, the mean emission efficiency lowered at room temperature and the frequency response worsened at liquid nitrogen temperature. These phenomena were interpreted in terms of trap states.

Collisional relaxation among rotation–inversion levels in the highly excited vibrational state of NH3

Pressure broadening parameters of the visible overtone transitions to the v=5 state in the N–H stretching mode of 14NH3 are measured. It is found that the parameters show good similarity to those of the microwave region both in the rotational quantum number dependence and in the absolute values. Even in the high vibrational state, the collision induced transitions between inversion levels give dominant contributions to the broadening parameters. Well‐defined rotational quantum number dependence obtained in the present analysis also gives confirmation of the previous assignments of rotational structure in the overtone transitions.