Takahiro Kuga

Takahiro Kuga
The University of Tokyo | Todai · College of Art and Science & Graduate School of Arts and Sciences

Doctor of Science

About

70
Publications
3,736
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2,459
Citations

Publications

Publications (70)
Article
The study of decoherence properties improves our understanding of the fundamental principles of quantum mechanics and advances the study of quantum information processing. Herein, we report a wave-optical experiment that can simulate the decoherence process of a Schrödinger’s cat state (SCS) by photon loss. The method is based on an analogy between...
Article
Full-text available
Continuous-variable (CV) qubits can be created on an optical longitudinal mode in which quantum information is encoded by the superposition of even and odd Schroedinger's cat states with quadrature amplitude. Based on the analogous features of paraxial optics and quantum mechanics, we propose a system to generate and detect CV qubits on an optical...
Article
Full-text available
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 injec...
Article
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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...
Article
Stable, phase-locked, sub 300fs soliton pairs and triplets are generated in a dispersion-managed mode-locked fiber laser using a taper fiber coated with a carbon nanotube (CNT)-polymer as a saturable absorber.
Article
Photodetachment of the positronium negative ion, a bound state of one positron and two electrons, has been observed. Development of a method to produce the ions efficiently using a Na coated tungsten surface has enabled the first observation of the photodetachment. The obtained lower limit of the photodetachment cross section for the wavelength of...
Article
We performed saturated absorption spectroscopy of acetylene (C2H2) nu(1) + nu(3) 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 (similar to 10mW) without a cavity. Our results showed that the simple ONF spectrometer is advantageous for performing s...
Conference Paper
We demonstrate a nonlinear optical device based on a fiber taper coated with a carbon nanotube/polymer composite. Four wave mixing-based wavelength conversion of 10 Gb/s Non-return-to-zero signal with the coated fiber-taper nonlinear device is achieved. Wavelength tuning is also investigated.
Article
Full-text available
We measured the intensity correlation of true thermal light scattered from cold atoms in an optical molasses. Using a single-mode fiber as a transverse mode filter, measurement with maximally high spatial coherence was realized, allowing us to observe ideal photon bunching with unprecedented precision. The measured intensity correlation functions s...
Article
Our recent work has demonstrated the superradiant writing and reading of collective coherence in a Bose-Einstein condensate [ Phys. Rev. Lett. 99 220407 (2007)]. This time we report the drastic improvement of the storage time realized by loading the atoms into an optical dipole trap, wherein an unfavorable spin-dependent phase shift and spatial dif...
Article
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Radiation force induced by an electromagnetic field stored in a two-dimensional photonic crystal slab is examined. We considered two different types of photonic crystal cavities, namely, double-layer slab cavity structures and air-slot cavity structures. With double-layer structures, induced force is attractive or repulsive depending on the spatial...
Article
Full-text available
We have presented a novel design of a photonic crystal slab (PCS) nanocavity, in which the electric field of the cavity mode is strongly localized in free space. The feature of the cavity is a linear air slot introduced to the center of the mode-gap confined PCS cavity. Owing to the discontinuity of the dielectric constant, the electric field of th...
Article
Full-text available
We demonstrate superradiant conversion between a two-mode collective atomic state and a single-mode light field in an elongated cloud of Bose-condensed atoms. Two off-resonant write beams induce superradiant Raman scattering, producing two independent coherence gratings with different wave vectors in the cloud. By applying phase-matched read beams...
Article
Full-text available
Superradiant light scattering from noncondensed, thermal atomic vapors was experimentally studied. We found that superradiant gain is independent of quantum degeneracy and determined only by the shape of the atomic cloud and a contained number of atoms. Superradiant pump-probe spectroscopy was also developed to measure the atomic correlation functi...
Article
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The superradiant Raman scattering in Bose-Einstein condensate which was irradiated by single off-resonant light beam was investigated. The probability of Raman scattering into particular Zeeman sublevel in F=1 state was considered to be greater than that of any other transitions. The probabilistic optical pumping was converted to stimulated Raman p...
Article
Full-text available
We present a simple modulation-free technique to stabilize a laser frequency to the Doppler-free spectra of an atomic vapor. Polarization spectroscopy with use of a balanced polarimeter allows us to obtain a background-free dispersion signal suitable for high-speed and robust frequency stabilization. We employed the method to the 5S(1/2) F = 2 -->...
Article
Propagation of a light pulse through a high-Q optical microcavity containing a few cold atoms (N<10) in its cavity mode is investigated experimentally. With less than ten cold rubidium atoms launched into an optical microcavity, up to 170 ns propagation lead time ("superluminal"), and 440 ns propagation delay time (subluminal) are observed. Compari...
Article
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We experimentally demonstrate the transformation from two-photon time entanglement to polarization entanglement using unbalanced interferometers. The polarization states of down-converted photon pairs from the interferometer depend on the path they have experienced. Therefore, the polarization entanglement is achieved by the coincidence measurement...
Article
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Quantum computers promise great improvements in solving problems such as factoring large integers, simulating quantum systems, and database searching. Using a photon as a quantum bit (qubit) is one of the most promising ways to realize a universal quantum computer because the coherent superposition state of a photon is very robust against various s...
Article
Full-text available
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 a...
Article
Full-text available
Wideband, highly noise-suppressed squeezing was observed by using a high-speed, high-quantum-efficiency light-emitting diode. The squeezing bandwidth extended over 200 MHz. We also have investigated the dependence of the squeezing bandwidth on the pump-current at low temperature. The experimental result was compared with the theoretical predictions...
Article
Full-text available
Optical amplifiers, such as pulsed-dye amplifiers, are key components in many laser systems and have been responsible for many important scientific advances. In such devices the optical field amplitude of a 'seed' laser pulse is amplified while its phase coherence is preserved. In the field of coherent matter waves it has long been speculated, in d...
Article
Summary form only given. The strong atom-photon coupling generated inside high-finesse optical micro cavity lets us detect a single atom trajectory in real time. Such a strong coupling is a candidate for trapping a single atom with a single photon. Dynamics of trapped atoms can be observed in situ and in real time by detecting intensity change on t...
Article
Quantum entangled state attracts a wide range of interest for potential application in novel communication and computation schemes. We propose the multi-term two-particle entangled state. To generate photon pairs, we made use of a waveguide type nonlinear device fabricated on a 1-cm-long LiNbO3 substrate. This device is originally developed for gen...
Article
Full-text available
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 propertie...
Conference Paper
We constructed a Mach-Zehnder interferometer for Bose-Einstein condensed rubidium atoms using optical Bragg diffraction. An interference pattern of almost 100% visibility was obtained. This scheme permits novel studies of condensate phase properties
Conference Paper
The generation of photon-number squeezed states or sub-Poissonian light using semiconductor devices is especially attractive because of its simple experimental configuration, small energy consumption, and the possibility of large amplitude noise reduction by using high quantum efficiency devices. Recently, 3 dB squeezing at liquid nitrogen temperat...
Conference Paper
Since the first experimental demonstration of Bose-Einstein condensation (BEC) in an alkali-metal vapor, this subject has been attracting remarkable interests in the physics society. Hundreds of theoretical works concerning this subject have been reported so far. However, the number of reports about BEC experiments is much less than that of the the...
Article
Full-text available
We utilized a blue-detuned Laguerre-Gaussian (doughnut) laser beam to trap cold rubidium atoms by optical dipole force. ”Pulsed” polarization gradient cooling was applied to the trapped atoms to suppress the trap loss due to heating caused by random photon scattering of the trapping light. In this trap about 108 atoms were initially captured and th...
Article
We have measured the squeezing bandwidth as a function of the operating temperature. The experimental setup is shown. A LED (Hitachi HE8812SG), which is a GaAlAs LED with a double heterojunction structure, is connected through a metal-film resistor (R) to a constant voltage power supply, and generates sub-Poissonian light
Article
We demonstrate experimentally and numerically that a laser system can be synchronized to any chaotic pulsation intrinsic to the system by modulating a system parameter. A sequence of pulses is prepared, the heights and interpulse intervals of which are artificially constructed from the correlation function of a prerecorded chaotic pulsation. When a...
Article
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.
Article
Full-text available
We have constructed a novel optical trap for neutral atoms by using a Laguerre-Gaussian (doughnut) beam whose frequency is blue detuned to the atomic transition. Laser-cooled rubidium atoms are trapped in the dark core of the doughnut beam with the help of two additional laser beams which limit the atomic motion along the optical axis. About 108 at...
Article
We have investigated the intensity squeezing bandwidth of sub-Poissonian light generated by a high-speed light-emitting diode. The squeezing bandwidth of a constant-current-driven light-emitting diode is proportional to a driving current in a lower-current regime and approaches a constant in a higher-current regime. This dependence on driving curre...
Article
Complete transparency via effect of quantum interference in a A-type three-level atomic system was observed by using laser-cooled rubidium atoms. We also demonstrated inversionless amplification of weak probe laser in this system.
Article
We find that photons from a light emitting diode injected with Poissonian electrons have superPoissonian statistics when the differential efficiency is larger than the mean emission efficiency of the device. This result shows the limitation of a conventional theory which describes the generation of sub-Poissonian light in terms of Poisson point pro...
Article
We investigate spectroscopic properties of cold 85Rb atoms in a magneto-optic trap. Both the transmission and the reflection spectra of the trapped atoms are recorded. The dressed-atom picture and the density-matrix formalism are appropriately used to explain the absorption profile around the F=3-F′=2,3,4 and F=2-F′=1 transitions. Around the trappi...
Article
We show that a chaotic pulsation in the passive Q-switched ${\mathrm{CO}}_{2}$ laser system can be made to synchronize to its prerecorded history. A signal based only on the prerecorded history is used to modulate the saturable absorber instead of the actual difference between the prerecorded signal and the current signal. Numerical calculation sho...
Article
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 r...
Article
Full-text available
Thermal muonium (positive muon mu+ and electron e-) atoms were produced from the surface of a hot (2300 K) and high purity tungsten foil placed at the primary beam of 500 MeV pulsed protons. Resonant ionization 1s-->2p-->unbound of the thermal muonium with pulsed laser sources was achieved with reference to thermal hydrogen isotopes (D and T) so as...
Article
We report on the generation of intense pulsed VUV laser radiation of the Lyman-α wavelength of hydrogen-like atoms and muonium by a four wave mixing process in a mixture of krypton and argon gas. The pulse energy was of the order of ∼ 1 μJ/pulse at a 20 Hz repetition rate. The 200 GHz band-width corresponds to the Doppler width of muonium at 2300 K...
Article
A concept, a construction and an account of commissioning experiments up to May 1993 are given for the recently completed ultra-slow positive muon facility at the pulsed muon facility of UT-MSL/KEK. The intense and ultra-slow + beam with an extremely small phase-space volume to be produced in this facility will open new muon sciences includingSR st...
Article
The time evolution of the luminescence of the colored form of a furylfulgide dispersed at various concentrations in a poly(methyl methacrylate) film was measured as a function of the luminescence photon energy. The observed decay time of the luminescence is about 1–2 ns and one order of magnitude shorter than the radiative lifetime (14 ns) estimate...
Article
A two-photon coincidence counting measurement is reported of a squeezed pulse train from a parametric amplifier pumped by a mode-locked laser. The intensity correlation function at zero time delay can be determined by measuring the ratio of two types of the coincidences, the one within the same mode-locked pulse and the one among different pulses....
Article
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.
Article
A concept, a design, a construction and an account of commissioning experiments are given for the recently completed ultra-slow muon facility at the pulsed muon facility of UT-MSL/KEK. The intense (more than 103/s) slow + beam with an extremely narrow phase-space volume (0.2 eV(3 cm)2) to be produced in this facility will open a new muon science in...
Article
A new two-photon interference method to measure coherence time and pulse width of ultrashort pulses is proposed. Experiments are performed on cw-modelocked dye-laser pulses.
Article
We have studied the time-integrated spectra and temporal behavior of Z3-exciton-polariton luminescence in the temperature range from 4 to 88 K under excitation at the Z1,2-exciton absorption region. The temperature-dependent spatial motion of polaritons at each temperature modifies the temporal behavior of polariton luminescence significantly. The...
Article
Full-text available
We have studied the spectra and the temporal behavior of the emissions from biexcitons in CuCl at 4-65 K created via two-photon absorption under weak-excitation conditions using high-repetition-rate (82-MHz) picosecond tunable uv light. Temperature dependences of the interband- and the intraband-relaxation rate of the biexciton are obtained. These...
Article
The relaxation mechanisms of biexcitons under weak excitation limit are studied in the temperature region 1.6 to 50 K. From the temporal responses of biexciton emissions, the biexciton radiative relaxation time is found to be 50 ps. Single acoustic phonon scattering determines the intraband relaxation at finite temperature. Single acoustic phonon e...
Chapter
This chapter describes bond localization of molecular vibration. Several vibrational quanta tend to localize in a bond and each bond oscillates independently in higher vibrational states. The vibrational characteristics in the high energy level are quite different from those in the ground state; the collisional dynamics in well-defined rotational l...
Article
Full-text available
We have observed at 4.2 K the emission of biexcitons in CuCl (the M emission) created via two-photon absorption under a weak excitation condition by means of a high-repetition-rate (82-MHz) picosecond-tunable uv light source obtained by sum frequency generation. The spectral shape and the time response are measured. The results are quite different...
Article
Full-text available
We have achieved reduction of the power fluctuation of the second harmonic of a cw mode-locked YAG laser from 5% to less than 0.1% using a feedback loop with an acoustic loss modulator inside the laser cavity. By using the second harmonic of this laser as a pumping source, we have also reduced the output power fluctuation of a synchronously pumped...
Article
We have developed a high repetition rate tunable uv picosecond light source by a sum frequency generation technique, a light source which eanbles us to observe transient phenomena without high density excitation effects. We have applied this light to the spectroscopy of CuCl and obtained several new resutls: pulsed propagation of polariton luminesc...
Article
Full-text available
Pulsed luminescence is observed in time- and frequency-resolved measurements of the Z(3) exciton luminescence of CuCl single crystals under excitation in the highly absorptive region of the Z(12) exciton resonance. A high-repetition-rate tunable picosecond UV light source was used. The pulse traveling time coincides well with the time calculated fr...
Article
Full-text available
We present evidence for the excitation of the two-photon Doppler-free 1S-2S transition in muonium. Thermal muonium atoms were produced in vacuum by a 27.5-MeV/c pulsed positive-muon beam incident on a powdered SiO/sub 2/ target. Transitions were induced by the light from a frequency-doubled pulsed dye laser and were detected by photoionization of t...
Article
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 ind...
Chapter
Muonium, the bound state of a positive muon and an electron, is one of the simplest systems for testing QED. Spectroscopic measurements in muonium are particularly attractive since the atom does not possess the proton structure effects present in hydrogen. Positronium is free of nuclear structure offsets, but the relativistic two-body problem prese...
Chapter
Molecules in highly excited vibrational states have been studied with great interest in recent years. Their energy level structure and dynamic behavior are directly concerned with mechanisms of multiphoton dissociation, chemical reaction, relaxation in the lasing medium, and so on. A localization of vibration energy into a particular bond may take...
Article
A sensitive spectrometer was constructed in order to observe very weak visible absorption lines of molecules using a ring dye laser, a multiple-pass absorption cell, and a Stark modulation technique. The minimum detectable absorption coefficient is 3× 10-9/cm and is limited by excess laser-radiation noise of the ring dye laser. This system has been...
Article
Full-text available
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...
Article
An intracavity time-resolved spectroscopic method has been developed for studies of collisional relaxation mechanisms associated with very weak infrared transitions. The Stark pulse applied on absorptive molecules modulates the loss in the laser cavity. Transient behaviors of the laser output provide information on the molecular relaxation. The exp...

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