Search Results (36 total)

Single photons from a coherent input are efficiently redirected to a separate output by way of a fiber-coupled microtoroidal cavity interacting with individual cesium atoms. By operating in an overcoupled regime for the input-output to a tapered fiber, our system functions as a quantum router with high efficiency for photon sorting. Single photons are reflected and excess photons transmitted, as confirmed by observations of photon antibunching (bunching) for the reflected (transmitted) light. Our photon router is robust against large variations of atomic position and input power, with the observed photon antibunching persisting for intracavity photon number 0.03≲n̅ ≲0.7.

We investigate co-evolving dynamics in a weighted network of phase oscillators in which the phases of the oscillators at the nodes and the weights of the links interact with each other. We find that depending on the type of the dynamics of the weights, the system exhibits three kinds of asymptotic behavior: a two-cluster state, a coherent state with a fixed phase relation, and a chaotic state with frustration. Because of its structural stability, it is believed that our model captures the essential characteristics of a class of co-evolving and adaptive networks.

We report on the first creation of ultracold bosonic heteronuclear molecules of two fermionic species, ⁶Li and ⁴⁰K, by a magnetic field sweep across an interspecies s-wave Feshbach resonance. This allows us to associate up to 4×10⁴ molecules with high efficiencies of up to 50%. Using direct imaging of the molecules, we measure increased lifetimes of the molecules close to resonance of more than 100 ms in the molecule-atom mixture stored in a harmonic trap.

We report on the generation of a quantum degenerate Fermi-Fermi mixture of two different atomic species. The quantum degenerate mixture is realized employing sympathetic cooling of fermionic ⁶Li and ⁴⁰K gases by an evaporatively cooled bosonic ⁸⁷Rb gas. We describe the combination of trapping and cooling methods that proved crucial to successfully cool the mixture. In particular, we study the last part of the cooling process and show that the efficiency of sympathetic cooling of the ⁶Li gas by ⁸⁷Rb is increased by the presence of ⁴⁰K through catalytic cooling. Because of the differing physical properties of the two components, the quantum degenerate ⁶Li-⁴⁰K Fermi-Fermi mixture is an excellent candidate for a stable, heteronuclear system allowing the study of several so far unexplored types of quantum matter.

We demonstrated δ-kick cooling of sodium atoms with a three-dimensional anisotropic harmonic potential. The radius and temperature of the atomic cloud oscillated with kick time, indicating that the elongated atomic distribution in phase space rotates in the Ioffe-Pritchard potential. The temperature of kicked atoms was reduced to below the recoil temperature.

We demonstrate unconditional telecloning for the first time. In particular, we symmetrically and unconditionally teleclone coherent states of light from one sender to two receivers, achieving a fidelity for each clone of F=0.58±0.01, which surpasses the classical limit. This is a manipulation of a new type of multipartite entanglement whose nature is neither purely bipartite nor purely tripartite.

Quantum teleportation of a squeezed state is demonstrated experimentally. Due to some inevitable losses in experiments, a squeezed vacuum necessarily becomes a mixed state which is no longer a minimum uncertainty state. We establish an operational method of evaluation for quantum teleportation of such a state using fidelity and discuss the classical limit for the state. The measured fidelity for the input state is 0.85±0.05, which is higher than the classical case of 0.73±0.04. We also verify that the teleportation process operates properly for the nonclassical state input and its squeezed variance is certainly transferred through the process. We observe the smaller variance of the teleported squeezed state than that for the vacuum state input.

We experimentally demonstrate continuous-variable quantum teleportation beyond the no-cloning limit. We teleport a coherent state and achieve the fidelity of 0.70±0.02 that surpasses the no-cloning limit of 2/3. Surpassing the limit is necessary to transfer the nonclassicality of an input quantum state. By using our high-fidelity teleporter, we demonstrate entanglement swapping, namely, teleportation of quantum entanglement, as an example of transfer of nonclassicality.

Berry’s phase of the atom in the state with a positive or negative g factor for partial cycles of a rotating magnetic field was determined free from the dynamical phase shift using a time-domain atom interferometer. The experimental phase shift is in good agreement with the prediction of Berry’s phase for partial cycles. It was found that the sense of Berry’s phase depends on the sign of the magnetic quantum number, the sense of the rotating magnetic field, and the sign of the g factor of the state.

A Reply to the Comment by Byoungchoo Park et al.

We have developed a polarimetry of ultrashort pulse γ rays based on the fact that γ rays penetrating in the forward direction through a magnetized iron carry information on the helicity of the original γ rays. Polarized, short-pulse γ rays of (1.1±0.2)×10⁶/bunch with a time duration of 31 ps and a maximum energy of 55.9 MeV were produced via Compton scattering of a circularly polarized laser beam of 532 nm off an electron beam of 1.28 GeV. The first demonstration of asymmetry measurements of short-pulse γ rays was conducted using longitudinally magnetized iron of 15 cm length. It is found that the γ-ray intensity is in good agreement with the simulated value of 1.0×10⁶. Varying the degree of laser polarization, the asymmetry for 100% laser polarization was derived to be (1.29±0.12)%, which is also consistent with the expected value of 1.3%.

Based on the requirements from a conceptual design of a polarized positron beam for future linear colliders, we constructed a special collision system with a short focal length of 150 mm of the laser beams so as to produce γ rays through inverse Compton scattering. In order to achieve efficient laser-electron collisions, we created a special optics to produce very small e^--beam sizes of σ_ex0=7.6   μm and σ_ey0=5.4   μm in the horizontal and vertical directions at the collision point. Using laser light with a wavelength of 532 nm and an e^- beam of 1.28 GeV, provided from the ATF-damping ring at KEK, we generated 2×10⁵ γ rays with a time duration of 26 ps in rms, leading to a peak brightness of 1.7×10¹⁸/(mrad² mm² 0.1%bandwidth s) near to the maximum energy of 56 MeV.

A continuous-variable tripartite entangled state is experimentally generated by combining three independent squeezed vacuum states, and the variances of its relative positions and total momentum are measured. We show that the measured values violate the separability criteria based on the sum of these quantities and prove the full inseparability of the generated state.

A time-domain atomic multiple-wave interferometer using laser-cooled and trapped sodium atoms has been developed under pulsed magnetic fields. Each atomic phase was shifted due to the scalar Aharonov-Bohm effect by applying spatially homogeneous pulsed magnetic fields between numerous Raman excitation laser pulses. Interference fringes with a finesse of 11 were demonstrated for 11 successive Raman pulses and ten magnetic-field pulses.

The scalar Aharonov-Bohm effect with a time-dependent magnetic field was investigated for ultracold sodium atoms using the time-domain atom interferometer. The 38 interference fringes with almost the same amplitude verified the nondispersivity of this effect. The measured phase shift as a function of the strength of the applied magnetic field agreed with the predicted one within an accuracy of 3%. With a weak magnetic field orthogonal to the quantization axis, the phase shift was in proportion to the variation of the strength of the resultant magnetic field.

Electron beams with the lowest, normalized transverse emittance recorded so far were produced and confirmed in single-bunch-mode operation of the Accelerator Test Facility at KEK. We established a tuning method of the damping ring which achieves a small vertical dispersion and small x-y orbit coupling. The vertical emittance was less than 1% of the horizontal emittance. At the zero-intensity limit, the vertical normalized emittance was less than 2.8×10^-8 rad m at beam energy 1.3 GeV. At high intensity, strong effects of intrabeam scattering were observed, which had been expected in view of the extremely high particle density due to the small transverse emittance.

Spectrally resolved and time-integrated four-wave mixing are used to measure the polarization dependence of biexcitonic signals and quantum beats between two-A-exciton (X_AX_A^*) and A-biexciton (X_AX_A) states in a high-quality GaN epilayer. Mixed beats with two periods are observed: the first beating period corresponds to the energy splitting between X_AX_A^* and X_AX_A; the second period corresponds to beating between A excitons (X_A) and donor bound excitons (D⁰X). We also measure the polarization-dependent B-biexciton (X_BX_B) signal. The effective masses for the A and B holes are deduced from the binding energy.

Molecular orientational order parameters have been obtained by Raman scattering in two types of liquid crystal materials showing the V-shaped switching in thin homogeneous cells. One is the Mitsui mixture and the other is one component of the Inui mixture. The antiferroelectric phase exists in the bulk of both materials but, in thin homogeneous cells, the stability is distinct from each other. The obtained distribution of the local in-plane directors at the tip of the V is considerably broad in the former, while it is narrow in the latter. These differences have been explained by the barrier between the ferroelectric and antiferroelectric orderings, the chiral twisting power, and the interface induced destruction of the antiferroelectric ordering.

We report on the study of quantum beats in the time-integrated four-wave mixing signal in reflection geometry from a pair of excitons with different valence orbitals in GaN and ZnSe. We observe a π-phase shift between quantum beats in the co- and cross-linear polarization configuration, and nearly 100% modulation of the signal for both materials. In the co-circular polarization configuration, the observed phases of the quantum beats at the frequency of the A exciton in GaN and heavy-hole exciton in ZnSe are 0.2π and 0.1π, respectively. The phases of the quantum beats change sign for co- and counter-circular polarization configurations and also for A- (heavy-hole) and B-(light-hole) excitons in GaN(ZnSe). We describe the third-order coherent optical response of the exciton pair with different valence orbitals by taking into account the finite memory depth of the four-particle correlation. In particular, our experimental findings indicate that excitons with different valence orbitals and equal angular momentum attract each other similar to excitons with the same valence orbitals but opposite angular momentum. Excitons with different orbitals and opposite angular momenta repel one another. The comparison between experimental and theoretical results allows us to develop a quantitative analysis of the four-particle correlation in the presence of an exciton pair with different valence orbitals. We show that the observed π-phase shift and nearly 100% modulation of the signal in the co- and cross-linear polarization configuration impose restraints on the memory functions, which describe the exciton-exciton interaction. These restraints imply, in particular, that electron spins play a more important role in the exciton-exciton interaction in comparison to hole spins. We show that a striking similarity, which we observe in the quantum beat signal from GaN and ZnSe, originates from the strong four-particle correlation contribution to the third-order excitonic nonlinearity.

A pseudo-simple-cubic photonic crystal lattice was fabricated in silicon. The phase-shift spectra and the amplitude spectra of the transmitted electromagnetic wave are studied by Terahertz time-domain spectroscopy measurements. The π phase shifts between Fabry-Perot modulation peaks are clearly observed. Our results are in good correspondence with transfer-matrix calculations. The dispersion curves were qualitatively extracted from the frequency dependence of the phase shifts. The gradient of the phase shift becomes large at the band-gap edges. This suggests that the group velocity is correspondingly small near the Brillouin-zone boundary.

To clarify the thresholdless, hysteresis free V-shaped switching due to frustration between ferro- and antiferroelectricity, we have studied a prototype binary mixture system. The apparent orientational order parameters, 〈P₂〉 and 〈P₄〉, obtained from polarized Raman scattering in thin homogeneous cells indicate that substrate interfaces induce some randomization of local in-plane directors at the tip of the V. Their correlation lengths, ξ_∥≈3.5 nm and ξ_⊥≈75 nm, have been estimated by assuming the Langevin-like reorientation. Because of the much shorter ξ_∥ and ξ_⊥ than the visible light wavelength, the switching process looks uniform.

An atomic multiple-beam interferometer comprised of numerous copropagating stimulated Raman transitions has been developed using laser-cooled and trapped sodium atoms. The interference fringes of the interferometer with multiple rf fields were calculated by a method based on atomic trajectory and were compared with the experimental results. An interference fringe with a finesse of 180 was demonstrated for 200 successive Raman pulses.

Spectrally resolved four-wave mixing measurements in wurtzite GaN show a dependence of the quantum beats' phase on incident polarizations. We observe different phases at the A-exciton and the B-exciton resonances when exciting with circular polarized light. The observed phase difference indicates that exciton-exciton interaction plays a major role in the quantum beat process. The developed analysis allows us to conclude that the spins of the electrons rather than the holes give the major contribution to the exciton-exciton interaction in GaN.

Decreases in both dark conductivity and photoconductivity after intense and prolonged photoirradiation have been observed in the narrow-band-gap amorphous Sb₂Se₃; they are similar to those observed in hydrogenated amorphous silicon (a-Si:H) (the Staebler-Wronski effect). The ac conductivity, on the other hand, decreases after illumination, which is in contrast to that observed in a-Si:H. Unlike a-Si:H, these photoinduced changes are not interpreted in terms of light-induced defect creation. The broadening of energy levels of preexisting thermal-equilibrium defects by illumination could produce all of these photoinduced changes in the present system.

The dc and ac photoconductivities σ_p(0) and σ_p(f) of hydrogenated amorphous germanium have been measured in the temperature range 18–300 K. The ratio σ_p(f)/σ_p(0) for different intensities of illumination follows a universal function of frequency at temperatures below 50 K. The low-temperature photoconductivity is discussed in terms of dispersive diffusion of electrons in band tails.