Hong-Ou-Mandel interference of two phonons in trapped ions.

The quantum statistics of bosons and fermions manifest themselves in the manner in which two indistinguishable particles interfere quantum mechanically. When two photons, which are bosonic particles, enter a beam-splitter with one photon in each input port, they bunch together at either of the two output ports. The corresponding disappearance of the coincidence count is the Hong-Ou-Mandel effect. Here we show the phonon counterpart of this effect in a system of trapped-ion phonons, which are collective excitations derived by quantizing vibrational motions that obey Bose-Einstein statistics. We realize a beam-splitter transformation of the phonons by employing the mutual Coulomb repulsion between ions, and perform a two-phonon quantum interference experiment using that transformation. We observe an almost perfect disappearance of the phonon coincidence between two ion sites, confirming that phonons can be considered indistinguishable bosonic particles. The two-particle interference demonstrated here is purely a quantum effect, without a classical counterpart, hence it should be possible to demonstrate the existence of entanglement on this basis. We attempt to generate an entangled state of phonons at the centre of the Hong-Ou-Mandel dip in the coincidence temporal profile, under the assumption that the entangled phonon state is successfully generated if the fidelity of the analysis pulses is taken into account adequately. Two-phonon interference, as demonstrated here, proves the bosonic nature of phonons in a trapped-ion system. It opens the way to establishing phonon modes as carriers of quantum information in their own right, and could have implications for the quantum simulation of bosonic particles and analogue quantum computation via boson sampling.

Measuring the charge density of a tapered optical fiber using trapped microparticles.

We report the measurements of charge density of tapered optical fibers using charged particles confined in a linear Paul trap at ambient pressure. A tapered optical fiber is placed across the trap axis at a right angle, and polystyrene microparticles are trapped along the trap axis. The distance between the equilibrium position of a positively charged particle and the tapered fiber is used to estimate the amount of charge per unit length of the fiber without knowing the amount of charge of the trapped particle. The charge per unit length of a tapered fiber with a diameter of 1.6 µm was measured to be 2-1+3×10-11 C/m.

Experimental realization of a quantum phase transition of polaritonic excitations.

We report an experimental realization of the Jaynes-Cummings-Hubbard model using the internal and radial phonon states of two trapped ions. An adiabatic transfer corresponding to a quantum phase transition from a localized insulator ground state to a delocalized superfluid (SF) ground state is demonstrated. The SF phase of polaritonic excitations characteristic of the interconnected Jaynes-Cummings (JC) system is experimentally explored, where a polaritonic excitation refers to a combination of an atomic excitation and a phonon interchanged via a JC coupling.

Generation of a decoherence-free entangled state using a radio-frequency dressed state.

We propose the generation of entangled states with trapped calcium ions using a combination of an rf dressed state and a spin-dependent force. By using this method, a decoherence-free entangled state of rf qubits can be directly generated, and ideally its fidelity is close to unity. We demonstrate an rf entangled state with a fidelity of 0.68±0.08, which has a coherence time of more than 200 ms by virtue of the fact that it is an eigenstate with energy gaps between adjacent levels. Using the same technique, we also produce a qutrit-qutrit entangled state with a fidelity of 0.77±0.09, which exceeds the threshold value for separability of 2/3.

Generation of Dicke states with phonon-mediated multilevel stimulated Raman adiabatic passage.

We generate half-excited symmetric Dicke states of two and four ions. We use multilevel stimulated Raman adiabatic passage whose intermediate states are phonon Fock states. This process corresponds to the spin squeezing operation and half-excited Dicke states are generated during multilevel stimulated Raman adiabatic passage. This method does not require local access for each ion or the preparation of phonon Fock states. Furthermore, it is robust since it is an adiabatic process. We evaluate the Dicke state using a witness operator and determine the upper and lower bounds of the fidelity without using full quantum tomography.

Molecular characterization of MHC class I and beta-2 microglobulin in a clonal strain of ginbuna crucian carp, Carassius auratus langsdorfii.

Clonal ginbuna crucian carp is, a naturally gynogenetic fish, and is a useful model animal for studying T-cell-mediated immunity. To gain molecular information on MHC class I molecules from this species, we have identified four types of MHC class I (caauUA-S3n, caauUF-S3n, caauZE-S3n, and caauZB-S3n) and five beta 2-microglobulin (ß(2)m) (caauß2m-1a, caauß2m-1b, caauß2m-2, caauß2m-3a and caauß2m-3b) by an expressed sequence tag (EST) analysis and using homology cloning with degenerated primers. Like UA class I genes in other cyprinid fish, the caauUA-S3n shows features of classical MHC class I, such as conservation of all key amino acids interacting with antigenic peptides, and ubiquitous tissue expression. A phylogenetic analysis shows that the ß(2)m-1 and ß(2)m-2 isoforms are clustered with those of other cyprinid fishes, while ß(2)m-3 isoforms make a cluster that is separated from a common ancestor of salmonid and cyprinid fishes. This finding suggests that the ß(2)m isoforms of ginbuna cruician carp comprise two lineages and may possess different functions. The MHC class I and ß(2)m sequences from one clonal strain will facilitate our understanding of the interaction of MHC class I with ß(2)m in teleosts.

Frequency Measurement System of Optical Clocks Without a Flywheel Oscillator.

We developed a system for the remote frequency comparison of optical clocks. The system does not require a flywheel oscillator at the remote end, making it possible to evaluate optical frequencies even in laboratories, where no stable microwave reference, such as an Rb clock, a Cs clock, or a hydrogen maser exists. The system is established by the integration of several systems: a portable carrier-phase two-way satellite frequency transfer station and a microwave signal generation system by an optical frequency comb from an optical clock. The measurement was as quick as a conventional method that employs a local microwave reference. We confirmed the system uncertainty and instability to be at the low 10-15 level using an Sr lattice clock.

Aharonov-Bohm effect in the tunnelling of a quantum rotor in a linear Paul trap.

Quantum tunnelling is a common fundamental quantum mechanical phenomenon that originates from the wave-like characteristics of quantum particles. Although the quantum tunnelling effect was first observed 85 years ago, some questions regarding the dynamics of quantum tunnelling remain unresolved. Here we realize a quantum tunnelling system using two-dimensional ionic structures in a linear Paul trap. We demonstrate that the charged particles in this quantum tunnelling system are coupled to the vector potential of a magnetic field throughout the entire process, even during quantum tunnelling, as indicated by the manifestation of the Aharonov-Bohm effect in this system. The tunnelling rate of the structures periodically depends on the strength of the magnetic field, whose period is the same as the magnetic flux quantum φ0 through the rotor [(0.99 ± 0.07) × φ0].

Acceleration effect of sulfate ion on the dissolution of amorphous silica.

The dissolution rate of amorphous silica is enhanced by sulfate ions. The zeta potential for silica particles in Na(2)SO(4) solution was lower than that in NaCl solution with the same ionic strength. These facts indicate that the specific adsorption of sulfate ions occurred by overcoming repulsion between negative charges of the SO(4)(2-) ion and SiO(-) on the surface of silica. The dissolution rate of amorphous silica may be accelerated by the specific adsorption of SO(4)(2-) ions because of a decrease in the strength of the [triple bond]Si-O-Si[triple bond] bond in amorphous silica due to donation of electron density from the adsorbed SO(4)(2-) ions.

Thin-cell sub-Doppler spectroscopy by spatially separated beam method and pump-probe method.

We experimentally demonstrate two methods that improve the resolution of sub-Doppler spectroscopy using a 1 mm thick vapor cell. The linewidths of the observed spectra are approximately 1 order of magnitude narrower than the Doppler width. The first method involves using a 1 mm thick cell filled with Rb atomic vapor and two spatially separated laser beams. By employing the same principle, we also demonstrate that it is possible to achieve the same resolution by using the pump and probe pulses of a single beam. The latter method enables us to construct a simple and robust optical setup for sub-Doppler spectroscopy.

Phase-locked laser system for a metastable states qubit in 40 Ca+.

We describe the development of a phase-locked laser system tailored to an ion-trap-based quantum information processor with (40)Ca(+). Laser outputs from an extended cavity diode laser and a Ti:sapphire laser with output laser wavelengths of approximately 850 and 854 nm, respectively, were phase locked and used to excite a Raman transition between the D(3/2) and D(5/2) metastable states qubit. Development and the performance of the laser system are described. We also compare the characteristics and the benefits of the developed qubit coupling with those in the conventional approaches.

Sideband-resolved spectroscopy on the 4 2S(1/2)-3 2D(5/2) transition in single calcium ions by use of fundamental waves of diode lasers.

The 4 2S(1/2)-3 2D(5/2) electric quadrupole transition in calcium ions, which is expected to be used in calcium-ion optical frequency standards, is spectroscopically investigated with a laser system that uses only fundamental waves of diode lasers as light sources. Four Zeeman components allowed by the selection rules for the electric quadrupole transition are identified. As for one Zeeman component, one large carrier and accompanying smaller first sidebands are observed, which implies that Lamb-Dicke confinement of the ion is achieved. The compact and reliable system for spectroscopy of single calcium ions described is advantageous for realization of practical optical frequency standards.