An on-demand coherent single-electron source

We report on the electron analog of the single-photon gun. On-demand single-electron injection in a quantum conductor was obtained using a quantum dot connected to the conductor via a tunnel barrier. Electron emission was triggered by the application of a potential step that compensated for the dot-charging energy. Depending on the barrier transparency, the quantum emission time ranged from 0.1 to 10 nanoseconds. The single-electron source should prove useful for the use of quantum bits in ballistic conductors. Additionally, periodic sequences of single-electron emission and absorption generate a quantized alternating current.     

Violation of Kirchhoff's laws for a coherent RC circuit

What is the complex impedance of a fully coherent quantum resistance-capacitance (RC) circuit at gigahertz frequencies in which a resistor and a capacitor are connected in series? While Kirchhoff's laws predict addition of capacitor and resistor impedances, we report on observation of a different behavior. The resistance, here associated with charge relaxation, differs from the usual transport resistance given by the Landauer formula. In particular, for a single-mode conductor, the charge-relaxation resistance is half the resistance quantum, regardless of the transmission of the mode. The new mesoscopic effect reported here is relevant for the dynamical regime of all quantum devices.     

Detection of quantum noise from an electrically driven two-level system

The electrical noise of mesoscopic devices can be strongly influenced by the quantum motion of electrons. To probe this effect, we have measured the current fluctuations at high frequency (5 to 90 gigahertz) using a superconductor-insulator-superconductor tunnel junction as an on-chip spectrum analyzer. By coupling this frequency-resolved noise detector to a quantum device, we can measure the high-frequency, nonsymmetrized noise as demonstrated for a Josephson junction. The same scheme is used to detect the current fluctuations arising from coherent charge oscillations in a two-level system, a superconducting charge qubit. A narrow band peak is observed in the spectral noise density at the frequency of the coherent charge oscillations.     

Spin-charge separation and localization in one dimension

We report on measurements of quantum many-body modes in ballistic wires and their dependence on Coulomb interactions, obtained by tunneling between two parallel wires in an GaAs/AlGaAs heterostructure while varying electron density. We observed two spin modes and one charge mode of the coupled wires and mapped the dispersion velocities of the modes down to a critical density, at which spontaneous localization was observed. Theoretical calculations of the charge velocity agree well with the data, although they also predict an additional charge mode that was not observed. The measured spin velocity was smaller than theoretically predicted.     

Spin Hall effect transistor

The field of semiconductor spintronics explores spin-related quantum relativistic phenomena in solid-state systems. Spin transistors and spin Hall effects have been two separate leading directions of research in this field. We have combined the two directions by realizing an all-semiconductor spin Hall effect transistor. The device uses diffusive transport and operates without electrical current in the active part of the transistor. We demonstrate a spin AND logic function in a semiconductor channel with two gates. Our study shows the utility of the spin Hall effect in a microelectronic device geometry, realizes the spin transistor with electrical detection directly along the gated semiconductor channel, and provides an experimental tool for exploring spin Hall and spin precession phenomena in an electrically tunable semiconductor layer.     

Strongly interacting Rydberg excitations of a cold atomic gas

Highly excited Rydberg atoms have many exaggerated properties. In particular, the interaction strength between such atoms can be varied over an enormous range. In a mesoscopic ensemble, such strong, long-range interactions can be used for fast preparation of desired many-particle states. We generated Rydberg excitations in an ultra-cold atomic gas and subsequently converted them into light. As the principal quantum number n was increased beyond ~70, no more than a single excitation was retrieved from the entire mesoscopic ensemble of atoms. These results hold promise for studies of dynamics and disorder in many-body systems with tunable interactions and for scalable quantum information networks.     

Squeezed states in a Bose-Einstein condensate

We report manipulation of the atom number statistics associated with Bose-Einstein condensed atoms confined in an array of weakly linked mesoscopic traps. We used the interference of atoms released from the traps as a sensitive probe of these statistics. By controlling relative strengths of the tunneling rate between traps and atom-atom interactions within each trap, we observed trap states characterized by sub-Poissonian number fluctuations and adiabatic transitions between these number-squeezed states and coherent states of the atom field. The quantum states produced in this work may enable substantial gains in sensitivity for atom interference-based instruments as well as fundamental studies of quantum phase transitions.     

All-optical switching in rubidium vapor

We report on an all-optical switch that operates at low light levels. It consists of laser beams counterpropagating through a warm rubidium vapor that induce an off-axis optical pattern. A switching laser beam causes this pattern to rotate even when the power in the switching beam is much lower than the power in the pattern. The observed switching energy density is very low, suggesting that the switch might operate at the single-photon level with system optimization. This approach opens the possibility of realizing a single-photon switch for quantum information networks and for improving transparent optical telecommunication networks.     

Low-noise current amplifier based on mesoscopic Josephson junction

We used the band structure of a mesoscopic Josephson junction to construct low-noise amplifiers. By taking advantage of the quantum dynamics of a Josephson junction, i.e., the interplay of interlevel transitions and the Coulomb blockade of Cooper pairs, we created transistor-like devices, Bloch oscillating transistors, with considerable current gain and high-input impedance. In these transistors, the correlated supercurrent of Cooper pairs is controlled by a small base current made up of single electrons. Our devices reached current and power gains on the order of 30 and 5, respectively. The noise temperature was estimated to be around 1 kelvin, but noise temperatures of less than 0.1 kelvin can be realistically achieved. These devices provide quantum-electronic building blocks that will be useful at low temperatures in low-noise circuit applications with an intermediate impedance level.     

Hanbury brown and twiss-type experiment with electrons

Fermion anti-bunching was directly observed by measuring the cross-covariance of the current fluctuations of partitioned electrons. A quantum point contact was used to inject single-mode electrons into a mesoscopic electron beam splitter device. The beam splitter output currents showed negative cross-covariance, indicating that the electrons arrived individually at the beam splitter and were randomly partitioned into two output channels. As the relative time delay between the outputs was changed, the observed ringing in the cross-covariance was consistent with the bandwidths used to monitor the fluctuations. The result demonstrates a fermion complement to the Hanbury Brown and Twiss experiment for photons.     

Local gate control of a carbon nanotube double quantum dot

We have measured carbon nanotube quantum dots with multiple electrostatic gates and used the resulting enhanced control to investigate a nanotube double quantum dot. Transport measurements reveal honeycomb charge stability diagrams as a function of two nearly independent gate voltages. The device can be tuned from weak to strong interdot tunnel-coupling regimes, and the transparency of the leads can be controlled independently. We extract values of energy-level spacings, capacitances, and interaction energies for this system. This ability to control electron interactions in the quantum regime in a molecular conductor is important for applications such as quantum computation.     

Ballistic 2-d imaging through scattering walls using an ultrafast optical kerr gate

An ultrafast optical shutter was used to image ultrasmall objects hidden behind scattering walls by a procedure that selects in time the ballistic component and rejects the scattered diffusive light. Scattering walls used in this experiment included human breast tissue, chicken breast tissue, and a water suspension of polystyrene particles with scattering coefficients up to 21.7. Submillimeter resolution was achieved for two-dimensional ballistic images of a single point, a double-point fluorescence source, and a bar test chart in or behind these different turbid media.     

量子波导理论下连一杂质的双环介观结构电子输运性质

利用量子波导理论研究连一杂质双环介观结构的电子输运性质,讨论了环臂长以及磁通量对电子透射几率和持续电流的影响.结果表明,在杂质位置、强度和磁通量一定时,透射几率和两环中的持续电流均随环臂长的改变做周期性振荡,且振荡周期相同;左环中持续电流的峰值明显大于右环中持续电流的峰值,而且左右两环持续电流的顺时针最大值明显比逆时针最大值大.当杂质位置、强度和环臂长一定时,透射几率和两环中的持续电流均随磁通量的改变做周期性振荡,左右两环持续电流的顺时针最大值与逆时针最大值相等.     

Broken-symmetry states in doubly gated suspended bilayer graphene

The single-particle energy spectra of graphene and its bilayer counterpart exhibit multiple degeneracies that arise through inherent symmetries. Interactions among charge carriers should spontaneously break these symmetries and lead to ordered states that exhibit energy gaps. In the quantum Hall regime, these states are predicted to be ferromagnetic in nature, whereby the system becomes spin polarized, layer polarized, or both. The parabolic dispersion of bilayer graphene makes it susceptible to interaction-induced symmetry breaking even at zero magnetic field. We investigated the underlying order of the various broken-symmetry states in bilayer graphene suspended between top and bottom gate electrodes. We deduced the order parameter of the various quantum Hall ferromagnetic states by controllably breaking the spin and sublattice symmetries. At small carrier density, we identified three distinct broken-symmetry states, one of which is consistent with either spontaneously broken time-reversal symmetry or spontaneously broken rotational symmetry.     

Communication through a diffusive medium: coherence and capacity

Coherent wave propagation in disordered media gives rise to many fascinating phenomena as diverse as universal conductance fluctuations in mesoscopic metals and speckle patterns in light scattering. Here, the theory of electromagnetic wave propagation in diffusive media is combined with information theory to show how interference affects the information transmission rate between antenna arrays. Nontrivial dependencies of the information capacity on the nature of the antenna arrays are found, such as the dimensionality of the arrays and their direction with respect to the local scattering medium. This approach provides a physical picture for understanding the importance of scattering in the transfer of information through wireless communications.     

Direct evidence for R- rotons having antiparallel momentum and velocity

Experimental evidence is presented that shows that the momentum of a R- roton (a particle-like excitation in liquid helium-4) is antiparallel to its velocity. Although this is anticipated from the negative slope of the dispersion curve for these excitations, it has only been possible to test since the development of a source of ballistic R- rotons. The backward refraction of the quantum evaporation process, which is the signature of antiparallel momentum and velocity, is observed.     

Phase-coherent transport in graphene quantum billiards

As an emergent electronic material and model system for condensed-matter physics, graphene and its electrical transport properties have become a subject of intense focus. By performing low-temperature transport spectroscopy on single-layer and bilayer graphene, we observe ballistic propagation and quantum interference of multiply reflected waves of charges from normal electrodes and multiple Andreev reflections from superconducting electrodes, thereby realizing quantum billiards in which scattering only occurs at the boundaries. In contrast to the conductivity of conventional two-dimensional materials, graphene's conductivity at the Dirac point is geometry-dependent because of conduction via evanescent modes, approaching the theoretical value 4e(2)/pih (where e is the electron charge and h is Planck's constant) only for short and wide devices. These distinctive transport properties have important implications for understanding chaotic quantum systems and implementing nanoelectronic devices, such as ballistic transistors.     

Gaseous-diffusion porometer for continuous measurement of diffusive resistance of leaves

We describe a porometer that enables continuous monitoring of the stomatal diffusive resistance of leaves. The flux is measured of a gas-such as nitrous oxide-diffusing through a leaf that divides an enclosing chamber into two compartments. Nitrous oxide is added in known concentration to the airstream passing through the compartment on one side of the leaf and is recovered from the airstream passing through the opposite compartment. From measurements of the difference in concentration across the leaf and of the flux, the diffusive resistance of the leaf to N(2)O is calculated; this value, adjusted for resistance external to the leaf, gives a continuous record of internal diffusive resistance. This record can be made simultaneously with measurements of transpiration and photosynthesis.     

Quisqualate activates a rapidly inactivating high conductance ionic channel in hippocampal neurons

Glutamate activates a number of different receptor-channel complexes, each of which may contribute to generation of excitatory postsynaptic potentials in the mammalian central nervous system. The rapid application of the selective glutamate agonist, quisqualate, activates a large rapidly inactivating current (3 to 8 milliseconds), which is mediated by a neuronal ionic channel with high unitary conductance (35 picosiemens). The current through this channel shows pharmacologic characteristics similar to those observed for the fast excitatory postsynaptic current (EPSC); it reverses near 0 millivolts and shows no significant voltage dependence. The amplitude of the current through this channel is many times larger than that through the other non-NMDA (N-methyl-D-aspartate) channels. These results suggest that this high-conductance quisqualate-activated channel may mediate the fast EPSC in the mammalian central nervous system.     

A Bose-Einstein condensate of metastable atoms

We report the realization of a Bose-Einstein condensate of metastable atoms (helium in the lowest triplet state). The excitation energy of each atom with respect to the ground state is 20 electron volts, but inelastic processes that would destroy the sample are suppressed strongly enough in a spin-polarized sample to allow condensation. Our detection scheme takes advantage of the metastability to achieve detection of individual atoms as well as of the decay products of inelastic processes. This detection opens the way toward new studies in mesoscopic quantum statistical physics, as well as in atomic quantum optics.