Photon-assisted tunneling of electrons through an insulating barrier may be used to detect long-wavelength radiation with a sensitivity approaching the limit imposed by the Heisenberg uncertainty principle. A new generation of ultra-low-noise millimeter-wave receivers, currently being developed for astronomical observation, utilizes the extremely sharp nonlinearity produced by single-electron quasiparticle tunneling between two superconductors in a superconductor-insulator-superconductor (SIS) tunnel junction. At millimeter wavelengths, the quantum energy fico/e may be larger than the voltage width for onset of quasiparticle tunneling in a SIS junction; and under these conditions the absorption of a single photon can cause one additional electron to tunnel through the barrier. Several newly discovered quantum effects become possible in this regime, including power amplification of an incoming signal during the process of frequency downconversion in a heterodyne receiver. The experimental development of SIS millimeter-wave receivers is reviewed, along with the quantum theory of mixing which predicts their performance. CONTENTS

Water, like any polarizable medium, responds to a nonuniform electric field by collecting preferentially in regions of maximum field intensity. This manifestation of dielectrophoresis (DEP) makes possible a variety of microelectromechanical liquid actuation schemes. In particular, we demonstrate a new class of high-speed DEP actuators, including “wall-less” flow structures, siphons, and nanodroplet dispensers that operate with water. Liquid in these microfluidic devices rests on a thin, insulating, polyimide layer that covers the coplanar electrodes. Microliter volumes of water, deposited on these substrates from a micropipette, are manipulated, transported, and subdivided into droplets as small as ∼7 nl by sequences of voltage application and appropriate changes of electrode connections. The finite conductivity of the water and the capacitance of the dielectric layer covering the electrodes necessitate use of rf voltage above ∼60 kHz. A simple RC circuit model explains this frequency-dependent behavior. DEP actuation of small water volumes is very fast. We observe droplet formation in less than 0.1 s and transient, voltage-driven movement of water fingers at speeds exceeding 5 cm/s. Such speed suggests that actuation can be accomplished using preprogrammed, short applications of the rf voltage to minimize Joule heating.

Most quantum computer realizations require the ability to apply local fields and tune the couplings between qubits, in order to realize single bit and two bit gates which are necessary for universal quantum computation. We present a scheme to remove the necessity of switching the couplings between qubits for two bit gates, which are more costly in many cases. Our strategy is to compute in and out of carefully designed interaction free subspaces analogous to decoherence free subspaces, which allows us to effectively turn off and turn on the interactions between the encoded qubits. We give two examples to show how universal quantum computation is realized in our scheme with local manipulations to physical qubits only, for both diagonal and off diagonal interactions. Quantum computation is generally formulated in terms of a collection of qubits subject to a sequence of single and two bit operations [1]. This implies that the effective local fields applied to individual qubits, and the couplings between the qubits, are variable functions subject to external control. In many cases, two bit operations, whose implementation depends on certain interactions between qubits, are more difficult than single bit gates. They can require more sophisticated manipulations, therefore may take a longer time and cause stronger decoherence. This usually results from the requirement to vary (in the simplest case just switch on and off) the couplings between qubits, which is not always possible, or easy to realize. One such example is quantum computing with Josephson junction devices, both charge and flux type [2,3,4,5,6]. In this case, the coupling between qubits is most naturally realized with a hard wired capacitor or inductor, whose value is fixed by the fabrication and cannot be tuned during the computation. The superconducting quantum computing community has been working hard to devise variable coupling schemes [5,7,8,9], but it is generally agreed that none of these proposed switches is completely satisfactory [9]. Most of them [5,7] require external controls, thus are likely to be major decoherence sources. Others were designed to avoid such external controls, but may suffer other problems, for instance the number of qubits that can be incorporated into the system can be limited [8,9], which is at odd with the supposed scalability of a solid state quantum computer.An always on and un-tunable coupling causes certain problems for quantum computation, depending on the particular form of the interaction. If the interaction Hamiltonian is diagonal in the computational basis, each qubit state will gain additional phases depending on the states of the qubits to which it is coupled, even in the idle mode. It is then necessary to keep track of these phases, or suppress them by repeated refocusing pulses like those used in NMR, which requires high precisions and complicates the operation [8,9]. The situation is more serious in the case of off diagonal interactions, because these interactions will cause the states of the qubits to propaga...

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An unbiased Josephson tunnel junction, irradiated with microwaves, can spontaneously develop quantized dc voltages. We explain this effect with reference to the shunted junction model and to analog experiments. This effect may be utilized to significantly improve both the accuracy and the simplicity of the Josephson voltage standard.

Nonlinear electrical characteristics of nanostructured T-branch junctions (TBJs) made of two-dimensional electron gas in an InGaAs∕InAlAs heterostructure were studied by a systematic variation of both the device size and the operating temperature. We have found that two distinct mechanisms are responsible for the electronic transport in TBJs and their resulting nonlinear characteristics, namely, the nonlinear ballistic effect at low applied voltages and the intervalley transfer at high voltages. Detailed experimental analysis for each mechanism and their contributions with respect to the TBJ’s nanochannel length and operating temperature are discussed.

The theory of parametric amplification by a series of unbiased Josephson junctions is presented. The various regimes of operation are explored. This device is in many ways different from the usual varactor parametric amplifier. In particular, a very small pump power is required and a large bandwidth is obtained. Comparison with experimental data is made. PACS numbers: 74.50.T, 85.25., 84.30.L V = dt [LJ (1)1], (1) L (I) =L sin-1 (I/IJ ) J J I/I J (2)The ideal Josephson element is then a nonlinear inductor, with characteristic inductance L J at zero bias current.We wish to operate with zero bias current. An unbiased Josephson element between identical superconductors is symmetric to a physical inversion. This symmetry operation, which carries 1--I and V --V simultaneously, cannot change the equations which de-

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