493 research outputs found
Continuous measurements of two qubits
We develop a theory of coherent quantum oscillations in two, in general
interacting, qubits measured continuously by a mesoscopic detector with
arbitrary non-linearity and discuss an example of SQUID magnetometer that can
operate as such a detector. Calculated spectra of the detector output show that
the detector non-linearity should lead to mixing of the oscillations of the two
qubits. For non-interacting qubits oscillating with frequencies and
, the mixing manifests itself as spectral peaks at the combination
frequencies . Additional nonlinearity introduced by the
qubit-qubit interaction shifts all the frequencies. In particular, for
identical qubits, the interaction splits coherent superposition of the
single-qubit peaks at . Quantum mechanics of the measurement
imposes limitations on the height of the spectral peaks.Comment: 14 pages, 6 figure
FQHE interferometers in strong tunneling regime. The role of compactness of edge fields
We consider multiple-point tunneling in the interferometers formed between
edges of electron liquids with in general different filling factors in the
regime of the Fractional Quantum Hall effect (FQHE). We derive an effective
matrix Caldeira-Leggett models for the multiple tunneling contacts connected by
the chiral single-mode FQHE edges. It is shown that the compactness of the Wen-
Fr\"ohlich chiral boson fields describing the FQHE edge modes plays a crucial
role in eliminating the spurious non-locality of the electron transport
properties of the FQHE interferometers arising in the regime of strong
tunneling.Comment: 5 page
AC Josephson effect and resonant Cooper pair tunneling emission of a Cooper Pair Transistor
We measure the high-frequency emission of a single Cooper pair
transistor(SCPT) in the regime where transport is only due to tunneling of
Cooper pairs. This is achieved by coupling on-chip the SCPT to a
superconductor-insulator-superconductor junction and by measuring the photon
assisted tunneling current of quasiparticles across the junction. This
technique allows a direct detection of the AC Josephson effect of the SCPT and
provides evidence of Landau-Zener transitions for proper gate voltage. The
emission in the regime of resonant Cooper pair tunneling is also investigated.
It is interpreted in terms of transitions between charge states coupled by the
Josephson effect.Comment: Revtex4, 5 pages, 4 figures, final versio
System of Programmed Modules for Measuring Photographs with a Gamma-Telescope
Physical experiments using tracking cameras resulted in hundreds of thousands of stereo photographs of events being received. To process such a large volume of information, automatic and semiautomatic measuring systems are required. At the Institute of Space Research of the Academy of Science of the USSR, a system for processing film information from the spark gamma-telescope was developed. The system is based on a BPS-75 projector in line with the minicomputer Elektronika 1001. The report describes this system. The various computer programs available to the operators are discussed
Coulomb Charging Effects for Finite Channel Number
We consider quantum fluctuations of the charge on a small metallic grain
caused by virtual electron tunneling to a nearby electrode. The average
electron number and the effective charging energy are determined by means of
perturbation theory in the tunneling Hamiltonian. In particular we discuss the
dependence of charging effects on the number N of tunneling channels. Earlier
results for N>>1 are found to be approached rather rapidly with increasing N.Comment: 6 pages, 5 figure
Coulomb blockade in superconducting quantum point contacts
Amplitude of the Coulomb blockade oscillations is calculated for a
single-mode Josephson junction with arbitrary electron transparency . It is
shown that the Coulomb blockade is suppressed in ballistic junctions with . The suppression is described quantitatively as the Landau-Zener transition
in imaginary time.Comment: 5 pages, 3 figures include
Statistics of the dissipated energy in driven single-electron transitions
We analyze the distribution of heat generated in driven single-electron
transitions and discuss the related non-equilibrium work theorems. In the
adiabatic limit, the heat distribution is shown to become Gaussian, with the
heat noise that, in spite of thermal fluctuations, vanishes together with the
average dissipated energy. We show that the transitions satisfy Jarzynski
equality for arbitrary drive and calculate the probability of the negative heat
values. We also derive a general condition on the heat distribution that
generalizes the Bochkov-Kuzovlev equality and connects it to the Jarzynski
equality.Comment: 5 pages, 2 figure
Theory of the Franck-Condon blockade regime
Strong coupling of electronic and vibrational degrees of freedom entails a
low-bias suppression of the current through single-molecule devices, termed
Franck-Condon blockade. In the limit of slow vibrational relaxation, transport
in the Franck-Condon-blockade regime proceeds via avalanches of large numbers
of electrons, which are interrupted by long waiting times without electron
transfer. The avalanches consist of smaller avalanches, leading to a
self-similar hierarchy which terminates once the number of transferred
electrons per avalanche becomes of the order of unity. Experimental signatures
of self-similar avalanche transport are strongly enhanced current (shot) noise,
as expressed by giant Fano factors, and a power-law noise spectrum. We develop
a theory of the Franck-Condon-blockade regime with particular emphasis on
effects of electron cotunneling through highly excited vibrational states. As
opposed to the exponential suppression of sequential tunneling rates for
low-lying vibrational states, cotunneling rates suffer only a power-law
suppression. This leads to a regime where cotunneling dominates the current for
any gate voltage. Including cotunneling within a rate-equation approach to
transport, we find that both the Franck-Condon blockade and self-similar
avalanche transport remain intact in this regime. We predict that cotunneling
leads to absorption-induced vibrational sidebands in the Coulomb-blockaded
regime as well as intrinsic telegraph noise near the charge degeneracy point.Comment: 20 pages, 10 figures; minor changes, version published in Phys. Rev.
Coulomb blockade in two island systems with highly conductive junctions
We report measurements on single-electron pumps, consisting of two metallic
islands formed by three tunnel junctions in series. We focus on the
linear-response conductance as a function of gate voltage and temperature of
three samples with varying system parameters. In all cases, strong quantum
fluctuation phenomena are observed by a log(k_B T/(2 E_co)) reduction of the
maximal conductance, where E_co measures the coupling strength between the
islands. The samples display a rich phenomenology, culminating in a
non-monotonic behavior of the maximal conductance as a function of temperature
Transport in the Laughlin quasiparticle interferometer: Evidence for topological protection in an anyonic qubit
We report experiments on temperature and Hall voltage bias dependence of the
superperiodic conductance oscillations in the novel Laughlin quasiparticle
interferometer, where quasiparticles of the 1/3 fractional quantum Hall fluid
execute a closed path around an island of the 2/5 fluid. The amplitude of the
oscillations fits well the quantum-coherent thermal dephasing dependence
predicted for a two point-contact chiral edge channel interferometer in the
full experimental temperature range 10.2<T<141 mK. The temperature dependence
observed in the interferometer is clearly distinct from the behavior in
single-particle resonant tunneling and Coulomb blockade devices. The 5h/e flux
superperiod, originating in the anyonic statistical interaction of Laughlin
quasiparticles, persists to a relatively high T~140 mK. This temperature is
only an order of magnitude less than the 2/5 quantum Hall gap. Such protection
of quantum logic by the topological order of fractional quantum Hall fluids is
expected to facilitate fault-tolerant quantum computation with anyons.Comment: 13 pages, 10 figure
- …