184 research outputs found
Certainly Unsupervisable States
This paper proposes an abstraction method for compositional synthesis. Synthesis is a method to automatically compute a control program or supervisor that restricts the behaviour of a given system to ensure safety and liveness. Compositional synthesis uses repeated abstraction and simplification to combat the state-space explosion problem for large systems. The abstraction method proposed in this paper finds and removes the so-called certainly unsupervisable states. By removing these states at an early stage, the final state space can be reduced substantially. The paper describes an algorithm with cubic time complexity to compute the largest possible set of removable states. A practical example demonstrates the feasibility of the method to solve real-world problems
Coherent quantum phase slip
A hundred years after discovery of superconductivity, one fundamental
prediction of the theory, the coherent quantum phase slip (CQPS), has not been
observed. CQPS is a phenomenon exactly dual to the Josephson effect: whilst the
latter is a coherent transfer of charges between superconducting contacts, the
former is a coherent transfer of vortices or fluxes across a superconducting
wire. In contrast to previously reported observations of incoherent phase slip,
the CQPS has been only a subject of theoretical study. Its experimental
demonstration is made difficult by quasiparticle dissipation due to gapless
excitations in nanowires or in vortex cores. This difficulty might be overcome
by using certain strongly disordered superconductors in the vicinity of the
superconductor-insulator transition (SIT). Here we report the first direct
observation of the CQPS in a strongly disordered indium-oxide (InOx)
superconducting wire inserted in a loop, which is manifested by the
superposition of the quantum states with different number of fluxes. Similarly
to the Josephson effect, our observation is expected to lead to novel
applications in superconducting electronics and quantum metrology.Comment: 14 pages, 3 figure
Quantum Measurement and the Aharonov-Bohm Effect with Superposed Magnetic Fluxes
We consider the magnetic flux in a quantum mechanical superposition of two
values and find that the Aharonov-Bohm effect interference pattern contains
information about the nature of the superposition, allowing information about
the state of the flux to be extracted without disturbance. The information is
obtained without transfer of energy or momentum and by accumulated nonlocal
interactions of the vector potential with many charged particles
forming the interference pattern, rather than with a single particle. We
suggest an experimental test using already experimentally realized superposed
currents in a superconducting ring and discuss broader implications.Comment: 6 pages, 4 figures; Changes from version 3: corrected typo (not
present in versions 1 and 2) in Eq. 8; Changes from version 2: shortened
abstract; added refs and material in Section IV. The final publication is
available at: http://link.springer.com/article/10.1007/s11128-013-0652-
Quantum phase slip phenomenon in ultra-narrow superconducting nanorings
The smaller the system, typically - the higher is the impact of fluctuations.
In narrow superconducting wires sufficiently close to the critical temperature
Tc thermal fluctuations are responsible for the experimentally observable
finite resistance. Quite recently it became possible to fabricate sub-10 nm
superconducting structures, where the finite resistivity was reported within
the whole range of experimentally obtainable temperatures. The observation has
been associated with quantum fluctuations capable to quench zero resistivity in
superconducting nanowires even at temperatures T-->0. Here we demonstrate that
in tiny superconducting nanorings the same phenomenon is responsible for
suppression of another basic attribute of superconductivity - persistent
currents - dramatically affecting their magnitude, the period and the shape of
the current-phase relation. The effect is of fundamental importance
demonstrating the impact of quantum fluctuations on the ground state of a
macroscopically coherent system, and should be taken into consideration in
various nanoelectronic applications.Comment: 20 pages, 4 figure
Reversing non-local transport through a superconductor by electromagnetic excitations
Superconductors connected to normal metallic electrodes at the nanoscale
provide a potential source of non-locally entangled electron pairs. Such states
would arise from Cooper pairs splitting into two electrons with opposite spins
tunnelling into different leads. In an actual system the detection of these
processes is hindered by the elastic transmission of individual electrons
between the leads, yielding an opposite contribution to the non-local
conductance. Here we show that electromagnetic excitations on the
superconductor can play an important role in altering the balance between these
two processes, leading to a dominance of one upon the other depending on the
spatial symmetry of these excitations. These findings allow to understand some
intriguing recent experimental results and open the possibility to control
non-local transport through a superconductor by an appropriate design of the
experimental geometry.Comment: 6 pages, 3 figure
Topologically protected quantum bits from Josephson junction arrays
All physical implementations of quantum bits (qubits), carrying the
information and computation in a putative quantum computer, have to meet the
conflicting requirements of environmental decoupling while remaining
manipulable through designed external signals. Proposals based on quantum
optics naturally emphasize the aspect of optimal isolation, while those
following the solid state route exploit the variability and scalability of
modern nanoscale fabrication techniques. Recently, various designs using
superconducting structures have been successfully tested for quantum coherent
operation, however, the ultimate goal of reaching coherent evolution over
thousands of elementary operations remains a formidable task. Protecting qubits
from decoherence by exploiting topological stability, a qualitatively new
proposal due to Kitaev, holds the promise for long decoherence times, but its
practical physical implementation has remained unclear so far. Here, we show
how strongly correlated systems developing an isolated two-fold degenerate
quantum dimer liquid groundstate can be used in the construction of
topologically stable qubits and discuss their implementation using Josephson
junction arrays.Comment: 6 pages, 4 figure
Electrical detection of 31P spin quantum states
In recent years, a variety of solid-state qubits has been realized, including
quantum dots, superconducting tunnel junctions and point defects. Due to its
potential compatibility with existing microelectronics, the proposal by Kane
based on phosphorus donors in Si has also been pursued intensively. A key issue
of this concept is the readout of the P quantum state. While electrical
measurements of magnetic resonance have been performed on single spins, the
statistical nature of these experiments based on random telegraph noise
measurements has impeded the readout of single spin states. In this letter, we
demonstrate the measurement of the spin state of P donor electrons in silicon
and the observation of Rabi flops by purely electric means, accomplished by
coherent manipulation of spin-dependent charge carrier recombination between
the P donor and paramagnetic localized states at the Si/SiO2 interface via
pulsed electrically detected magnetic resonance. The electron spin information
is shown to be coupled through the hyperfine interaction with the P nucleus,
which demonstrates the feasibility of a recombination-based readout of nuclear
spins
Sisyphus cooling and amplification by a superconducting qubit
Laser cooling of the atomic motion paved the way for remarkable achievements
in the fields of quantum optics and atomic physics, including Bose-Einstein
condensation and the trapping of atoms in optical lattices. More recently
superconducting qubits were shown to act as artificial two-level atoms,
displaying Rabi oscillations, Ramsey fringes, and further quantum effects.
Coupling such qubits to resonators brought the superconducting circuits into
the realm of quantum electrodynamics (circuit QED). It opened the perspective
to use superconducting qubits as micro-coolers or to create a population
inversion in the qubit to induce lasing behavior of the resonator. Furthering
these analogies between quantum optical and superconducting systems we
demonstrate here Sisyphus cooling of a low frequency LC oscillator coupled to a
near-resonantly driven superconducting qubit. In the quantum optics setup the
mechanical degrees of freedom of an atom are cooled by laser driving the atom's
electronic degrees of freedom. Here the roles of the two degrees of freedom are
played by the LC circuit and the qubit's levels, respectively. We also
demonstrate the counterpart of the Sisyphus cooling, namely Sisyphus
amplification. Parallel to the experimental demonstration we analyze the system
theoretically and find quantitative agreement, which supports the
interpretation and allows us to estimate system parameters.Comment: 7 pages, 4 figure
Phase-slip induced dissipation in an atomic Bose-Hubbard system
Phase slips play a primary role in dissipation across a wide spectrum of
bosonic systems, from determining the critical velocity of superfluid helium to
generating resistance in thin superconducting wires. This subject has also
inspired much technological interest, largely motivated by applications
involving nanoscale superconducting circuit elements, e.g., standards based on
quantum phase-slip junctions. While phase slips caused by thermal fluctuations
at high temperatures are well understood, controversy remains over the role of
phase slips in small-scale superconductors. In solids, problems such as
uncontrolled noise sources and disorder complicate the study and application of
phase slips. Here we show that phase slips can lead to dissipation for a clean
and well-characterized Bose-Hubbard (BH) system by experimentally studying
transport using ultra-cold atoms trapped in an optical lattice. In contrast to
previous work, we explore a low velocity regime described by the 3D BH model
which is not affected by instabilities, and we measure the effect of
temperature on the dissipation strength. We show that the damping rate of
atomic motion-the analogue of electrical resistance in a solid-in the confining
parabolic potential fits well to a model that includes finite damping at zero
temperature. The low-temperature behaviour is consistent with the theory of
quantum tunnelling of phase slips, while at higher temperatures a cross-over
consistent with the transition to thermal activation of phase slips is evident.
Motion-induced features reminiscent of vortices and vortex rings associated
with phase slips are also observed in time-of-flight imaging.Comment: published in Nature 453, 76 (2008
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