942 research outputs found
Public exhibit for demonstrating the quantum of electrical conductance
We present a new robust setup that explains and demonstrates the quantum of
electrical conductance for a general audience and which is continuously
available in a public space. The setup allows users to manually thin a gold
wire of several atoms in diameter while monitoring its conductance in real
time. During the experiment, a characteristic step-like conductance decrease
due to rearrangements of atoms in the cross-section of the wire is observed.
Just before the wire breaks, a contact consisting of a single atom with a
characteristic conductance close to the quantum of conductance can be
maintained up to several seconds. The setup is operated full-time, needs
practically no maintenance and is used on different educational levels
Entanglement between charge qubits induced by a common dissipative environment
We study entanglement generation between two charge qubits due to the strong
coupling with a common bosonic environment (Ohmic bath). The coupling to the
boson bath is a source of both quantum noise (leading to decoherence) and an
indirect interaction between qubits. As a result, two effects compete as a
function of the coupling strength with the bath: entanglement generation and
charge localization induced by the bath. These two competing effects lead to a
non-monotonic behavior of the concurrence as a function of the coupling
strength with the bath. As an application, we present results for charge qubits
based on double quantum dots.Comment: 9 pages, 7 figure
Asymmetry and decoherence in a double-layer persistent-current qubit
Superconducting circuits fabricated using the widely used shadow evaporation
technique can contain unintended junctions which change their quantum dynamics.
We discuss a superconducting flux qubit design that exploits the symmetries of
a circuit to protect the qubit from unwanted coupling to the noisy environment,
in which the unintended junctions can spoil the quantum coherence. We present a
theoretical model based on a recently developed circuit theory for
superconducting qubits and calculate relaxation and decoherence times that can
be compared with existing experiments. Furthermore, the coupling of the qubit
to a circuit resonance (plasmon mode) is explained in terms of the asymmetry of
the circuit. Finally, possibilities for prolonging the relaxation and
decoherence times of the studied superconducting qubit are proposed on the
basis of the obtained results.Comment: v.2: published version; 8 pages, 12 figures; added comparison with
experiment, improved discussion of T_ph
Decoherence of the Superconducting Persistent Current Qubit
Decoherence of a solid state based qubit can be caused by coupling to
microscopic degrees of freedom in the solid. We lay out a simple theory and use
it to estimate decoherence for a recently proposed superconducting persistent
current design. All considered sources of decoherence are found to be quite
weak, leading to a high quality factor for this qubit.Comment: 10 pages, 1 figure, Latex/revtex.To appear in proceedings of the
NATO-ASI on "Quantum Mesoscopic Phenomena and Mesoscopic Devices in
Microelectronics"; Corrections were made on Oct. 29th, 199
Interference effects in isolated Josephson junction arrays with geometric symmetries
As the size of a Josephson junction is reduced, charging effects become
important and the superconducting phase across the link turns into a periodic
quantum variable. Isolated Josephson junction arrays are described in terms of
such periodic quantum variables and thus exhibit pronounced quantum
interference effects arising from paths with different winding numbers
(Aharonov-Casher effects). These interference effects have strong implications
for the excitation spectrum of the array which are relevant in applications of
superconducting junction arrays for quantum computing. The interference effects
are most pronounced in arrays composed of identical junctions and possessing
geometric symmetries; they may be controlled by either external gate potentials
or by adding/removing charge to/from the array. Here we consider a loop of N
identical junctions encircling one half superconducting quantum of magnetic
flux. In this system, the ground state is found to be non-degenerate if the
total number of Cooper pairs on the array is divisible by N, and doubly
degenerate otherwise (after the stray charges are compensated by the gate
voltages).Comment: 9 pages, 6 figure
Decoherence and Relaxation of a Quantum Bit in the Presence of Rabi Oscillations
Dissipative dynamics of a quantum bit driven by a strong resonant field and
interacting with a heat bath is investigated. We derive generalized Bloch
equations and find modifications of the qubit's damping rates caused by Rabi
oscillations. Nonequilibrium decoherence of a phase qubit inductively coupled
to a LC-circuit is considered as an illustration of the general results. It is
argued that recent experimental results give a clear evidence of effective
suppression of decoherence in a strongly driven flux qubit.Comment: 14 pages; misprints correcte
Impact of time-ordered measurements of the two states in a niobium superconducting qubit structure
Measurements of thermal activation are made in a superconducting, niobium
Persistent-Current (PC) qubit structure, which has two stable classical states
of equal and opposite circulating current. The magnetization signal is read out
by ramping the bias current of a DC SQUID. This ramping causes time-ordered
measurements of the two states, where measurement of one state occurs before
the other. This time-ordering results in an effective measurement time, which
can be used to probe the thermal activation rate between the two states.
Fitting the magnetization signal as a function of temperature and ramp time
allows one to estimate a quality factor of 10^6 for our devices, a value
favorable for the observation of long quantum coherence times at lower
temperatures.Comment: 14 pages, 4 figure
Method for direct observation of coherent quantum oscillations in a superconducting phase qubit
Time-domain observations of coherent oscillations between quantum states in
mesoscopic superconducting systems were so far restricted to restoring the
time-dependent probability distribution from the readout statistics. We propose
a new method for direct observation of Rabi oscillations in a phase qubit. The
external source, typically in GHz range, induces transitions between the qubit
levels. The resulting Rabi oscillations of supercurrent in the qubit loop are
detected by a high quality resonant tank circuit, inductively coupled to the
phase qubit. Detailed calculation for zero and non-zero temperature are made
for the case of persistent current qubit. According to the estimates for
dephasing and relaxation times, the effect can be detected using conventional
rf circuitry, with Rabi frequency in MHz range.Comment: 5 pages, 1 figure, to appear in Phys.Rev.
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