5,536 research outputs found
Quantum Nondemolition Charge Measurement of a Josephson Qubit
In a qubit system, the measurement operator does not necessarily commute with
the qubit Hamiltonian, so that the readout process demolishes (mixes) the qubit
energy eigenstates. The readout time is therefore limited by such a mixing time
and its fidelity will be reduced. A quantum nondemolition readout scheme is
proposed in which the charge of a flux qubit is measured. The measurement
operator is shown to commute with the qubit Hamiltonian in the reduced
two-level Hilbert space, even though the Hamiltonian contains non-commuting
charge and flux terms.Comment: 4 pages, 3 figures, a paragraph added to describe how the scheme
works in charge regim
Quantum analysis of a nonlinear microwave cavity-embedded dc SQUID displacement detector
We carry out a quantum analysis of a dc SQUID mechanical displacement
detector, comprising a SQUID with mechanically compliant loop segment, which is
embedded in a microwave transmission line resonator. The SQUID is approximated
as a nonlinear, current dependent inductance, inducing an external flux
tunable, nonlinear Duffing self-interaction term in the microwave resonator
mode equation. Motion of the compliant SQUID loop segment is transduced
inductively through changes in the external flux threading SQUID loop, giving a
ponderomotive, radiation pressure type coupling between the microwave and
mechanical resonator modes. Expressions are derived for the detector signal
response and noise, and it is found that a soft-spring Duffing self-interaction
enables a closer approach to the displacement detection standard quantum limit,
as well as cooling closer to the ground state
Noncommutative fluid dynamics in the K\"{a}hler parametrization
In this paper, we propose a first order action functional for a large class
of systems that generalize the relativistic perfect fluids in the K\"{a}hler
parametrization to noncommutative spacetimes. We calculate the equations of
motion for the fluid potentials and the energy-momentum tensor in the first
order in the noncommutative parameter. The density current does not receive any
noncommutative corrections and it is conserved under the action of the
commutative generators but the energy-momentum tensor is not.
Therefore, we determine the set of constraints under which the energy-momentum
tensor is divergenceless. Another set of constraints on the fluid potentials is
obtained from the requirement of the invariance of the action under the
generalization of the volume preserving transformations of the noncommutative
spacetime. We show that the proposed action describes noncommutative fluid
models by casting the energy-momentum tensor in the familiar fluid form and
identifying the corresponding energy and momentum densities. In the commutative
limit, they are identical to the corresponding quantities of the relativistic
perfect fluids. The energy-momentum tensor contains a dissipative term that is
due to the noncommutative spacetime and vanishes in the commutative limit.
Finally, we particularize the theory to the case when the complex fluid
potentials are characterized by a function that is a deformation
of the complex plane and show that this model has important common features
with the commutative fluid such as infinitely many conserved currents and a
conserved axial current that in the commutative case is associated to the
topologically conserved linking number.Comment: References fixed. Typos corrected. 12 page
Quantum Phase Tomography of a Strongly Driven Qubit
The interference between repeated Landau-Zener transitions in a qubit swept
through an avoided level crossing results in Stueckelberg oscillations in qubit
magnetization. The resulting oscillatory patterns are a hallmark of the
coherent strongly-driven regime in qubits, quantum dots and other two-level
systems. The two-dimensional Fourier transforms of these patterns are found to
exhibit a family of one-dimensional curves in Fourier space, in agreement with
recent observations in a superconducting qubit. We interpret these images in
terms of time evolution of the quantum phase of qubit state and show that they
can be used to probe dephasing mechanisms in the qubit.Comment: 5 pgs, 4 fg
Probing Decoherence with Electromagnetically Induced Transparency in Superconductive Quantum Circuits
Superconductive quantum circuits (SQCs) comprise quantized energy levels that
may be coupled via microwave electromagnetic fields. Described in this way, one
may draw a close analogy to atoms with internal (electronic) levels coupled by
laser light fields. In this Letter, we present a superconductive analog to
electromagnetically induced transparency (S-EIT) that utilizes SQC designs of
present day experimental consideration. We discuss how S-EIT can be used to
establish macroscopic coherence in such systems and, thereby, utilized as a
sensitive probe of decoherence.Comment: 5 pages, 3 figure
Entanglement generation in persistent current qubits
In this paper we investigate the generation of entanglement between two
persistent current qubits. The qubits are coupled inductively to each other and
to a common bias field, which is used to control the qubit behaviour and is
represented schematically by a linear oscillator mode. We consider the use of
classical and quantum representations for the qubit control fields and how
fluctuations in the control fields tend to suppress entanglement. In
particular, we demonstrate how fluctuations in the bias fields affect the
entanglement generated between persistent current qubits and may limit the
ability to design practical systems.Comment: 7 pages, 4 figures, minor changes in reply to referees comment
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
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