5,988 research outputs found
Multiple junction biasing of superconducting tunnel junction detectors
We describe a new biasing scheme for single photon detectors based on
superconducting tunnel junctions. It replaces a single detector junction with a
circuit of three junctions and achieves biasing of a detector junction at
subgap currents without the use of an external magnetic field. The biasing
occurs through the nonlinear interaction of the three junctions, which we
demonstrate through numerical simulation. This nonlinear state is numerically
stable against external fluctuations and is compatible with high fidelity
electrical readout of the photon-induced current. The elimination of the
external magnetic field potentially increases the capability of these types of
photon detectors and eases constraints involved in the fabrication of large
detector arrays.Comment: 15 pages, including 3 figure
Fast partial decoherence of a superconducting flux qubit in a spin bath
The superconducting flux qubit has two quantum states with opposite magnetic
flux. Environment of nuclear spins can find out the direction of the magnetic
flux after a decoherence time inversely proportional to the magnitude
of the flux and the square root of the number of spins. When the Hamiltonian of
the qubit drives fast coherent Rabi oscillations between the states with
opposite flux, then flux direction is flipped at a constant rate and
the decoherence time is much longer than .
However, on closer inspection decoherence actually takes place on two
timescales. The long time is a time of full decoherence but a part of
quantum coherence is lost already after the short time . This fast
partial decoherence biases coherent flux oscillations towards the initial flux
direction and it can affect performance of the superconducting devices as
qubits.Comment: 7 page
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
Coupling of Josephson current qubits using a connecting loop
We propose a coupling scheme for the three-Josephson junction qubits which
uses a connecting loop, but not mutual inductance. Present scheme offers the
advantages of a large and tunable level splitting in implementing the
controlled-NOT (CNOT) operation. We calculate the switching probabilities of
the coupled qubits in the CNOT operations and demonstrate that present CNOT
gate can meet the criteria for the fault-tolerant quantum computing. We obtain
the coupling strength as a function of the coupling energy of the Josephson
junction and the length of the connecting loop which varies with selecting two
qubits from the scalable design.Comment: 5 pages with updates, version to appear in Phys. Rev.
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
Josephson junctions in thin and narrow rectangular superconducting strips
I consider a Josephson junction crossing the middle of a thin rectangular
superconducting strip of length L and width W subjected to a perpendicular
magnetic induction B. I calculate the spatial dependence of the gauge-invariant
phase difference across the junction and the resulting B dependence of the
critical current Ic(B).Comment: 4 pages, 6 figures, revised following referee's comment
Hamiltonian for coupled flux qubits
An effective Hamiltonian is derived for two coupled three-Josephson-junction
(3JJ) qubits. This is not quite trivial, for the customary "free" 3JJ
Hamiltonian is written in the limit of zero inductance L. Neglecting the
self-flux is already dubious for one qubit when it comes to readout, and
becomes untenable when discussing inductive coupling. First, inductance effects
are analyzed for a single qubit. For small L, the self-flux is a "fast
variable" which can be eliminated adiabatically. However, the commonly used
junction phases are_not_ appropriate "slow variables", and instead one
introduces degrees of freedom which are decoupled from the loop current to
leading order. In the quantum case, the zero-point fluctuations (LC
oscillations) in the loop current diverge as L->0. Fortunately, they merely
renormalize the Josephson couplings of the effective (two-phase) theory.
In the coupled case, the strong zero-point fluctuations render the full
(six-phase) wave function significantly entangled in leading order. However, in
going to the four-phase theory, this uncontrollable entanglement is integrated
out completely, leaving a computationally usable mutual-inductance term of the
expected form as the effective interaction.Comment: REVTeX4, 16pp., one figure. N.B.: "Alec" is my first, and "Maassen
van den Brink" my family name. Informal note. v2: completely rewritten;
correction of final result and major expansion. v3: added numerical
verification plus a discussion of Ref. [2
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