7,964 research outputs found
Hybridized solid-state qubit in the charge-flux regime
Most superconducting qubits operate in a regime dominated by either the
electrical charge or the magnetic flux. Here we study an intermediate case: a
hybridized charge-flux qubit with a third Josephson junction (JJ) added into
the SQUID loop of the Cooper-pair box. This additional JJ allows the optimal
design of a low-decoherence qubit. Both charge and flux noises are
considered. Moreover, we show that an efficient quantum measurement of either
the current or the charge can be achieved by using different area sizes for the
third JJ.Comment: 7 pages, 5 figures. Phys. Rev. B, in pres
Scalable quantum computing with Josephson charge qubits
A goal of quantum information technology is to control the quantum state of a
system, including its preparation, manipulation, and measurement. However,
scalability to many qubits and controlled connectivity between any selected
qubits are two of the major stumbling blocks to achieve quantum computing (QC).
Here we propose an experimental method, using Josephson charge qubits, to
efficiently solve these two central problems. The proposed QC architecture is
scalable since any two charge qubits can be effectively coupled by an
experimentally accessible inductance. More importantly, we formulate an
efficient and realizable QC scheme that requires only one (instead of two or
more) two-bit operation to implement conditional gates.Comment: 4 pages, 2 figure
Low-decoherence flux qubit
A flux qubit can have a relatively long decoherence time at the degeneracy
point, but away from this point the decoherence time is greatly reduced by
dephasing. This limits the practical applications of flux qubits. Here we
propose a new qubit design modified from the commonly used flux qubit by
introducing an additional capacitor shunted in parallel to the smaller
Josephson junction (JJ) in the loop. Our results show that the effects of noise
can be considerably suppressed, particularly away from the degeneracy point, by
both reducing the coupling energy of the JJ and increasing the shunt
capacitance. This shunt capacitance provides a novel way to improve the qubit.Comment: 4 pages, 4 figure
Exotic phase diagram of a topological quantum system
We study the quantum phase transitions (QPTs) in the Kitaev spin model on a
triangle-honeycomb lattice. In addition to the ordinary topological QPTs
between Abelian and non-Abelian phases, we find new QPTs which can occur
between two phases belonging to the same topological class, namely, either two
non-Abelian phases with the same Chern number or two Abelian phases with the
same Chern number. Such QPTs result from the singular behaviors of the nonlocal
spin-spin correlation functions at the critical points.Comment: 10 pages, 5 figure
Measuring the quality factor of a microwave cavity using superconduting qubit devices
We propose a method to create superpositions of two macroscopic quantum
states of a single-mode microwave cavity field interacting with a
superconducting charge qubit. The decoherence of such superpositions can be
determined by measuring either the Wigner function of the cavity field or the
charge qubit states. Then the quality factor Q of the cavity can be inferred
from the decoherence of the superposed states. The proposed method is
experimentally realizable within current technology even when the value is
relatively low, and the interaction between the qubit and the cavity field is
weak.Comment: 8 page
Weak and strong measurement of a qubit using a switching-based detector
We analyze the operation of a switching-based detector that probes a qubit's
observable that does not commute with the qubit's Hamiltonian, leading to a
nontrivial interplay between the measurement and free-qubit dynamics. In order
to obtain analytic results and develop intuitive understanding of the different
possible regimes of operation, we use a theoretical model where the detector is
a quantum two-level system that is constantly monitored by a macroscopic
system. We analyze how to interpret the outcome of the measurement and how the
state of the qubit evolves while it is being measured. We find that the answers
to the above questions depend on the relation between the different parameters
in the problem. In addition to the traditional strong-measurement regime, we
identify a number of regimes associated with weak qubit-detector coupling. An
incoherent detector whose switching time is measurable with high accuracy can
provide high-fidelity information, but the measurement basis is determined only
upon switching of the detector. An incoherent detector whose switching time can
be known only with low accuracy provides a measurement in the qubit's energy
eigenbasis with reduced measurement fidelity. A coherent detector measures the
qubit in its energy eigenbasis and, under certain conditions, can provide
high-fidelity information.Comment: 20 pages (two-column), 6 figure
The information about the state of a charge qubit gained by a weakly coupled quantum point contact
We analyze the information that one can learn about the state of a quantum
two-level system, i.e. a qubit, when probed weakly by a nearby detector. We
consider the general case where the qubit Hamiltonian and the qubit's operator
probed by the detector do not commute. Because the qubit's state keeps evolving
while being probed and the measurement data is mixed with a detector-related
background noise, one might expect the detector to fail in this case. We show,
however, that under suitable conditions and by proper analysis of the
measurement data useful information about the initial state of the qubit can be
extracted. Our approach complements the usual master-equation and
quantum-trajectory approaches, which describe the evolution of the qubit's
quantum state during the measurement process but do not keep track of the
acquired measurement information.Comment: 5 pages, 3 figures; Published in the proceedings of the Nobel
Symposium 141: Qubits for Future Quantum Informatio
Quantum two-level systems in Josephson junctions as naturally formed qubits
The two-level systems (TLSs) naturally occurring in Josephson junctions
constitute a major obstacle for the operation of superconducting phase qubits.
Since these TLSs can possess remarkably long decoherence times, we show that
such TLSs can themselves be used as qubits, allowing for a well controlled
initialization, universal sets of quantum gates, and readout. Thus, a single
current-biased Josephson junction (CBJJ) can be considered as a multiqubit
register. It can be coupled to other CBJJs to allow the application of quantum
gates to an arbitrary pair of qubits in the system. Our results indicate an
alternative way to realize superconducting quantum information processing.Comment: Reference adde
Resonant peak splitting for ballistic conductance in magnetic superlattices
We investigate theoretically the resonant splitting of ballistic conductance
peaks in magnetic superlattices. It is found that, for magnetic superlattices
with periodically arranged identical magnetic-barriers, there exists a
general -fold resonant peak splitting rule for ballistic conductance,
which is the analogy of the -fold resonant splitting for transmission in
-barrier electric superlattices (R. Tsu and L. Esaki, Appl. Phys. Lett. {\bf
22}, 562 (1973)).Comment: 9 pages, 3 figures, latex forma
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