7,957 research outputs found
Variable-frequency-controlled coupling in charge qubit circuits: Effects of microwave field on qubit-state readout
To implement quantum information processing, microwave fields are often used
to manipulate superconuducting qubits. We study how the coupling between
superconducting charge qubits can be controlled by variable-frequency magnetic
fields. We also study the effects of the microwave fields on the readout of the
charge-qubit states. The measurement of the charge-qubit states can be used to
demonstrate the statistical properties of photons.Comment: 7 pages, 3 figure
Optical selection rules and phase-dependent adiabatic state control in a superconducting quantum circuit
We analyze the optical selection rules of the microwave-assisted transitions
in a flux qubit superconducting quantum circuit (SQC). We show that the
parities of the states relevant to the superconducting phase in the SQC are
well-defined when the external magnetic flux , then the
selection rules are same as the ones for the electric-dipole transitions in
usual atoms. When , the symmetry of the potential of
the artificial "atom'' is broken, a so-called -type "cyclic"
three-level atom is formed, where one- and two-photon processes can coexist. We
study how the population of these three states can be selectively transferred
by adiabatically controlling the electromagnetic field pulses. Different from
-type atoms, the adiabatic population transfer in our three-level
-atom can be controlled not only by the amplitudes but also by the
phases of the pulses
A qubit strongly-coupled to a resonant cavity: asymmetry of the spontaneous emission spectrum beyond the rotating wave approximation
We investigate the spontaneous emission spectrum of a qubit in a lossy
resonant cavity. We use neither the rotating-wave approximation nor the Markov
approximation. The qubit-cavity coupling strength is varied from weak, to
strong, even to lower bound of the ultra-strong. For the weak-coupling case,
the spontaneous emission spectrum of the qubit is a single peak, with its
location depending on the spectral density of the qubit environment. Increasing
the qubit-cavity coupling increases the asymmetry (the positions about the
qubit energy spacing and heights of the two peaks) of the two spontaneous
emission peaks (which are related to the vacuum Rabi splitting) more.
Explicitly, for a qubit in a low-frequency intrinsic bath, the height asymmetry
of the splitting peaks becomes larger, when the qubit-cavity coupling strength
is increased. However, for a qubit in an Ohmic bath, the height asymmetry of
the spectral peaks is inverted from the same case of the low-frequency bath,
when the qubit is strongly coupled to the cavity. Increasing the qubit-cavity
coupling to the lower bound of the ultra-strong regime, the height asymmetry of
the left and right peak heights are inverted, which is consistent with the same
case of low-frequency bath, only relatively weak. Therefore, our results
explicitly show how the height asymmetry in the spontaneous emission spectrum
peaks depends not only on the qubit-cavity coupling, but also on the type of
intrinsic noise experienced by the qubit.Comment: 10pages, 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
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
Simultaneous cooling of an artificial atom and its neighboring quantum system
We propose an approach for cooling both an artificial atom (e.g., a flux
qubit) and its neighboring quantum system, the latter modeled by either a
quantum two-level system or a quantum resonator. The flux qubit is cooled by
manipulating its states, following an inverse process of state population
inversion, and then the qubit is switched on to resonantly interact with the
neighboring quantum system. By repeating these steps, the two subsystems can be
simultaneously cooled. Our results show that this cooling is robust and
effective, irrespective of the chosen quantum systems connected to the qubit.Comment: 5 pages, 3 figure
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
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
Producing cluster states in charge qubits and flux qubits
We propose a method to efficiently generate cluster states in charge qubits,
both semiconducting and superconducting, as well as flux qubits. We show that
highly-entangled cluster states can be realized by a `one-touch' entanglement
operation by tuning gate bias voltages for charge qubits. We also investigate
the robustness of these cluster states for non-uniform qubits, which are
unavoidable in solid-state systems. We find that quantum computation based on
cluster states is a promising approach for solid-state qubits.Comment: 4 pages, 1 figure
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