8,126 research outputs found
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
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
Implementing topological quantum manipulation with superconducting circuits
A two-component fermion model with conventional two-body interactions was
recently shown to have anyonic excitations. We here propose a scheme to
physically implement this model by transforming each chain of two two-component
fermions to the two capacitively coupled chains of superconducting devices. In
particular, we elaborate how to achieve the wanted operations to create and
manipulate the topological quantum states, providing an experimentally feasible
scenario to access the topological memory and to build the anyonic
interferometry.Comment: 4 pages with 3 figures; V2: published version with minor updation
Persistent single-photon production by tunable on-chip micromaser with a superconducting quantum circuit
We propose a tunable on-chip micromaser using a superconducting quantum
circuit (SQC). By taking advantage of externally controllable state
transitions, a state population inversion can be achieved and preserved for the
two working levels of the SQC and, when needed, the SQC can generate a single
photon. We can regularly repeat these processes in each cycle when the
previously generated photon in the cavity is decaying, so that a periodic
sequence of single photons can be produced persistently. This provides a
controllable way for implementing a persistent single-photon source on a
microelectronic chip.Comment: 8 pages, 4 figure
Cooling a mechanical resonator via coupling to a tunable double quantum dot
We study the cooling of a mechanical resonator (MR) that is capacitively
coupled to a double quantum dot (DQD). The MR is cooled by the dynamical
backaction induced by the capacitive coupling between the DQD and the MR. The
DQD is excited by a microwave field and afterwards a tunneling event results in
the decay of the excited state of the DQD. An important advantage of this
system is that both the energy level splitting and the decay rate of the DQD
can be well tuned by varying the gate voltage. We find that the steady average
occupancy, below unity, of the MR can be achieved by changing both the decay
rate of the excited state and the detuning between the transition frequency of
the DQD and the microwave frequency, in analogy to the laser sideband cooling
of an atom or trapped ion in atomic physics. Our results show that the cooling
of the MR to the ground state is experimentally implementable.Comment: 10 pages, 5 figure
Probing tiny motions of nanomechanical resonators: classical or quantum mechanical?
We propose a spectroscopic approach to probe tiny vibrations of a
nanomechanical resonator (NAMR), which may reveal classical or quantum behavior
depending on the decoherence-inducing environment. Our proposal is based on the
detection of the voltage-fluctuation spectrum in a superconducting transmission
line resonator (TLR), which is {\it indirectly} coupled to the NAMR via a
controllable Josephson qubit acting as a quantum transducer. The classical
(quantum mechanical) vibrations of the NAMR induce symmetric (asymmetric) Stark
shifts of the qubit levels, which can be measured by the voltage fluctuations
in the TLR. Thus, the motion of the NAMR, including if it is quantum mechanical
or not, could be probed by detecting the voltage-fluctuation spectrum of the
TLR.Comment: 4 pages, 3 figures. to appear in Physical Review Letter
Interqubit coupling mediated by a high-excitation-energy quantum object
We consider a system composed of two qubits and a high-excitation-energy
quantum object used to mediate coupling between the qubits. We treat the entire
system quantum mechanically and analyze the properties of the eigenvalues and
eigenstates of the total Hamiltonian. After reproducing well-known results
concerning the leading term in the mediated coupling, we obtain an expression
for the residual coupling between the qubits in the off state. We also analyze
the entanglement between the three objects, i.e. the two qubits and the
coupler, in the eigenstates of the total Hamiltonian. Although we focus on the
application of our results to the recently realized parametric-coupling scheme
with two qubits, we also discuss extensions of our results to
harmonic-oscillator couplers, couplers that are near resonance with the qubits
and multi-qubit systems. In particular, we find that certain errors that are
absent for a two-qubit system arise when dealing with multi-qubit systems.Comment: 15 pages (two-column
- …