415 research outputs found
Photon-Number Squeezing in Circuit Quantum Electrodynamics
A superconducting single-electron transistor (SSET) coupled to an anharmonic
oscillator, e.g., a Josephson junction-L-C circuit, can drive the latter to a
nonequilibrium photon number state. By biasing the SSET in a regime where the
current is carried by a combination of inelastic quasiparticle tunneling and
coherent Cooper-pair tunneling (Josephson quasiparticle cycle), cooling of the
oscillator as well as a laser like enhancement of the photon number can be
achieved. Here we show, that the cut-off in the quasiparticle tunneling rate
due to the superconducting gap, in combination with the anharmonicity of the
oscillator, may create strongly squeezed photon number distributions. For low
dissipation in the oscillator nearly pure Fock states can be produced.Comment: 5 pages, 5 figure
Phase diffusion and locking in single-qubit lasers
Motivated by recent experiments, which demonstrated lasing and cooling of the
electromagnetic field in an electrical resonator coupled to a superconducting
qubit, we study the phase coherence and diffusion of the system in the lasing
state. We also discuss phase locking and synchronization induced by an
additional {\sl ac} driving of the resonator. We extend earlier work to account
for the strong qubit-resonator coupling and to include the effects of
low-frequency qubit's noise. We show that the strong coupling may lead to a
double peak structure of the spectrum, while the shape and width are determined
to the low-frequency noise.Comment: Revised version with a new section about the validity of the model
when applied to describe experiment
Resonant all-electric spin pumping with spin-orbit coupling
All-electric devices for the generation and filtering of spin currents are of
crucial importance for spintronics experiments and applications. Here we
consider a quantum dot between two metallic leads in the presence of spin-orbit
coupling, and we analyze in the frame of a scattering matrix approach the
conditions for generating spin currents in an adiabatically driven two-terminal
device. We then focus on a dot with two resonant orbitals and show by specific
examples that both spin filtering and pure spin current generation can be
achieved. Finally, we discuss the effect of the Coulomb interaction.Comment: 4 pages, corrected typo in list of author
Single-qubit lasing and cooling at the Rabi frequency
For a superconducting qubit driven to perform Rabi oscillations and coupled
to a slow electromagnetic or nano-mechanical oscillator we describe previously
unexplored quantum optics effects. When the Rabi frequency is tuned to
resonance with the oscillator the latter can be driven far from equilibrium.
Blue detuned driving leads to a population inversion in the qubit and a
bi-stability with lasing behavior of the oscillator; for red detuning the qubit
cools the oscillator. This behavior persists at the symmetry point where the
qubit-oscillator coupling is quadratic and decoherence effects are minimized.
There the system realizes a "single-atom-two-photon laser".Comment: Replaced with final published version, fig. 2 compresse
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