195 research outputs found
Decoherence in adiabatic quantum evolution - application to Cooper pair pumping
One of the challenges of adiabatic control theory is the proper inclusion of
the effects of dissipation. Here, we study the adiabatic dynamics of an open
two-level quantum system deriving a generalized master equation to consistently
account for the combined action of the driving and dissipation. We demonstrate
that in the zero temperature limit the ground state dynamics is not affected by
environment. As an example, we apply our theory to Cooper pair pumping which
demonstrates the robustness of ground state adiabatic evolution.Comment: 7 pages, derivation of the master equation in the appendi
Quantum Circuits for General Multiqubit Gates
We consider a generic elementary gate sequence which is needed to implement a
general quantum gate acting on n qubits -- a unitary transformation with 4^n
degrees of freedom. For synthesizing the gate sequence, a method based on the
so-called cosine-sine matrix decomposition is presented. The result is optimal
in the number of elementary one-qubit gates, 4^n, and scales more favorably
than the previously reported decompositions requiring 4^n-2^n+1 controlled NOT
gates.Comment: 4 pages, 3 figure
Observation of the single-electron regime in a highly tunable silicon quantum dot
We report on low-temperature electronic transport measurements of a silicon
metal-oxide-semiconductor quantum dot, with independent gate control of
electron densities in the leads and the quantum dot island. This architecture
allows the dot energy levels to be probed without affecting the electron
density in the leads, and vice versa. Appropriate gate biasing enables the dot
occupancy to be reduced to the single-electron level, as evidenced by
magnetospectroscopy measurements of the ground state of the first two charge
transitions. Independent gate control of the electron reservoirs also enables
discrimination between excited states of the dot and density of states
modulations in the leads.Comment: 4 pages, 3 figures, accepted for Applied Physics Letter
Vortex splitting and phase separating instabilities of coreless vortices in F=1 spinor Bose-Einstein condensates
The low lying excitations of coreless vortex states in F = 1 spinor
Bose-Einstein condensates (BECs) are theoretically investigated using the
Gross-Pitaevskii and Bogoliubov-de Gennes equations. The spectra of the
elementary excitations are calculated for different spin-spin interaction
parameters and ratios of the number of particles in each sublevel. There exist
dynamical instabilities of the vortex state which are suppressed by
ferromagnetic interactions, and conversely, enhanced by antiferromagnetic
interactions. In both of the spin-spin interaction regimes, we find vortex
splitting instabilities in analogy with scalar BECs. In addition, a phase
separating instability is found in the antiferromagnetic regime.Comment: 11 pages, 9 figure
Photon assisted tunneling as an origin of the Dynes density of states
We show that the effect of a high-temperature environment in current
transport through a normal metal-insulator-superconductor tunnel junction can
be described by an effective density of states (DOS) in the superconductor. In
the limit of a resistive low-ohmic environment, this DOS reduces into the
well-known Dynes form. Our theoretical result is supported by experiments in
engineered environments. We apply our findings to improve the performance of a
single-electron turnstile, a potential candidate for a metrological current
source.Comment: 4+3 pages, 4 figures; updated to the published version, includes
EPAPS supplementary materia
A proposal for implementing an n-qubit controlled-rotation gate with three-level superconducting qubit systems in cavity QED
We present a way for implementing an n-qubit controlled-rotation gate with
three-level superconducting qubit systems in cavity QED. The two logical states
of a qubit are represented by the two lowest levels of each system while a
higher-energy level is used for the gate implementation. The method operates
essentially by preparing a state conditioned on the states of the control
qubits, creating a single photon in the cavity mode, and then performing an
arbitrary rotation on the states of the target qubit with assistance of the
cavity photon. It is interesting to note that the basic operational steps for
implementing the proposed gate do not increase with the number of qubits,
and the gate operation time decreases as the number of qubits increases. This
proposal is quite general, which can be applied to various types of
superconducting devices in a cavity or coupled to a resonator.Comment: Six figures, accepted by Journal of Physics: Condensed Matte
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