564 research outputs found
Optimal multiqubit operations for Josephson charge qubits
We introduce a method for finding the required control parameters for a
quantum computer that yields the desired quantum algorithm without invoking
elementary gates. We concentrate on the Josephson charge-qubit model, but the
scenario is readily extended to other physical realizations. Our strategy is to
numerically find any desired double- or triple-qubit gate. The motivation is
the need to significantly accelerate quantum algorithms in order to fight
decoherence.Comment: 4 pages, 5 figure
Information entropic superconducting microcooler
We consider a design for a cyclic microrefrigerator using a superconducting
flux qubit. Adiabatic modulation of the flux combined with thermalization can
be used to transfer energy from a lower temperature normal metal thin film
resistor to another one at higher temperature. The frequency selectivity of
photonic heat conduction is achieved by including the hot resistor as part of a
high frequency LC resonator and the cold one as part of a low-frequency
oscillator while keeping both circuits in the underdamped regime. We discuss
the performance of the device in an experimentally realistic setting. This
device illustrates the complementarity of information and thermodynamic entropy
as the erasure of the quantum bit directly relates to the cooling of the
resistor.Comment: 4 pages, 3 figure
Ground-state geometric quantum computing in superconducting systems
We present a theoretical proposal for the implementation of geometric quantum
computing based on a Hamiltonian which has a doubly degenerate ground state.
Thus the system which is steered adiabatically, remains in the ground-state.
The proposed physical implementation relies on a superconducting circuit
composed of three SQUIDs and two superconducting islands with the charge states
encoding the logical states. We obtain a universal set of single-qubit gates
and implement a non-trivial two-qubit gate exploiting the mutual inductance
between two neighboring circuits, allowing us to realize a fully geometric
ground-state quantum computing. The introduced paradigm for the implementation
of geometric quantum computing is expected to be robust against environmental
effects.Comment: 9 pages, 5 figures. Final version with notation and typos correcte
Decoherence of flux qubits due to 1/f flux noise
We have investigated decoherence in Josephson-junction flux qubits. Based on
the measurements of decoherence at various bias conditions, we discriminate
contributions of different noise sources. In particular, we present a Gaussian
decay function of the echo signal as evidence of dephasing due to flux
noise whose spectral density is evaluated to be about /Hz
at 1 Hz. We also demonstrate that at an optimal bias condition where the noise
sources are well decoupled the coherence observed in the echo measurement is
mainly limited by energy relaxation of the qubit.Comment: 4 pages, error in Fig.4 corrected, to appear in PR
Electron-phonon coupling and longitudinal mechanical-mode cooling in a metallic nanowire
We investigate electron-phonon coupling in a narrow suspended metallic wire,
in which the phonon modes are restricted to one dimension but the electrons
behave three-dimensionally. Explicit theoretical results related to the known
bulk properties are derived. We find out that longitudinal vibration modes can
be cooled by electronic tunnel refrigeration far below the bath temperature
provided the mechanical quality factors of the modes are sufficiently high. The
obtained results apply to feasible experimental configurations.Comment: 4+ pages, 3 figure
Exact solutions of the isoholonomic problem and the optimal control problem in holonomic quantum computation
The isoholonomic problem in a homogeneous bundle is formulated and solved
exactly. The problem takes a form of a boundary value problem of a variational
equation. The solution is applied to the optimal control problem in holonomic
quantum computer. We provide a prescription to construct an optimal controller
for an arbitrary unitary gate and apply it to a -dimensional unitary gate
which operates on an -dimensional Hilbert space with . Our
construction is applied to several important unitary gates such as the Hadamard
gate, the CNOT gate, and the two-qubit discrete Fourier transformation gate.
Controllers for these gates are explicitly constructed.Comment: 19 pages, no figures, LaTeX2
Calibration and High Fidelity Measurement of a Quantum Photonic Chip
Integrated quantum photonic circuits are becoming increasingly complex.
Accurate calibration of device parameters and detailed characterization of the
prepared quantum states are critically important for future progress. Here we
report on an effective experimental calibration method based on Bayesian
updating and Markov chain Monte Carlo integration. We use this calibration
technique to characterize a two qubit chip and extract the reflectivities of
its directional couplers. An average quantum state tomography fidelity of
93.79+/-1.05% against the four Bell states is achieved. Furthermore, comparing
the measured density matrices against a model using the non-ideal device
parameters derived from the calibration we achieve an average fidelity of
97.57+/-0.96%. This pinpoints non-ideality of chip parameters as a major factor
in the decrease of Bell state fidelity. We also perform quantum state
tomography for Bell states while continuously varying photon distinguishability
and find excellent agreement with theory
Electronic cooling of a submicron-sized metallic beam
We demonstrate electronic cooling of a suspended AuPd island using
superconductor-insulator-normal metal tunnel junctions. This was achieved by
developing a simple fabrication method for reliably releasing narrow submicron
sized metal beams. The process is based on reactive ion etching and uses a
conducting substrate to avoid charge-up damage and is compatible with e.g.
conventional e-beam lithography, shadow-angle metal deposition and oxide tunnel
junctions. The devices function well and exhibit clear cooling; up to factor of
two at sub-kelvin temperatures.Comment: 4 pages, 3 figure
Reconfigurable controlled two-qubit operation on a quantum photonic chip
Integrated quantum photonics is an appealing platform for quantum information
processing, quantum communication and quantum metrology. In all these
applications it is necessary not only to be able to create and detect Fock
states of light but also to program the photonic circuits that implements some
desired logical operation. Here we demonstrate a reconfigurable controlled
two-qubit operation on a chip using a multiwaveguide interferometer with a
tunable phase shifter. We find excellent agreement between theory and
experiment, with a 0.98 \pm 0.02 average similarity between measured and ideal
operations
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