389 research outputs found

### 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

### Dephasing of qubits by transverse low-frequency noise

We analyze the dissipative dynamics of a two-level quantum system subject to
low-frequency, e.g. 1/f noise, motivated by recent experiments with
superconducting quantum circuits. We show that the effect of transverse linear
coupling of the system to low-frequency noise is equivalent to that of
quadratic longitudinal coupling. We further find the decay law of quantum
coherent oscillations under the influence of both low- and high-frequency
fluctuations, in particular, for the case of comparable rates of relaxation and
pure dephasing

### Minimum construction of two-qubit quantum operations

Optimal construction of quantum operations is a fundamental problem in the
realization of quantum computation. We here introduce a newly discovered
quantum gate, B, that can implement any arbitrary two-qubit quantum operation
with minimal number of both two- and single-qubit gates. We show this by giving
an analytic circuit that implements a generic nonlocal two-qubit operation from
just two applications of the B gate. We also demonstrate that for the highly
scalable Josephson junction charge qubits, the B gate is also more easily and
quickly generated than the CNOT gate for physically feasible parameters.Comment: 4 page

### Relaxation of Josephson qubits due to strong coupling to two-level systems

We investigate the energy relaxation (T1) process of a qubit coupled to a
bath of dissipative two-level fluctuators (TLF). We consider the fluctuators
strongly coupled to the qubit both in the limit of spectrally separated single
TLF's as well as in the limit of spectrally dense TLF's. We conclude that the
avoided level crossings, usually attributed to very strongly coupled single
TLF's, could also be caused by many weakly coupled spectrally dense
fluctuators.Comment: 11+ pages, 10 figures, citations added, discussion extende

### Non-adiabatically detecting the geometric phase of the macroscopic quantum state with symmetric SQUID

We give a simple way to detect the geometric phase shift and the conditional
geometric phase shift with Josephson junction system. Comparing with the
previous work(Falcl G, Fazio R, Palma G.M., Siewert J and Verdal V, {\it
Nature} {\bf 407}, 355(2000)), our scheme has two advantages. We use the
non-adiabatic operation, thus the detection is less affected by the
decoherence. Also, we take the time evolution on zero dynamic phase loop, we
need not take any extra operation to cancel the dynamic phase.Comment: 8 pages, 4 figure

### Topological surface states in three-dimensional magnetic insulators

An electron moving in a magnetically ordered background feels an effective
magnetic field that can be both stronger and more rapidly varying than typical
externally applied fields. One consequence is that insulating magnetic
materials in three dimensions can have topologically nontrivial properties of
the effective band structure. For the simplest case of two bands, these "Hopf
insulators" are characterized by a topological invariant as in quantum Hall
states and Z_2 topological insulators, but instead of a Chern number or parity,
the underlying invariant is the Hopf invariant that classifies maps from the
3-sphere to the 2-sphere. This paper gives an efficient algorithm to compute
whether a given magnetic band structure has nontrivial Hopf invariant, a
double-exchange-like tight-binding model that realizes the nontrivial case, and
a numerical study of the surface states of this model.Comment: 4 pages, 2 figures; published versio

### Statistics and noise in a quantum measurement process

The quantum measurement process by a single-electron transistor or a quantum
point contact coupled to a quantum bit is studied. We find a unified
description of the statistics of the monitored quantity, the current, in the
regime of strong measurement and expect this description to apply for a wide
class of quantum measurements. We derive the probability distributions for the
current and charge in different stages of the process. In the parameter regime
of the strong measurement the current develops a telegraph-noise behavior which
can be detected in the noise spectrum.Comment: 4 pages, 2 figure

### Quantum logic operations and creation of entanglement in a scalable superconducting quantum computer with long-range constant interaction between qubits

We consider a one-dimensional chain of many superconducting quantum
interference devices (SQUIDs), serving as charge qubits. Each SQUID is coupled
to its nearest neighbors through constant capacitances. We study the quantum
logic operations and implementation of entanglement in this system.
Arrays with two and three qubits are considered in detail. We show that the
creation of entanglement with an arbitrary number of qubits can be implemented,
without systematic errors, even when the coupling between qubits is not small.
A relatively large coupling constant allows one to increase the clock speed of
the quantum computer. We analytically and numerically demonstrate the creation
of the entanglement for this case, which can be a good test for the
experimental implementation of a relatively simple quantum protocol with many
qubits. We discuss a possible application of our approach for implementing
universal quantum logic for more complex algorithms by decreasing the coupling
constant and, correspondingly, decreasing the clock speed. The errors
introduced by the long-range interaction for the universal logic gates are
estimated analytically and calculated numerically. Our results can be useful
for experimental implementation of quantum algorithms using controlled magnetic
fluxes and gate voltages applied to the SQUIDs. The algorithms discussed in
this paper can be implemented using already existing technologies in
superconducting systems with constant inter-qubit coupling.Comment: 24 page

### Optimal quantum circuit synthesis from Controlled-U gates

From a geometric approach, we derive the minimum number of applications
needed for an arbitrary Controlled-Unitary gate to construct a universal
quantum circuit. A new analytic construction procedure is presented and shown
to be either optimal or close to optimal. This result can be extended to
improve the efficiency of universal quantum circuit construction from any
entangling gate. Specifically, for both the Controlled-NOT and Double-CNOT
gates, we develop simple analytic ways to construct universal quantum circuits
with three applications, which is the least possible.Comment: 4 pages, 3 figure

### Full Frequency Back-Action Spectrum of a Single Electron Transistor during Qubit read-out

We calculate the spectral density of voltage fluctuations in a Single
Electron Transistor (SET), biased to operate in a transport mode where
tunneling events are correlated due to Coulomb interaction. The whole spectrum
from low frequency shot noise to quantum noise at frequencies comparable to the
SET charging energy $(E_{C}/\hbar)$ is considered. We discuss the back-action
during read-out of a charge qubit and conclude that single-shot read-out is
possible using the Radio-Frequency SET.Comment: 4 pages, 5 figures, submitted to PR

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