46 research outputs found
Spin-echo entanglement protection from random telegraph noise
We analyze local spin-echo procedures to protect entanglement between two
non-interacting qubits, each subject to pure-dephasing random telegraph noise.
For superconducting qubits this simple model captures characteristic features
of the effect of bistable impurities coupled to the device. An analytic
expression for the entanglement dynamics is reported. Peculiar features related
to the non-Gaussian nature of the noise already observed in the single qubit
dynamics also occur in the entanglement dynamics for proper values of the ratio
, between the qubit-impurity coupling strength and the switching
rate of the random telegraph process, and of the separation between the pulses
. We find that the echo procedure may delay the disappearance of
entanglement, cancel the dynamical structure of entanglement revivals and dark
periods, and induce peculiar plateau-like behaviors of the concurrence.Comment: 9 pages, 2 figure
Memory effects in a Markov chain dephasing channel
We study a dephasing channel with memory, modelled by a Markov chain. We show
that even weak memory effects have a detrimental impact on the performance of
quantum error correcting schemes designed for uncorrelated errors. We also
discuss an alternative scheme that takes advantage of memory effects to protect
quantum information.Comment: 5 pages, 1 figure, NIC@QS proceeding
Detection of finite frequency photo-assisted shot noise with a resonant circuit
Photo-assisted transport through a mesoscopic conductor occurs when an
oscillatory (AC) voltage is superposed to the constant (DC) bias which is
imposed on this conductor. Of particular interest is the photo assisted shot
noise, which has been investigated theoretically and experimentally for several
types of samples. For DC biased conductors, a detection scheme for finite
frequency noise using a dissipative resonant circuit, which is inductively
coupled to the mesoscopic device, was developped recently. We argue that the
detection of the finite frequency photo-assisted shot noise can be achieved
with the same setup, despite the fact that time translational invariance is
absent here. We show that a measure of the photo-assisted shot noise can be
obtained through the charge correlator associated with the resonant circuit,
where the latter is averaged over the AC drive frequency. We test our
predictions for a point contact placed in the fractional quantum Hall effect
regime, for the case of weak backscattering. The Keldysh elements of the
photo-assisted noise correlator are computed. For simple Laughlin fractions,
the measured photo-assisted shot noise displays peaks at the frequency
corresponding to the DC bias voltage, as well as satellite peaks separated by
the AC drive frequency
Entanglement between two superconducting qubits via interaction with non-classical radiation
We propose a scheme to physically interface superconducting nano-circuits and
quantum optics. We address the transfer of quantum information between systems
having different physical natures and defined in Hilbert spaces of different
dimensions. In particular, we investigate the transfer of the entanglement
initially in a non-classical state of a continuous-variable system to a pair of
superconducting charge qubits. This set-up is able to drive an initially
separable state of the qubits into an almost pure, highly entangled state
suitable for quantum information processing.Comment: 4 pages, RevTeX; revised versio
Second spectrum of charge carrier density fluctuations in graphene due to trapping/detrapping processes
We investigate the second spectrum of charge carrier density fluctuations in
graphene within the McWorther model, where noise is induced by electron traps
in the substrate. Within this simple picture, we obtain a closed-form
expression including both Gaussian and non-Gaussian fluctuations. We show that
a very extended distribution of switching rates of the electron traps in the
substrate leads to a carrier density power spectrum with a non-trivial
structure on the scale of the measurement bandwidth. This explains the
appearance of a component in the Gaussian part of the second spectrum,
which adds up to the expected frequency-independent term. Finally, we find that
the non-Gaussian part of the second spectrum can become quantitatively relevant
by approaching extremely low temperatures.Comment: 10 pages, 2 figure
Low-frequency critical current noise in graphene Josephson junctions in the open-circuit gate voltage limit
We investigate critical current noise in short ballistic graphene Josephson
junctions in the open-circuit gate-voltage limit within the McWorther model. We
find flicker noise in a wide frequency range and discuss the temperature
dependence of the noise amplitude as a function of the doping level. At the
charge neutrality point we find a singular temperature dependence ,
strikingly different from the linear dependence expected for short ballistic
graphene Josephson junctions under fixed gate voltage
Entanglement dynamics in superconducting qubits affected by local bistable impurities
We study the entanglement dynamics for two independent superconducting qubits, each affected by a bistable impurity generating random telegraph noise (RTN) at pure dephasing. The relevant parameter is the ratio g between the qubit–RTN coupling strength and the RTN switching rate, which captures the physics of the crossover between Markovian and non-Markovian features of the dynamics. For identical qubit–RTN subsystems, a threshold value gth of the crossover parameter separates exponential decay and the onset of revivals; different qualitative behaviors also show up by changing the initial conditions of the RTN. We also show that, for different qubit–RTN subsystems, when both qubits are very strongly coupled to the RTN, an increase of entanglement revival amplitude may occur during the dynamics