19 research outputs found
Nonlinearity and nonclassicality in a nanomechanical resonator
We address quantitatively the relationship between the nonlinearity of a
mechanical resonator and the nonclassicality of its ground state. In
particular, we analyze the nonclassical properties of the nonlinear Duffing
oscillator (being driven or not) as a paradigmatic example of a nonlinear
nanomechanical resonator. We first discuss how to quantify the nonlinearity of
this system and then show that the nonclassicality of the ground state, as
measured by the volume occupied by the negative part of the Wigner function,
monotonically increases with the nonlinearity in all the working regimes
addressed in our study. Our results show quantitatively that nonlinearity is a
resource to create nonclassical states in mechanical systems.Comment: 6 pages; 7 figures; RevTeX4-
Quantifying the nonlinearity of a quantum oscillator
We address the quantification of nonlinearity for quantum oscillators and
introduce two measures based on the properties of the ground state rather than
on the form of the potential itself. The first measure is a fidelity-based one,
and corresponds to the renormalized Bures distance between the ground state of
the considered oscillator and the ground state of a reference harmonic
oscillator. Then, in order to avoid the introduction of this auxiliary
oscillator, we introduce a different measure based on the non-Gaussianity (nG)
of the ground state. The two measures are evaluated for a sample of significant
nonlinear potentials and their properties are discussed in some detail. We show
that the two measures are monotone functions of each other in most cases, and
this suggests that the nG-based measure is a suitable choice to capture the
anharmonic nature of a quantum oscillator, and to quantify its nonlinearity
independently on the specific features of the potential. We also provide
examples of potentials where the Bures measure cannot be defined, due to the
lack of a proper reference harmonic potential, while the nG-based measure
properly quantify their nonlinear features. Our results may have implications
in experimental applications where access to the effective potential is
limited, e.g., in quantum control, and protocols rely on information about the
ground or thermal state.Comment: 8 pages, 5 figures, published versio
Quantum phase communication channels assisted by non-deterministic noiseless amplifiers
We address quantum -ary phase-shift keyed (PSK) communication channels in
the presence of phase diffusion, and analyze the use of probabilistic noiseless
linear amplifiers (NLA) to enhance performance of coherent signals. We consider
both static and dynamical phase diffusion and assess the performances of the
channel for ideal and realistic phase receivers. Our results show that NLA
employed at the stage of signal preparations is a useful resource, especially
in the regime of weak signals. We also discuss the interplay between the use of
NLA, and the memory effects occurring with dynamical noise, in determining the
capacity of the channel.Comment: to appear in JOSA
Joint quantum estimation of loss and nonlinearity in driven-dissipative Kerr resonators
We address multiparameter quantum estimation for coherently driven nonlinear
Kerr resonators in the presence of loss. In particular, we consider the
realistic situation in which the parameters of interest are the loss rate and
the nonlinear coupling, whereas the amplitude of the coherent driving is known
and externally tunable. Our results show that this driven-dissipative model is
asymptotically classical, i.e. the Uhlmann curvature vanishes, and the two
parameters may be jointly estimated without any additional noise of quantum
origin. We also find that the ultimate bound to precision, as quantified by the
quantum Fisher information (QFI), increases with the interaction time and the
driving amplitude for both parameters. Finally, we investigate the performance
of quadrature detection, and show that for both parameters the Fisher
information oscillates in time, repeatedly approaching the corresponding QFI
Experimental estimation of one-parameter qubit gates in the presence of phase diffusion
We address estimation of one-parameter qubit gates in the presence of phase
diffusion. We evaluate the ultimate quantum limits to precision, seek for
optimal probes and measurements, and demonstrate an optimal estimation scheme
for polarization qubits. An adaptive method to achieve optimal estimation in
any working regime is also analyzed in details and experimentally implemented.Comment: revised version, to appear on PR
Bayesian estimation of one-parameter qubit gates
We address estimation of one-parameter unitary gates for qubit systems and
seek for optimal probes and measurements. Single- and two-qubit probes are
analyzed in details focusing on precision and stability of the estimation
procedure. Bayesian inference is employed and compared with the ultimate
quantum limits to precision, taking into account the biased nature of Bayes
estimator in the non asymptotic regime. Besides, through the evaluation of the
asymptotic a posteriori distribution for the gate parameter and the comparison
with the results of Monte Carlo simulated experiments, we show that asymptotic
optimality of Bayes estimator is actually achieved after a limited number of
runs. The robustness of the estimation procedure against fluctuations of the
measurement settings is investigated and the use of entanglement to improve the
overall stability of the estimation scheme is also analyzed in some details.Comment: 10 pages, 5 figure