4 research outputs found
Quantum Probes for Ohmic Environments at Thermal Equilibrium
It is often the case that the environment of a quantum system may be
described as a bath of oscillators with Ohmic density of states. In turn, the
precise characterization of these classes of environments is a crucial tool to
engineer decoherence or to tailor quantum information protocols. Recently, the
use of quantum probes in characterizing Ohmic environments at zero-temperature
has been discussed, showing that a single qubit provides precise estimation of
the cutoff frequency. On the other hand, thermal noise often spoil quantum
probing schemes, and for this reason we here extend the analysis to complex
system at thermal equilibrium. In particular, we discuss the interplay between
thermal fluctuations and time evolution in determining the precision
{attainable by} quantum probes. Our results show that the presence of thermal
fluctuations degrades the precision for low values of the cutoff frequency,
i.e. values of the order (in natural units). For larger
values of decoherence is mostly due to the structure of environment,
rather than thermal fluctuations, such that quantum probing by a single qubit
is still an effective estimation procedure.Comment: Entropy, special issue on Open Quantum Systems (OQS) for quantum
technologies (S. Lorenzo and M. G. Palma, Eds
Quantum probes for the cutoff frequency of Ohmic environments
Quantum probing consists of suitably exploiting a simple, small, and
controllable quantum system to characterize a larger and more complex system.
Here, we address the estimation of the cutoff frequency of the Ohmic spectral
density of a harmonic reservoir by quantum probes. To this aim, we address the
use of single-qubit and two-qubit systems and different kinds of coupling with
the bath of oscillators. We assess the estimation precision by the quantum
Fisher information of the sole quantum probe as well as the corresponding
quantum signal-to-noise ratio. We prove that, for most of the values of the
Ohmicity parameter, a simple probe such as a single qubit is already optimal
for the precise estimation of the cutoff frequency. Indeed for those values,
upon considering a two-qubit probe either in a Bell or in separable state, we
do not find improvement to the estimation precision. However, we also showed
that there exist few conditions where employing two qubits in a Bell state
interacting with a common bath is more suitable for precisely estimating the
cutoff frequency.Comment: 8 pages, 5 figures, 1 tabl