564 research outputs found
Low-control and robust quantum refrigerator and applications with electronic spins in diamond
We propose a general protocol for low-control refrigeration and thermometry
of thermal qubits, which can be implemented using electronic spins in diamond.
The refrigeration is implemented by a probe, consisting of a network of
interacting spins. The protocol involves two operations: (i) free evolution of
the probe; and (ii) a swap gate between one spin in the probe and the thermal
qubit we wish to cool. We show that if the initial state of the probe falls
within a suitable range, and the free evolution of the probe is both unital and
conserves the excitation in the -direction, then the cooling protocol will
always succeed, with an efficiency that depends on the rate of spin dephasing
and the swap gate fidelity. Furthermore, measuring the probe after it has
cooled many qubits provides an estimate of their temperature. We provide a
specific example where the probe is a Heisenberg spin chain, and suggest a
physical implementation using electronic spins in diamond. Here the probe is
constituted of nitrogen vacancy (NV) centers, while the thermal qubits are dark
spins. By using a novel pulse sequence, a chain of NV centers can be made to
evolve according to a Heisenberg Hamiltonian. This proposal allows for a range
of applications, such as NV-based nuclear magnetic resonance of photosensitive
molecules kept in a dark spot on a sample, and it opens up possibilities for
the study of quantum thermodynamics, environment-assisted sensing, and
many-body physics
Quantum sensing of temperature close to absolute zero in a Bose-Einstein condensate
We propose a theoretical scheme for quantum sensing of temperature close to
absolute zero in a quasi-one-dimensional Bose-Einstein condensate (BEC). In our
scheme, a single-atom impurity qubit is used as a temper-ature sensor. We
investigate the sensitivity of the single-atom sensor in estimating the
temperature of the BEC. We demonstrate that the sensitivity of the temperature
sensor can saturate the quantum Cramer-Rao bound by means of measuring quantum
coherence of the probe qubit. We study the temperature sensing performance by
the use of quantum signal-to-noise ratio (QSNR). It is indicated that there is
an optimal encoding time that the QSNR can reach its maximum in the
full-temperature regime. In particular, we find that the QSNR reaches a finite
upper bound in the weak coupling regime even when the temperature is close to
absolute zero, which implies that the sensing-error-divergence problem is
avoided in our scheme. Our work opens a way for quantum sensing of temperature
close to absolute zero in the BEC.Comment: 9 pages,9 figure
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
Temperature-heat uncertainty relation for quantum thermometry
We investigate the resource theory for temperature estimation. We demonstrate
that it is the fluctuation of heat that fundamentally determines temperature
precision through the temperature-heat uncertainty relation. Specifically, we
find that heat is divided into trajectory heat and correlation heat, which are
associated with the heat exchange along thermometer's evolution path and the
correlation between the thermometer and the sample, respectively. Based on two
type of thermometers, we show that both of these heat terms are resources for
enhancing temperature precision. Additionally, we demonstrate that the
temperature-heat uncertainty relation is consistent with the well known
temperature-energy uncertainty relation in thermodynamics. By clearly
distinguishing the resources for enhancing estimation precision, our findings
not only explain why various quantum features are crucial for accurate
temperature sensing but also provide valuable insights for designing
ultrahigh-sensitive quantum thermometers.Comment: 6 pages, 1 figur
Criticality-Enhanced Precision in Phase Thermometry
Temperature estimation of interacting quantum many-body systems is both a
challenging task and topic of interest in quantum metrology, given that
critical behavior at phase transitions can boost the metrological sensitivity.
Here we study non-invasive quantum thermometry of a finite, two-dimensional
Ising spin lattice based on measuring the non-Markovian dephasing dynamics of a
spin probe coupled to the lattice. We demonstrate a strong critical enhancement
of the achievable precision in terms of the quantum Fisher information, which
depends on the coupling range and the interrogation time. Our numerical
simulations are compared to instructive analytic results for the critical
scaling of the sensitivity in the Curie-Weiss model of a fully connected
lattice and to the mean-field description in the thermodynamic limit, both of
which fail to describe the critical spin fluctuations on the lattice the spin
probe is sensitive to. Phase metrology could thus help to investigate the
critical behaviour of finite many-body systems beyond the validity of
mean-field models.Comment: 11 pages, 8 figure
Quantum metrology out of equilibrium
We address open quantum systems out-of-equilibrium as effective quantum
probes for the characterisation of their environment. We discuss estimation
schemes for parameters driving a de-phasing evolution of the probe and then
focus on qubits, establishing a relationship between the quantum Fisher
information and the residual coherence of the probe. Finally, we apply our
results to the characterisation of the ohmicity parameter of a bosonic
environment
Quantum Limits of Thermometry
The precision of typical thermometers consisting of particles is shot
noise limited, improving as . For high precision thermometry
and thermometric standards this presents an important theoretical noise floor.
Here it is demonstrated that thermometry may be mapped onto the problem of
phase estimation, and using techniques from optimal phase estimation, it
follows that the scaling of the precision of a thermometer may in principle be
improved to , representing a Heisenberg limit to thermometry.Comment: 4 page
In situ thermometry of a cold Fermi gas via dephasing impurities
The precise measurement of low temperatures is a challenging, important and
fundamental task for quantum science. In particular, in-situ thermometry is
highly desirable for cold atomic systems due to their potential for quantum
simulation. Here we demonstrate that the temperature of a non-interacting Fermi
gas can be accurately inferred from the non-equilibrium dynamics of impurities
immersed within it, using an interferometric protocol and established
experimental methods. Adopting tools from the theory of quantum parameter
estimation, we show that our proposed scheme achieves optimal precision in the
relevant temperature regime for degenerate Fermi gases in current experiments.
We also discover an intriguing trade-off between measurement time and
thermometric precision that is controlled by the impurity-gas coupling, with
weak coupling leading to the greatest sensitivities. This is explained as a
consequence of the slow decoherence associated with the onset of the Anderson
orthogonality catastrophe, which dominates the gas dynamics following its local
interaction with the immersed impurity.Comment: 6+5 pages, 4+4 figures. Final author versio
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