200 research outputs found
Exact dimension estimation of interacting qubit systems assisted by a single quantum probe
Estimating the dimension of an Hilbert space is an important component of
quantum system identification. In quantum technologies, the dimension of a
quantum system (or its corresponding accessible Hilbert space) is an important
resource, as larger dimensions determine e.g. the performance of quantum
computation protocols or the sensitivity of quantum sensors. Despite being a
critical task in quantum system identification, estimating the Hilbert space
dimension is experimentally challenging. While there have been proposals for
various dimension witnesses capable of putting a lower bound on the dimension
from measuring collective observables that encode correlations, in many
practical scenarios, especially for multiqubit systems, the experimental
control might not be able to engineer the required initialization, dynamics and
observables.
Here we propose a more practical strategy, that relies not on directly
measuring an unknown multiqubit target system, but on the indirect interaction
with a local quantum probe under the experimenter's control. Assuming only that
the interaction model is given and the evolution correlates all the qubits with
the probe, we combine a graph-theoretical approach and realization theory to
demonstrate that the dimension of the Hilbert space can be exactly estimated
from the model order of the system. We further analyze the robustness in the
presence of background noise of the proposed estimation method based on
realization theory, finding that despite stringent constrains on the allowed
noise level, exact dimension estimation can still be achieved.Comment: v3: accepted version. We would like to offer our gratitudes to the
editors and referees for their helpful and insightful opinions and feedback
Quantifying precision loss in local quantum thermometry via diagonal discord
When quantum information is spread over a system through nonclassical
correlation, it makes retrieving information by local measurements
difficult---making global measurement necessary for optimal parameter
estimation. In this paper, we consider temperature estimation of a system in a
Gibbs state and quantify the separation between the estimation performance of
the global optimal measurement scheme and a greedy local measurement scheme by
diagonal quantum discord. In a greedy local scheme, instead of global
measurements, one performs sequential local measurement on subsystems, which is
potentially enhanced by feed-forward communication. We show that, for
finite-dimensional systems, diagonal discord quantifies the difference in the
quantum Fisher information quantifying the precision limits for temperature
estimation of these two schemes, and we analytically obtain the relation in the
high-temperature limit. We further verify this result by employing the examples
of spins with Heisenberg's interaction.Comment: 5+4 pages, 4 figures, We thank the referees and editors for helpful
opinions. Accepted by Phys. Rev. A (accepted version
Hamiltonian identifiability assisted by a single-probe measurement
We study the Hamiltonian identifiability of a many-body spin-1/2 system assisted by the measurement on a single quantum probe based on the eigensystem realization algorithm approach employed in Zhang and Sarovar, Phys. Rev. Lett. 113, 080401 (2014). We demonstrate a potential application of Gröbner basis to the identifiability test of the Hamiltonian, and provide the necessary experimental resources, such as the lower bound in the number of the required sampling points, the upper bound in total required evolution time, and thus the total measurement time. Focusing on the examples of the identifiability in the spin-chain model with nearest-neighbor interaction, we classify the spin-chain Hamiltonian based on its identifiability, and provide the control protocols to engineer the nonidentifiable Hamiltonian to be an identifiable Hamiltonian.United States. Army Research Office (W911NF-11-1-0400)United States. Army Research Office (W911NF-15-1-0548)National Science Foundation (U.S.) (PHY0551153
Quantum and classical ergotropy from relative entropies
The quantum ergotropy quantifies the maximal amount of work that can be
extracted from a quantum state without changing its entropy. We prove that the
ergotropy can be expressed as the difference of quantum and classical relative
entropies of the quantum state with respect to the thermal state. This insight
is exploited to define the classical ergotropy, which quantifies how much work
can be extracted from distributions that are inhomogeneous on the energy
surfaces. A unified approach to treat both, quantum as well as classical
scenarios, is provided by geometric quantum mechanics, for which we define the
geometric relative entropy. The analysis is concluded with an application of
the conceptual insight to conditional thermal states, and the correspondingly
tightened maximum work theorem.Comment: 4.5 pages v3. fixed typo
Detailed Fluctuation Theorem from the One-Time Measurement Scheme
The quantum fluctuation theorem can be regarded as the first principle of
quantum nonequilibrium thermodynamics. However, many different formulations
have been proposed, which depend on how quantum work is defined. In this
context, we have seen that for some situations the one-time measurement (OTM)
scheme can be more informative than the two-time measurement (TTM) scheme. Yet,
so far the focus of OTM has been on integral fluctuation theorems, since, the
work distribution of the backward process has been lacking. To this end, we
prove that the OTM scheme is the quantum nondemolition (QND) TTM scheme, in
which the final state is a pointer state of the second measurement whose
Hamiltonian is conditioned on the first measurement outcome. This insight leads
to a derivation of the detailed fluctuation theorem for the characteristic
functions of the forward and backward work distributions, which captures the
detailed information about the irreversibility and can be applied to quantum
thermometry. Finally, our conceptual findings are experimentally verified with
the IBM quantum computerComment: v2: added new analyses on irreversibilit
Achieving quantum advantages for image filtering
Image processing is a fascinating field for exploring quantum algorithms.
However, achieving quantum speedups turns out to be a significant challenge. In
this work, we focus on image filtering to identify a class of images that can
achieve a substantial speedup. We show that for images with efficient encoding
and a lower bound on the signal-to-noise ratio, a quantum filtering algorithm
can be constructed with a polynomial complexity in terms of the qubit number.
Our algorithm combines the quantum Fourier transform with the amplitude
amplification technique. To demonstrate the advantages of our approach, we
apply it to three typical filtering problems. We highlight the importance of
efficient encoding by illustrating that for images that cannot be efficiently
encoded, the quantum advantage will diminish. Our work provides insights into
the types of images that can achieve a substantial quantum speedup.Comment: 8 pages, 9 figure
Jarzynski-like Equality of Nonequilibrium Information Production Based on Quantum Cross Entropy
The two-time measurement scheme is well studied in the context of quantum
fluctuation theorem. However, it becomes infeasible when the random variable
determined by a single measurement trajectory is associated with the
von-Neumann entropy of the quantum states. We employ the one-time measurement
scheme to derive a Jarzynski-like equality of nonequilibrium information
production by proposing an information production distribution based on the
quantum cross entropy. The derived equality further enables one to explore the
roles of the quantum cross entropy in quantum communications, quantum machine
learning and quantum thermodynamics.Comment: v2: We removed the results of two-time measurement scheme, and added
the relations of our main result of the one-time measurement scheme to the
cost function of quantum autoencoder and maximum available work theore
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