16 research outputs found
Thermal Baths as Quantum Resources: More Friends than Foes?
In this article we argue that thermal reservoirs (baths) are potentially
useful resources in processes involving atoms interacting with quantized
electromagnetic fields and their applications to quantum technologies. One may
try to suppress the bath effects by means of dynamical control, but such
control does not always yield the desired results. We wish instead to take
advantage of bath effects, that do not obliterate "quantumness" in the
system-bath compound. To this end, three possible approaches have been pursued
by us: (i) Control of a quantum system faster than the correlation time of the
bath to which it couples: Such control allows us to reveal
quasi-reversible/coherent dynamical phenomena of quantum open systems, manifest
by the quantum Zeno or anti-Zeno effects (QZE or AZE, respectively). Dynamical
control methods based on the QZE are aimed not only at protecting the
quantumness of the system, but also diagnosing the bath spectra or transferring
quantum information via noisy media. By contrast, AZE-based control is useful
for fast cooling of thermalized quantum systems. (ii) Engineering the coupling
of quantum systems to selected bath modes: This approach, based on field -atom
coupling control in cavities, waveguides and photonic band structures, allows
to drastically enhance the strength and range of atom-atom coupling through the
mediation of the selected bath modes. More dramatically, it allows us to
achieve bath-induced entanglement that may appear paradoxical if one takes the
conventional view that coupling to baths destroys quantumness. (iii)
Engineering baths with appropriate non-flat spectra: This approach is a
prerequisite for the construction of the simplest and most efficient quantum
heat machines (engines and refrigerators). We may thus conclude that often
thermal baths are "more friends than foes" in quantum technologies.Comment: 27 pages, 17 figure
Criticality of environmental information obtainable by dynamically controlled quantum probes
A universal approach to decoherence control combined with quantum estimation
theory reveals a critical behavior, akin to a phase transition, of the
information obtainable by a qubit probe concerning the memory time of
environmental fluctuations. This criticality emerges only when the probe is
subject to dynamical control. It gives rise to a sharp transition between two
dynamical phases characterized by either a short or long memory time compared
to the probing time. This phase-transition of the environmental information is
a fundamental feature that facilitates the attainment of the highest estimation
precision of the environment memory-time and the characterization of probe
dynamics.Comment: 3 pages, 4 figure
Maximizing information on the environment by dynamically controlled qubit probes
We explore the ability of a qubit probe to characterize unknown parameters of
its environment. By resorting to quantum estimation theory, we analytically
find the ultimate bound on the precision of estimating key parameters of a
broad class of ubiquitous environmental noises ("baths") which the qubit may
probe. These include the probe-bath coupling strength, the correlation time of
generic bath spectra, the power laws governing these spectra, as well as their
dephasing times T2. Our central result is that by optimizing the dynamical
control on the probe under realistic constraints one may attain the maximal
accuracy bound on the estimation of these parameters by the least number of
measurements possible. Applications of this protocol that combines dynamical
control and estimation theory tools to quantum sensing are illustrated for a
nitrogen-vacancy center in diamond used as a probe.Comment: 8 pages + 6 pages (appendix), 3 Figure
Quantum state transfer in disordered spin chains: How much engineering is reasonable?
The transmission of quantum states through spin chains is an important
element in the implementation of quantum information technologies. Speed and
fidelity of transfer are the main objectives which have to be achieved by the
devices even in the presence of imperfections which are unavoidable in any
manufacturing process. To reach these goals, several kinds of spin chains have
been suggested, which differ in the degree of fine-tuning, or engineering, of
the system parameters. In this work we present a systematic study of two
important classes of such chains. In one class only the spin couplings at the
ends of the chain have to be adjusted to a value different from the bulk
coupling constant, while in the other class every coupling has to have a
specific value. We demonstrate that configurations from the two different
classes may perform similarly when subjected to the same kind of disorder in
spite of the large difference in the engineering effort necessary to prepare
the system. We identify the system features responsible for these similarities
and we perform a detailed study of the transfer fidelity as a function of chain
length and disorder strength, yielding empirical scaling laws for the fidelity
which are similar for all kinds of chain and all disorder models. These results
are helpful in identifying the optimal spin chain for a given quantum
information transfer task. In particular, they help in judging whether it is
worthwhile to engineer all couplings in the chain as compared to adjusting only
the boundary couplings.Comment: 20 pages, 13 figures. Revised version, title changed, accepted by
Quantum Information & Computatio
Robustness of spin-chain state-transfer schemes
This is a shortened and slightly edited version of a chapter in the
collection "Quantum State Transfer and Network Engineering", edited by G.M.
Nikolopoulos and I. Jex, where we review our own research about the robustness
of spin-chain state-transfer schemes along with other approaches to the topic.
Since our own research is documented elsewhere to a large extent we here
restrict ourselves to a review of other approaches which might be useful to
other researchers in the field
Enhanced precision bound of low-temperature quantum thermometry via dynamical control
High-precision low-temperature thermometry is a challenge for experimental
quantum physics and quantum sensing. Here we consider a thermometer modelled by
a dynamically-controlled multilevel quantum probe in contact with a bath.
Dynamical control in the form of periodic modulation of the energy-level
spacings of the quantum probe can dramatically increase the maximum accuracy
bound of low-temperatures estimation, by maximizing the relevant quantum Fisher
information. As opposed to the diverging relative error bound at low
temperatures in conventional quantum thermometry, periodic modulation of the
probe allows for low-temperature thermometry with temperature-independent
relative error bound. The proposed approach may find diverse applications
related to precise probing of the temperature of many-body quantum systems in
condensed matter and ultracold gases, as well as in different branches of
quantum metrology beyond thermometry, for example in precise probing of
different Hamiltonian parameters in many-body quantum critical systems.Comment: 8 pages, 4 figure
Spin chains for robust state transfer: Modified boundary couplings vs. completely engineered chains
Quantum state transfer in the presence of noise is one of the main challenges
in building quantum computers. We compare the quantum state transfer properties
for two classes of qubit chains under the influence of static randomness. In
fully engineered chains all nearest-neighbor couplings are tuned in such a way
that a single-qubit state can be transferred perfectly between the ends of the
chain, while in boundary-controlled chains only the two couplings between the
transmitting and receiving qubits and the remainder of the chain can be
optimized. We study how the noise in the couplings affects the state transfer
fidelity depending on the noise model and strength as well as the chain type
and length. We show that the desired level of fidelity and transfer time are
important factors in designing a chain. In particular we demonstrate that
transfer efficiency comparable or better than that of the most robust
engineered systems can also be reached in boundary-controlled chains without
the demanding engineering of a large number of couplings.Comment: 6 pages, 4 figure
Quantum state transfer in a XX chain with impurities
One spin excitation states are involved in the transmission of quantum states
and entanglement through a quantum spin chain, the localization properties of
these states are crucial to achieve the transfer of information from one
extreme of the chain to the other. We investigate the bipartite entanglement
and localization of the one excitation states in a quantum chain with one
impurity. The bipartite entanglement is obtained using the Concurrence and the
localization is analyzed using the inverse participation ratio. Changing the
strength of the exchange coupling of the impurity allows us to control the
number of localized or extended states. The analysis of the inverse
participation ratio allows us to identify scenarios where the transmission of
quantum states or entanglement can be achieved with a high degree of fidelity.
In particular we identify a regime where the transmission of quantum states
between the extremes of the chain is executed in a short transmission time
, where is the number of spins in the chain, and with a large
fidelity