15 research outputs found
Minimising the heat dissipation of quantum information erasure
Quantum state engineering and quantum computation rely on information erasure
procedures that, up to some fidelity, prepare a quantum object in a pure state.
Such processes occur within Landauer's framework if they rely on an interaction
between the object and a thermal reservoir. Landauer's principle dictates that
this must dissipate a minimum quantity of heat, proportional to the entropy
reduction that is incurred by the object, to the thermal reservoir. However,
this lower bound is only reachable for some specific physical situations, and
it is not necessarily achievable for any given reservoir. The main task of our
work can be stated as the minimisation of heat dissipation given probabilistic
information erasure, i.e., minimising the amount of energy transferred to the
thermal reservoir as heat if we require that the probability of preparing the
object in a specific pure state be no smaller than
. Here is the maximum
probability of information erasure that is permissible by the physical context,
and the error. To determine the achievable minimal heat
dissipation of quantum information erasure within a given physical context, we
explicitly optimise over all possible unitary operators that act on the
composite system of object and reservoir. Specifically, we characterise the
equivalence class of such optimal unitary operators, using tools from
majorisation theory, when we are restricted to finite-dimensional Hilbert
spaces. Furthermore, we discuss how pure state preparation processes could be
achieved with a smaller heat cost than Landauer's limit, by operating outside
of Landauer's framework
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
Universal validity of the second law of information thermodynamics
Feedback control and erasure protocols have often been considered as a model
to embody Maxwell's Demon paradox and to study the interplay between
thermodynamics and information processing. Such studies have led to the
conclusion, now widely accepted in the community, that Maxwell's Demon and the
second law of thermodynamics can peacefully coexist because any gain provided
by the demon must be offset by the cost of performing measurement and resetting
the demon's memory to its initial state. Statements of this kind are
collectively referred to as second laws of information thermodynamics and have
recently been extended to include quantum theoretical scenarios. However,
previous studies in this direction have made several assumptions, in particular
about the feedback process and the measurement performed on the demon's memory,
and thus arrived at statements that are not universally applicable and whose
range of validity is not clear. In this work, we fill this gap by precisely
characterizing the full range of quantum feedback control and erasure protocols
that are overall consistent with the second law of thermodynamics. This leads
us to conclude that the second law of information thermodynamics is indeed
universal: it must hold for any quantum feedback control and erasure protocol,
regardless of the measurement process involved, as long as the protocol is
overall compatible with thermodynamics. Our comprehensive analysis not only
encompasses new scenarios but also retrieves previous ones, doing so with fewer
assumptions. This simplification contributes to a clearer understanding of the
theory. Additionally, our work identifies the Groenewold--Ozawa information
gain as the correct information measure characterizing the work extractable by
feedback control.Comment: 30 pages, 1 figure. The title is changed from the previous version
and one author is added. The contents are significantly update
Thermodynamic uncertainty relation in slowly driven quantum heat engines
Thermodynamic Uncertainty Relations express a trade-off between precision,
defined as the noise-to-signal ratio of a generic current, and the amount of
associated entropy production. These results have deep consequences for
autonomous heat engines operating at steady-state, imposing an upper bound for
their efficiency in terms of the power yield and its fluctuations. In the
present manuscript we analyse a different class of heat engines, namely those
which are operating in the periodic slow-driving regime. We show that an
alternative TUR is satisfied, which is less restrictive than that of
steady-state engines: it allows for engines that produce finite power, with
small power fluctuations, to operate close to the Carnot efficiency. The bound
further incorporates the effect of quantum fluctuations, which reduces engine
efficiency relative to the average power and reliability. We finally illustrate
our findings in the experimentally relevant model of a single-ion heat engine.Comment: 11 pages, 2 figures. Updated to published version with additional
mathematical background in the supplementary material. Some additional
results from a previous draft have now been incorporated into another
article, see arXiv:2011.1158
Quantum control of hybrid nuclear-electronic qubits
Pulsed magnetic resonance is a wide-reaching technology allowing the quantum
state of electronic and nuclear spins to be controlled on the timescale of
nanoseconds and microseconds respectively. The time required to flip either
dilute electronic or nuclear spins is orders of magnitude shorter than their
decoherence times, leading to several schemes for quantum information
processing with spin qubits. We investigate instead the novel regime where the
eigenstates approximate 50:50 superpositions of the electronic and nuclear spin
states forming "hybrid nuclear-electronic" qubits. Here we demonstrate quantum
control of these states for the first time, using bismuth-doped silicon, in
just 32 ns: this is orders of magnitude faster than previous experiments where
pure nuclear states were used. The coherence times of our states are five
orders of magnitude longer, reaching 4 ms, and are limited by the
naturally-occurring 29Si nuclear spin impurities. There is quantitative
agreement between our experiments and no-free-parameter analytical theory for
the resonance positions, as well as their relative intensities and relative
Rabi oscillation frequencies. In experiments where the slow manipulation of
some of the qubits is the rate limiting step, quantum computations would
benefit from faster operation in the hybrid regime.Comment: 20 pages, 8 figures, new data and simulation
Thermodynamically free quantum measurements
Thermal channels -- the free processes allowed in the resource theory of
quantum thermodynamics -- are generalised to thermal instruments, which we
interpret as implementing thermodynamically free quantum measurements; a
Maxwellian demon using such measurements never violates the second law of
thermodynamics. The properties of thermal instruments are investigated and, in
particular, it is shown that they only measure observables commuting with the
Hamiltonian, and they thermalise the measured system when performing a complete
measurement, the latter of which indicates a thermodynamically induced
information-disturbance trade-off. The demarcation of measurements that are not
thermodynamically free paves the way for a resource-theoretic quantification
for the thermodynamic cost of quantum measurements