213 research outputs found
A quantum Szilard engine without heat from a thermal reservoir
We study a quantum Szilard engine that is not powered by heat drawn from a
thermal reservoir, but rather by projective measurements. The engine is
constituted of a system , a weight , and a Maxwell
demon , and extracts work via measurement-assisted feedback
control. By imposing natural constraints on the measurement and feedback
processes, such as energy conservation and leaving the memory of the demon
intact, we show that while the engine can function without heat from a thermal
reservoir, it must give up at least one of the following features that are
satisfied by a standard Szilard engine: (i) repeatability of measurements; (ii)
invariant weight entropy; or (iii) positive work extraction for all measurement
outcomes. This result is shown to be a consequence of the Wigner-Araki-Yanase
(WAY) theorem, which imposes restrictions on the observables that can be
measured under additive conservation laws. This observation is a first-step
towards developing "second-law-like" relations for measurement-assisted
feedback control beyond thermality
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
Classicality of the heat produced by quantum measurements
Quantum measurement is ultimately a physical process, resulting from an
interaction between the measured system and a measuring apparatus. Considering
the physical process of measurement within a thermodynamic context naturally
raises the following question: How can the work and heat be interpreted? In the
present paper we model the measurement process for an arbitrary discrete
observable as a measurement scheme. Here the system to be measured is first
unitarily coupled with an apparatus and subsequently the compound system is
objectified with respect to a pointer observable, thus producing definite
measurement outcomes. The work can therefore be interpreted as the change in
internal energy of the compound system due to the unitary coupling. By the
first law of thermodynamics, the heat is the subsequent change in internal
energy of this compound due to pointer objectification. We argue that the
apparatus serves as a stable record for the measurement outcomes only if the
pointer observable commutes with the Hamiltonian and show that such
commutativity implies that the uncertainty of heat will necessarily be
classical
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
First principles structures and circular dichroism spectra for the close-packed and the 7/2 motif of collagen
The recently proposed close-packed motif for collagen is investigated using
first principles semi-empirical wave function theory and Kohn-Sham density
functional theory. Under these refinements the close-packed motif is shown to
be stable. For the case of the 7/2 motif a similar stability exists. The
electronic circular dichroism of the close-packed model has a significant
negative bias and a large signal. An interesting feature of the close-packed
structure is the existence of a central channel. Simulations show that, if
hydrogen atoms are placed in the cavity, a chain of molecular hydrogens is
formed suggesting a possible biological function for molecular hydrogen.Comment: 12 pages, 3 figures; 3(PPG)_6 xyz file attached; v2: minor
modification
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
- …