363 research outputs found
Super Quantum Discord with Weak Measurements
Weak measurements cause small change to quantum states, thereby opening up
the possibility of new ways of manipulating and controlling quantum systems. We
ask, can weak measurements reveal more quantum correlation in a composite
quantum state? We prove that the weak measurement induced quantum discord,
called as the "super quantum discord", is always larger than the quantum
discord captured by the strong measurement. Moreover, we prove the monotonicity
of the super quantum discord as a function of the measurement strength. We find
that unlike the normal quantum discord, for pure entangled states, the super
quantum discord can exceed the quantum entanglement. Our result shows that the
notion of quantum correlation is not only observer dependent but also depends
on how weakly one perturbs the composite system.Comment: Latex, 5 pages, 2 Figs, Monotonicity of the super discord with the
measurement strength is included. Application to the entropic uncertainty
relation will be reported separatel
Energy Cost of Creating Quantum Coherence
We consider the physical situations where the resource theories of coherence
and thermodynamics play competing roles. In particular, we study the creation
of quantum coherence using unitary operations with limited thermodynamic
resources. We first find the maximal coherence that can be created under
unitary operations starting from a thermal state and find explicitly the
unitary transformation that creates the maximal coherence. Since coherence is
created by unitary operations starting from a thermal state, it requires some
amount of energy. This motivates us to explore the trade-off between the amount
of coherence that can be created and the energy cost of the unitary process. We
find the maximal achievable coherence under the constraint on the available
energy. Additionally, we compare the maximal coherence and the maximal total
correlation that can be created under unitary transformations with the same
available energy at our disposal. We find that when maximal coherence is
created with limited energy, the total correlation created in the process is
upper bounded by the maximal coherence and vice versa. For two qubit systems we
show that there does not exist any unitary transformation that creates maximal
coherence and maximal total correlation simultaneously with a limited energy
cost.Comment: 8 pages, Accepted for publication in Physical Review
A resource theory of activity for quantum thermodynamics in the absence of heat baths
Active states, from which work can be extracted by time-dependent
perturbations, are an important resource for quantum thermodynamics in the
absence of heat baths. Here we characterize this resource, establishing a
resource theory that captures the operational scenario where an experimenter
manipulates a quantum system by means of energy-preserving operations and
resets to non-active states. Our resource theory comes with simple conditions
for state convertibility and an experimentally accessible resource quantifier
that determines the maximum advantage of active states in the task of producing
approximations of the maximally coherent state by means of energy-preserving
quantum operations.Comment: 6+9 pages, 0 Figure
- …