36 research outputs found
Adiabatic freezing of long-range quantum correlations in spin chains
We consider a process to create quasi long-range quantum discord between the
non-interacting end spins of a quantum spin chain, with the end spins weakly
coupled to the bulk of the chain. The process is not only capable of creating
long-range quantum correlation but the latter remains frozen, when certain weak
end-couplings are adiabatically varied below certain thresholds. We term this
phenomenon as adiabatic freezing of quantum correlation. We observe that the
freezing is robust to moderate thermal fluctuations and is intrinsically
related to the cooperative properties of the quantum spin chain. In particular,
we find that the energy gap of the system remains frozen for these adiabatic
variations, and moreover, considering the end spins as probes, we show that the
interval of freezing can detect the anisotropy transition in quantum XY spin
chains. Importantly, the adiabatic freezing of long-range quantum correlations
can be simulated with contemporary experimental techniques.Comment: Main text (6 pages, 6 figures) and Supplemental material (4 pages, 4
figures), REVTeX 4-
Maximally coherent mixed states: Complementarity between maximal coherence and mixedness
Quantum coherence is a key element in topical research on quantum resource
theories and a primary facilitator for design and implementation of quantum
technologies. However, the resourcefulness of quantum coherence is severely
restricted by environmental noise, which is indicated by the loss of
information in a quantum system, measured in terms of its purity. In this work,
we derive the limits imposed by the mixedness of a quantum system on the amount
of quantum coherence that it can possess. We obtain an analytical trade-off
between the two quantities that upperbound the maximum quantum coherence for
fixed mixedness in a system. This gives rise to a class of quantum states,
"maximally coherent mixed states," whose coherence cannot be increased further
under any purity-preserving operation. For the above class of states, quantum
coherence and mixedness satisfy a complementarity relation, which is crucial to
understand the interplay between a resource and noise in open quantum systems.Comment: 8 pages, 1 figure, REVTex 4.1, close to published versio