10,502 research outputs found
Correlation and nonlocality measures as indicators of quantum phase transitions in several critical systems
We have investigated the quantum phase transitions in the ground states of
several critical systems, including transverse field Ising and XY models as
well as XY with multiple spin interactions, XXZ and the collective system
Lipkin-Meshkov-Glick models, by using different quantumness measures, such as
entanglement of formation, quantum discord, as well as its classical
counterpart, measurement-induced disturbance and the
Clauser-Horne-Shimony-Holt-Bell function. Measurement-induced disturbance is
found to detect the first and second order phase transitions present in these
critical systems, while, surprisingly, it is found to fail to signal the
infinite-order phase transition present in the XXZ model. Remarkably, the
Clauser-Horne-Shimony-Holt-Bell function is found to detect all the phase
transitions, even when quantum and classical correlations are zero for the
relevant ground state
Achieving quantum precision limit in adaptive qubit state tomography
The precision limit in quantum state tomography is of great interest not only
to practical applications but also to foundational studies. However, little is
known about this subject in the multiparameter setting even theoretically due
to the subtle information tradeoff among incompatible observables. In the case
of a qubit, the theoretic precision limit was determined by Hayashi as well as
Gill and Massar, but attaining the precision limit in experiments has remained
a challenging task. Here we report the first experiment which achieves this
precision limit in adaptive quantum state tomography on optical polarization
qubits. The two-step adaptive strategy employed in our experiment is very easy
to implement in practice. Yet it is surprisingly powerful in optimizing most
figures of merit of practical interest. Our study may have significant
implications for multiparameter quantum estimation problems, such as quantum
metrology. Meanwhile, it may promote our understanding about the
complementarity principle and uncertainty relations from the information
theoretic perspective.Comment: 9 pages, 4 figures; titles changed and structure reorganise
Information Tradeoff Relations for Finite-Strength Quantum Measurements
In this paper we give a new way to quantify the folklore notion that quantum
measurements bring a disturbance to the system being measured. We consider two
observers who initially assign identical mixed-state density operators to a
two-state quantum system. The question we address is to what extent one
observer can, by measurement, increase the purity of his density operator
without affecting the purity of the other observer's. If there were no
restrictions on the first observer's measurements, then he could carry this out
trivially by measuring the initial density operator's eigenbasis. If, however,
the allowed measurements are those of finite strength---i.e., those
measurements strictly within the interior of the convex set of all
measurements---then the issue becomes significantly more complex. We find that
for a large class of such measurements the first observer's purity increases
the most precisely when there is some loss of purity for the second observer.
More generally the tradeoff between the two purities, when it exists, forms a
monotonic relation. This tradeoff has potential application to quantum state
control and feedback.Comment: 15 pages, revtex3, 3 eps figure
Feedback control of spin systems
The feedback stabilization problem for ensembles of coupled spin 1/2 systems
is discussed from a control theoretic perspective. The noninvasive nature of
the bulk measurement allows for a fully unitary and deterministic closed loop.
The Lyapunov-based feedback design presented does not require spins that are
selectively addressable. With this method, it is possible to obtain control
inputs also for difficult tasks, like suppressing undesired couplings in
identical spin systems.Comment: 16 pages, 15 figure
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