809 research outputs found
A Direct Coupling Coherent Quantum Observer for a Qubit, including Observer Measurements
This paper proposes a direct coupling coherent quantum observer for a quantum plant which consists of a two level quantum system. The quantum observer, which is a quantum harmonic oscillator, includes homodyne detection measurements. It is shown that the observer can be designed so that it does not affect the quantum variable of interest in the quantum plant and that measured output converges in a given sense to the plant variable of interest. Also, the plant variable of interest-observer system can be described by a set of linear quantum stochastic differential equations. A minimum variance unbiased estimator form of the Kalman filter is derived for linear quantum systems and applied to the direct coupled coherent quantum observerThis work was supported by the Australian Research Council(ARC) under grant FL110100020 and the Air Force Office of Scientific Research(AFOSR),under agreement number FA2386-16-1-4065
Environment-Induced Decoherence and the Transition From Quantum to Classical
We study dynamics of quantum open systems, paying special attention to those
aspects of their evolution which are relevant to the transition from quantum to
classical. We begin with a discussion of the conditional dynamics of simple
systems. The resulting models are straightforward but suffice to illustrate
basic physical ideas behind quantum measurements and decoherence. To discuss
decoherence and environment-induced superselection einselection in a more
general setting, we sketch perturbative as well as exact derivations of several
master equations valid for various systems. Using these equations we study
einselection employing the general strategy of the predictability sieve.
Assumptions that are usually made in the discussion of decoherence are
critically reexamined along with the ``standard lore'' to which they lead.
Restoration of quantum-classical correspondence in systems that are classically
chaotic is discussed. The dynamical second law -it is shown- can be traced to
the same phenomena that allow for the restoration of the correspondence
principle in decohering chaotic systems (where it is otherwise lost on a very
short time-scale). Quantum error correction is discussed as an example of an
anti-decoherence strategy. Implications of decoherence and einselection for the
interpretation of quantum theory are briefly pointed out.Comment: 80 pages, 7 figures included, Lectures given by both authors at the
72nd Les Houches Summer School on "Coherent Matter Waves", July-August 199
Quantum data gathering
Measurement of a quantum system – the process by which an observer gathers information about it – provides a link between the quantum and classical worlds. The nature of this process is the central issue for attempts to reconcile quantum and classical descriptions of physical processes. Here, we show that the conventional paradigm of quantum measurement is directly responsible for a well-known disparity between the resources required to extract information from quantum and classical systems. We introduce a simple form of quantum data gathering, “coherent measurement”, that eliminates this disparity and restores a pleasing symmetry between classical and quantum statistical inference. To illustrate the power of quantum data gathering, we demonstrate that coherent measurements are optimal and strictly more powerful than conventional one-at-a-time measurements for the task of discriminating quantum states, including certain entangled many-body states (e.g., matrix product states)
Stochastic Master Equation Analysis of Optimized Three-Qubit Nondemolition Parity Measurement
We analyze a direct parity measurement of the state of three superconducting
qubits in circuit quantum electrodynamics. The parity is inferred from a
homodyne measurement of the reflected/transmitted microwave radiation and the
measurement is direct in the sense that the parity is measured without the need
for any quantum circuit operations or for ancilla qubits. Qubits are coupled to
two resonant cavity modes, allowing the steady state of the emitted radiation
to satisfy the necessary conditions to act as a pointer state for the parity.
However, the transient dynamics violates these conditions and we analyze this
detrimental effect and show that it can be overcome in the limit of weak
measurement signal. Our analysis shows that, with a moderate degree of
post-selection, it is possible to achieve post-measurement states with fidelity
of order 95%. We believe that this type of measurement could serve as a
benchmark for future error-correction protocols in a scalable architecture
A Unitary Model of The Black Hole Evaporation
A unitary effective field model of the black hole evaporation is proposed to
satisfy almost the four postulates of the black hole complementarity (BHC). In
this model, we enlarge a black hole-scalar field system by adding an extra
radiation detector that couples with the scalar field. After performing a
partial trace over the scalar field space, we obtain an effective entanglement
between the black hole and the detector (or radiation in it). As the whole
system evolves, the S-matrix formula can be constructed formally step by step.
Without local quantum measurements, the paradoxes of the information loss and
AMPS's firewall can be resolved. However, the information can be lost due to
quantum decoherence, as long as some local measurement has been performed on
the detector to acquire the information of the radiation in it. But unlike
Hawking's completely thermal spectrum, some residual correlations can be found
in the radiations. All these considerations can be simplified in a qubit model
that provides a \emph{modified quantum teleportation} to transfer the
information via an EPR pairs.Comment: 42 pages,two figures,published versio
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