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Gregg Jaeger

Publications and source records attributed to Gregg Jaeger.

17 recordsLinked to original sources

Measurement and Macroscopicity: Overcoming Conceptual Imprecision in Quantum Measurement Theory

John S. Bell is well known for the result now referred to simply as "Bell's theorem," which removed from serious consideration by physics of local hidden-variable theories. Under these circumstances, if quantum theory is to serve as a truly {\em fundamental} theory, conceptual precision in its interpretation is not only even more desirable but paramount. John Bell was accordingly concerned about what he viewed as conceptual imprecision, from the physical point of view, in the standard approaches to the theory. He saw this as most acute in the case of their treatment of {\em measurement at the level of principle}. Bell pointed out that conceptual imprecision is reflected in the terminology of the theory, a great deal of which he deemed worthy of banishment from discussions of principle. For him, it corresponded to a set of what he saw as vague and, in some instances, outright destructive concepts. Here, I consider this critique of standard quantum measurement theory and some alternative treatments wherein he saw greater conceptual precision, and make further suggestions as to how to proceed along the lines he advocated.

quant-ph

In Appreciation of Abner Shimony

Abner Shimony was an exceptional human being and a remarkably lucid and penetrating thinker whose work centered on some of the most significant physical and philosophical questions of his era at their nexus. He approached these questions with an open, agile and critical mind, something quickly evident to anyone who had the privilege of conversing with him. His choice of problems to pursue, which for the most part involved epistemology and the relationships between mind, matter and space-time, was visionary. Undoubtedly, most of the discussions of Abner Shimony's work in future will center on his enormous contribution to the investigation of the significance to physics, both theoretical and experimental, of quantum entanglement, the characteristic of physical entities on which his work came to concentrate, well represented in the Clauser--Horne--Shimony--Holt inequality. Also significant for physics is his pioneering development of the quantification of entanglement in the early-to-mid 1990s. Shimony focused largely on the question of the relationship between knowledge and the world, as he did not view these two philosophical concerns as distinct from study of the implications of entanglement per se. Indeed, the most well known phrase he coined is "experimental metaphysics." He is also known for the reapplication of the expression "peaceful coexistence" as pertaining to the relationship between quantum mechanics and special relativity. Shimony's less well known intellectual engagement was with the question of the relationship between mind and matter, which only a few physicists of our time have engaged seriously, likely because of its more philosophical and less mathematically tractable character. This appreciation reflects all these aspects of Abner Shimony's long and successful intellectual career, in addition to details of his education, including mention of some of his teachers.

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Coherent State Quantum Key Distribution with Entanglement Witnessing

An entanglement witness approach to quantum coherent state key distribution and a system for its practical implementation are described. In this approach, eavesdropping can be detected by a change in sign of either of two witness functions, an entanglement witness S or an eavesdropping witness W. The effects of loss and eavesdropping on system operation are evaluated as a function of distance. Although the eavesdropping witness W does not directly witness entanglement for the system, its behavior remains related to that of the true entanglement witness S. Furthermore, W is easier to implement experimentally than S. W crosses the axis at a finite distance, in a manner reminiscent of entanglement sudden death. The distance at which this occurs changes measurably when an eavesdropper is present. The distance dependance of the two witnesses due to amplitude reduction and due to increased variance resulting from both ordinary propagation losses and possible eavesdropping activity is provided. Finally, the information content and secure key rate of a continuous variable protocol using this witness approach are given.

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Unsharp Quantum Reality

The positive operator (valued) measures (POMs) allow one to generalize the notion of observable beyond the traditional one based on projection valued measures (PVMs). Here, we argue that this generalized conception of observable enables a consistent notion of unsharp reality and with it an adequate concept of joint properties. A sharp or unsharp property manifests itself as an element of sharp or unsharp reality by its tendency to become actual or to actualize a specific measurement outcome. This actualization tendency - or potentiality - of a property is quantified by the associated quantum probability. The resulting single-case interpretation of probability as a degree of reality will be explained in detail and its role in addressing the tensions between quantum and classical accounts of the physical world will be elucidated. It will be shown that potentiality can be viewed as a causal agency that evolves in a well-defined way.

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Disentanglement and decoherence in a pair of qutrits under dephasing noise

We relate disentanglement and decoherence rates in a pair of three-level atoms subjected to multi-local and collective pure dephasing noise acting in a preferred basis. The bipartite entanglement decay rate, as bounded from above by the negativity, is found to be greater than or equal to the dephasing-decoherence rates characterized by the decay of off-diagonal elements in the corresponding full density matrix describing the system or the reduced density matrix describing either qutrit, extending previous results for qubit pairs subject to such noise.

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Finite-time destruction of entanglement and non-locality by environmental influences

Entanglement and non-locality are non-classical global characteristics of quantum states important to the foundations of quantum mechanics. Recent investigations have shown that environmental noise, even when it is entirely local in influence, can destroy both of these properties in finite time despite giving rise to full quantum state decoherence only in the infinite time limit. These investigations, which have been carried out in a range of theoretical and experimental situations, are reviewed here.

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Generic tripartite Bell nonlocality sudden death under local phase noise

We definitively show, using an explicit and broadly applicable model, that local phase noise that is capable of eliminating state coherence only in the infinite-time limit is capable of eliminating nonlocality in finite time in three two-level systems prepared in the Bell-nonlocal tripartite states of the generic entanglement class.

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Local basis-dependent noise-induced Bell-nonlocality sudden death in tripartite systems

We demonstrate that multipartite Bell-inequality violations can be fully destroyed in finite time in three-qubit systems subject only to the mechanism of local external asymptotic dephasing noise. This broadens the study of local-noise-induced sudden death of nonlocal behavior, extending it beyond the realm of bipartite systems, to which it had previously been restricted.

quant-ph

Local-dephasing-induced entanglement sudden death in two-component finite-dimensional systems

Entanglement sudden death (ESD), the complete loss of entanglement in finite time, is demonstrated to occur in a class of bipartite states of qu-d-it pairs of any finite dimension d > 2, when prepared in so-called `isotropic states' and subject to multi-local dephasing noise alone. This extends previous results for qubit pairs [T. Yu, J. H. Eberly, Phys. Rev. Lett. 97, 140403 (2006)] to all qu-d-it pairs with d > 2.

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Fractal states in quantum information processing

The fractal character of some quantum properties has been shown for systems described by continuous variables. Here, a definition of quantum fractal states is given that suits the discrete systems used in quantum information processing, including quantum coding and quantum computing. Several important examples are provided.

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Entanglement sudden death in qubit-qutrit systems

We demonstrate the existence of entanglement sudden death (ESD), the complete loss of entanglement in finite time, in qubit-qutrit systems. In particular, ESD is shown to occur in such systems initially prepared in a one-parameter class of entangled mixed states and then subjected to local dephasing noise. Together with previous results, this proves the existence of ESD for some states in all quantum systems for which rigorously defined mixed-state entanglement measures have been identified. We conjecture that ESD exists in all quantum systems prepared in appropriate bipartite states.

quant-ph

Disentanglement and decoherence in two-spin and three-spin systems under dephasing

We compare disentanglement and decoherence rates within two-spin and three-spin entangled systems subjected to all possible combinations of local and collective pure dephasing noise combinations. In all cases, the bipartite entanglement decay rate is found to be greater than or equal to the dephasing-decoherence rates and often significantly greater. This sharpens previous results for two-spin systems [T. Yu and J. H. Eberly Phys. Rev. B 68, 165322 (2003)] and extends them to the three-spin context.

quant-ph

Bell Gems: the Bell basis generalized

A class of self-similar sets of entangled quantum states is introduced, for which a recursive definition is provided. These sets, the "Bell gems," are defined by the subsystem exchange symmetry characteristic of the Bell states. Each Bell gem is shown to be an orthonormal basis of maximally entangled elements. A non-trivial example Bell gem is presented. Quantum circuits for producing the elements of this example from the computational basis states are provided.

quant-ph

Entanglement, Mixedness, and Spin-Flip Symmetry in Multiple-Qubit Systems

A relationship between a recently introduced multipartite entanglement measure, state mixedness, and spin-flip symmetry is established for any finite number of qubits. It is also shown that, within those classes of states invariant under the spin-flip transformation, there is a complementarity relation between multipartite entanglement and mixedness. A number of example classes of multiple-qubit systems are studied in light of this relationship.

quant-ph

Invariants of multiple-qubit systems under stochastic local operations

We investigate the behavior of quantum states under stochastic local quantum operations and classical communication (SLOCC) for fixed numbers of qubits. We explicitly exhibit the homomorphism between complex and real groups for two-qubits, and use the latter to describe the effect of SLOCC operations on two-qubit states. We find an expression for the polarization Lorentz group invariant length, which is the Minkowskian analog of the quantum state purity, the corresponding Euclidean length. The construction presented is immediately generalizable to any finite number of qubits.

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A multi-photon Stokes-parameter invariant for entangled states

We consider the Minkowskian norm of the n-photon Stokes tensor, a scalar invariant under the group realized by the transformations of stochastic local quantum operations and classical communications (SLOCC). This invariant is offered as a candidate entanglement measure for n-qubit states and discussed in relation to measures of quantum state entanglement for certain important classes of two-qubit and three-qubit systems. This invariant can be directly estimated via a quantum network, obviating the need to perform laborious quantum state tomography. We also show that this invariant directly captures the extent of entanglement purification due to SLOCC filters.

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Book Review: Bohmian Mechanics and Quantum Theory: An Appraisal

A review of "Bohmian Mechanics and Quantum Theory: An Appraisal" (James Cushing, Arthur Fine and Sheldon Goldstein, Eds.), an extensive collection of articles on Bohmian mechanics. In addition to broad, critical overviews of Bohmian mechanics, the reviewed collection contains extensions and hybrid versions of the theory and several detailed applications to practical situtations.

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