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Gregg Jaeger carries out research in quantum computing, quantum cryptography, foundations of quantum mechanics, quantum metrology, history and philosophy of science and population genetics.

RESEARCH INTERESTS:

"Motivated by a strong interest in the foundations of quantum theory, which addresses such questions as the nature of quantum entitities and their interrelations, I have been investigating such practical issues as how information can be encoded and transformed using quantum physics, both in theory and practice, for example, for the purposes of quantum computing and quantum cryptography. In my earliest work, I explored ways in which quantum two-level systems ("qubits") can be used to probe physical systems to perform quantum metrology (click here to see a review article). From this basis, I have gone to study increasing larger quantum systems through a broader array of techniques and to find more adequate concepts for conceiving of them as being and consisting of physical objects.

During the period 2001-2006, the Quantum Imaging Laboratory and collaborators at Harvard University and BBN Technologies built a practical metro area quantum cryptographic network, known as the DARPA Quantum Network Testbed, to which I contributed as the principal quantum entanglement theorist. During this project, for the first time 1550 nm (the main telecomm. wavelength) entangled photon pairs were shown to violate Bell inequalities (using superconducting single-photon detectors) and to be of practical use for quantum cryptography in a real-world computer network environment. (See the "Patents and News" section for more information.) In the last two years we have pursued a new, coherent-state based approach that will allow much greater rates and distances of secure QKD to be achieved through the use of entanglement witnesses and related quantities as an eavesdropping indicators.

As mentioned above, my early work focused on the entanglement of dichotomic quantum observables in two-particle systems, that is on entangled pairs of two-level systems. These results included two new fundamental quantum complementarity relations. These are the relation between single-particle interference visibility and path distinguishability and the relation between single-particle and two-particle interference visibilities. Together with Abner Shimony, I also found a fundamental bound on the distinguishability of non-orthogonal quantum states. (See the "Articles" section for articles in .pdf format.) The quantum metrology techniques currently being developed in the Quantum Imaging Lab primarily use such systems.

More recently, I have been investigating coherence and entanglement in the larger quantum mechanical systems used for quantum information processing. This involves clarifying the character of entanglement in many-particle systems, such as by determining the relationship between entanglement and mixedness in arbitrarily large quantum systems. For example, complementarity relations were found and published in "Entanglement, mixedness, and spin-flip symmetry in multiple-qubit states." Most recent directions include the discovery of higher multipartite states most strongly exhibiting quantum behavior and the discovery of the effect of local noise on multipartite and semi-classical quantum systems.

Currently, my students, collaborators and I have been pursuing the question of the general relationship between quantum entanglement and decoherence in multiple-qubit and mulitiple qu-d-it systems. For example, our recent article "Local-dephasing-induced entanglement sudden death in two-component finite-dimensional systems" shows that local noise alone is capable of destroying all entanglement in classes of pairs of d-level systems ("qudits") in finite times, that is, of inducing entanglement sudden death.

I have most recently also been explicitly investigating the issues of probabilistic causation,
quantum measurement, and the individuation of
systems in quantum mechanics, as reported in a series of papers in
*Foundations of Physics* and elsewhere."

His 2007 book * Quantum Information: An Overview * is a concise
look at the new field of quantum information science.

His 2009 book, *Entanglement, Information, and the Interpretation of Quantum Mechanics* focuses on the relationship of information and entanglement to the foundations of
quantum mechanics.

His 2010 edited volume,

*Philosophy of Quantum Information and Entanglement*

was published by Cambridge University Press.

In 2014, he published a book outlining a program for achieving an explanatory, realist interpretation of quantum mechanics,

*Quantum objects*