John Jeffers
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Research

John's past research has been in quantum optics, more specifically on the quantum theory of dielectrics, quantum optical amplification and attenuation. He doesn't do this any more, but still uses the results regularly. More recently he has worked in quantum retrodiction and its applications, which as you can guess, uses quantum theory backwards in time to assess the likelihood of things having happened. This topic has pretty much come to an end as well, but survives in the use that he puts it to in new research topics. At present John works mainly in two areas: Postselection and Measurement-driven evolution. Other topics include spatial effects in quantum and classical optics, atom optics and open systems.

Postselection is the conditional generation of novel quantum states from partial measurements made on the system, and is the basis of measurement-based models of quantum computation. New quality (fidelity) measures proposed give tighter bounds on the postselecting system requirements, and have led to simple proposals to improve the overall fidelity for photodetection-based systems.

Increase in postselector fidelity as a function of detector loss (1-eta) and preamplifier gain (G) , showing that optical amplification can increase fidelity of postselectors based on optical photodetection. See J. Jeffers, Phys. Rev. A 75, 012335 (2007).

There are two types of evolution in our quantum world - unitary, time-reversible evolution associated with Hamiltonian driving, and nonunitary, irreversible evolution associated with loss and measurements. Measurement-driven evolution is associated with the second of these. The most famous repeated measurement effect is the quantum Zeno effect. This is called after Zeno of Elea, whose famous paradoxes challenged the existence of motion. The Zeno effect can be loosely described by the paraphrased proverb "A watched /quantum/ pot never boils", and is an example of measurement inducing less evolution, not more. The effect of continuous weak measurements on a quantum system could the basis of the appearance of the deterministic classical world that we all see (why don't we see superpositions?). There is also a link between measurements and strong damping, a topic which is not treated correctly by standard quantum open systems theory.

Evolution of a Schrodinger cat state of a particle via measurement-based friction. The first is the initial state, with a significant off-diagonal component. The second is the state after a very short (much less than the system decay time) evolution. The final state is diagonal.
No quantum coherences remain and the classical world asserts itself. See B. Bellomo et al, J. Phys. A: Math. Theor. 40, 9437-9453 .

Highlights

- Organising committee for Quantum Communication, Measurement and Computing 2004

- Invited talk at Imaging at the Limits, Corsica 2004

- Invited book chapters in “Quantum Imaging” and “Structured light and it’s Applications”