Published in Advanced Optical Technologies—A double feature

Two of my papers were published in Advanced Optical Technologies, recently, as part of a topical issue on applied quantum technologies.

The first paper deals with encoding the polarization of light signals for quantum key distribution (QKD). In principle, light is very good at maintaining its polarization, but in practice things like thermal effects in optical fibers and physical orientations causes polarizations to get rotated in sometimes unpredictable ways. There are various techniques to control and correct for these effects. This paper proposes an approach based on sampling the QKD signals themselves, and analyzes the performance in terms of how much light needs to be sampled. It turns out you can do very well to preserve the polarization with a relatively few signals.

The second paper looks at whether ‘adaptive optics’ techniques can be used to help transmit QKD signals from ground to an orbiting satellite. Adaptive optics uses fast …

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Published in Optics Express: Genuine time-bin-encoded quantum key distribution over a turbulent depolarizing free-space channel

Light can be used to encode information in a variety of ways. Polarization, for example: a ‘0’ bit could be represented by a pulse of horizontally polarized light, and a ‘1’ bit could be vertically polarized. This generally works well for transmissions over free-space. Also, by allowing superposition states and reducing the intensity to single-photon levels, one can start to access interesting quantum protocols such as quantum key distribution (QKD). You can do this with other encodings, too—“time bin”, for example, where you encode information in the arrival time, early or late, relative to a reference. But because of the way the superposition state (that is, the early “and” late state) is measured, it doesn't generally work well over air because of turbulence.

A recently discovered enhancement of the measurement device by my colleagues intrinsically bypasses the turbulence problem, and in this paper, we couple this improved apparatus with …

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Published in Physical Review A: Using weak values to experimentally determine “negative probabilities” in a two-photon state with Bell correlations

It's well known that quantum entangled systems can exhibit correlations that go beyond those that could be seen if Nature worked in intuitive “classical” ways. However, as Richard Feynman noted, classical theory can support exhibiting such correlations if we invoke negative probabilities to describe their properties. What he did not do was specify how these negative probabilities ought to be chosen, and without any justification, an infinite number of different combinations could be chosen that will satisfy the relevant equations.

The concept of negative probabilities seems nonsensical because they cannot actually be observed—indeed, they cannot be observed even within the framework of quantum theory due to the effects of measurement back-action. Here, we show how they can instead be inferred through the use of weak measurements, where a meter is only weakly coupled to the property of interest, thereby avoiding the back-action problem. Each individual weak measurement has a …

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Published in Nature Photonics: Experimental three-photon quantum nonlocality under strict locality conditions

Quantum mechanics implies properties of Nature that clash with our intuitive notions of how the universe ought to work. Testing these properties (to see if quantum mechanics is, indeed, true) involves generating entangled quantum states of two or more particles and measuring them under a number of strict conditions. While work is progressing to meet all of these conditions when using only two particles, no one has yet met even one of these conditions for more than two particles, which is considerably more difficult experimentally. Here, we conduct an experiment where we meet two of the most challenging conditions—namely measurement locality and freedom of choice—while generating triplet entangled photon states. We demonstrate that quantum mechanics wins out over intuition, measuring a violation of Mermin's inequality outside the classical bound by nine standard deviations.

C. Erven, E. Meyer-Scott, K. Fisher, J. Lavoie, B. L. Higgins, Z. Yan, C. J …

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Published in Optics Express: Generating polarization-entangled photon pairs using cross-spliced birefringent fibers

Generating entangled photon states is vital for numerous quantum communications and quantum computation primitives. Here we pioneer a new approach to in-fiber generation of entangled photon pairs. We take inspiration from a technique in bulk-optics, where two nonlinear crystals are sandwiched close together, and splice two pieces of birefringent optical fiber together at 90 degree orientation. With suitable compensation optics, all of which could be implemented in fiber, we show fidelity with a maximally-entangled Bell state of better than 92%.

E. Meyer-Scott, V. Roy, J.-P. Bourgoin, B. L. Higgins, L. K. Shalm, and T. Jennewein
Optics Express 21, 6205–12 (2013)

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Published in Journal of Lightwave Technology: Novel high-speed polarization source for decoy-state BB84 quantum key distribution over free space and satellite links

Here we detail our work on optoelectronics implementing a high-speed high-fidelity source of optical quantum states for quantum encryption.

Z. Yan, E. Meyer-Scott, J.-P. Bourgoin, B. L. Higgins, N. Gigov, A. MacDonald, H. Hübel, and T. Jennewein
J. Lightwave Tech. 31, 1399–408 (2013)

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Published in Physical Review Letters: Mixed state discrimination using optimal control

It's known to be impossible to, with 100% accuracy, discriminate between two different quantum states that are not orthogonal. In this paper we look at how accurately you can make this determination when you are given multiple identical copies of one of the two nonorthogonal states. We consider different measurements you can perform, and find that a measurement strategy that performs optimally when the states in question are pure actually performs poorer than a naive “majority vote” scheme when the states have some mixture. We experimentally demonstrate these schemes and derive (and also demonstrate) an adaptive measurement scheme that performs optimally in all conditions, and compare it to the fundamental limit.

B. L. Higgins, B. M. Booth, A. C. Doherty, S. D. Bartlett, H. M. Wiseman, and G. J. Pryde
Phys. Rev. Lett. 103, 220503 (2009)

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Published in IEEE Journal of Selected Topics in Quantum Electronics: Adaptive Measurements in the Optical Quantum Information Laboratory

We were (or more specifically, our theory collaborator Prof. Howard Wiseman was) invited to write a paper for IEEE Journal of Selected Topics in Quantum Electronics. Here we describe several of the experiments recently taking place in our (or more specifically, Prof. Geoff Pryde's) laboratory, of which Howard is an integral part. It discusses the recent work on phase measurement, with some bonus theoretical details, as well as touching briefly on some soon-to-be-published work on adaptive quantum state discrimination.

H. M. Wiseman, D. W. Berry, S. D. Bartlett, B. L. Higgins, and G. J. Pryde
IEEE J. Sel. Top. Quantum Electron. 15, 1661–72 (2009)

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BSce (Hons) thesis: Advanced photonic measurements for quantum information

This post marks the completion of my honours thesis, as part of my BSc. (Hons) degree. It was a pleasure to work and learn under the supervision of Prof. Geoff Pryde at Griffith University, in close collaboration with Ben Lanyon in the laboratory of Prof. Andrew White at The University of Queensland.

The project served as my introduction to the world of experimental photonic implementations of quantum information. The aim of the project was to attempt to combine two techniques—quantum nondemolition measurement, and unambiguous state discrimination. The project was successful insofar as the experiment reproduced what we expected to obtain theoretically, though considerably more work would have been required in order to achieve complete nondemolishing unambiguous state discrimination. The PDF of the thesis can be found here.

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