Mapping a New World for Light

The scientific revolution is often dated back to Copernicus's model of the solar system and before that, the re-recognition by the Western world that the Earth is round. Yet in the strange domain of quantum physics, scientists have once more been studying a flat world for the past several years. Our latest result, published in Nature 457, 67 (Jan 1, 2009), has now shown what riches can be found by circumnavigating the quantum globe. The three-masted schooner of this new adventure is an odd object called a "triphoton." The photon, a staple of high-power science-fiction weapons, is actually the smallest particle of light -- so small than an ordinary light bulb emits billions of photons in a trillionth of a second. Despite the unimaginably effervescent nature of these tiny particles, modern quantum technologies rely on single photons to store and manipulate information. A strange feature of quantum physics is that when several identical photons are combined, say in an optical fibre like those used to carry the internet to our homes, they undergo an "identity crisis" and one can no longer talk about what an individual photon is doing; on this voyage there are no Nina, Pinta, and Santa Maria, but merely one ephemeral "tri-ship." This "quantum indistinguishability" is the same effect which lies behind important technologies such as superconductivity, and is believed to be related to the very origin of mass.

In this new work, our team at the University of Toronto's Centre for Quantum Information & Quantum Control (Krister Shalm, Rob Adamson, and Aephraim Steinberg) have generated "triphotons" and studied the quantum information which can be encoded in their polarization. Polarization is a normally invisible (at least to humans -- bees in fact use it to navigate) property of light which is at the basis of 3D movies, glare-reducing sunglasses, and a coming wave of advanced technologies such as quantum cryptography.
The Heisenberg Uncertainty Principle teaches us that in the quantum world, nothing is certain ... but some things are more uncertain than others. An important area of study for over 20 years has been "squeezing," a set of methods for reducing this fundamental quantum uncertainty; for instance, such techniques are now being used to increase the sensitivity of LIGO, a gravity-wave detector which is expected to open up a new set of eyes on the universe. But even in the quantum world, there is no free lunch. and when you "squeeze" the uncertainty of one property, the uncertainty of another "complementary" property inevitably grows: think of a balloon which stretches out lengthwise when you squeeze it sideways. In all previous work, it was assumed that one could nevertheless squeeze indefinitely, simply tolerating the inexorable growth of uncertainty in some "uninteresting" direction.
But the Toronto results have shown for the first time the importance of the following fact: the world of polarization, like the Earth, is not flat. In fact, a state of polarization can be thought of as a small continent floating on a sphere, and Heisenberg tells us that while the continent can have any possible shape, it has to have at least a certain minimum area. When we "squeezed" our triphoton continent, at first all proceeded as in earlier experiments -- but when we squeezed sufficiently hard, the continent lengthened so much that it began to "wrap around" the surface of the sphere. Where all previous experiments were confined to such small areas that the sphere, like your home town, looked as though it was flat, this work needed to map the triphoton on a globe. In so doing, it showed for the first time that the spherical nature of polarization creates qualitatively different states and places a limit on how much squeezing is possible.
This work demonstrates the deep relationship between squeezing and other mysterious quantum properties such as "entanglement," highlighting for the first time the importance of the topology (the technical term for "shape") of the worlds being mapped out. The new sorts of quantum states our techniques can generate may be applicable to high-precision measurement, next-generation atomic clocks, and novel quantum systems for information processing.

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