Squeeze my uncertainty - Quantum noise hits its limits

This poses a great challenge to researchers who work to solve mysteries at the tiniest of scales, the quantum level: The smaller the scale, the more elusive an accurate measurement and the more challenging it is to reduce uncertainty. To acquire useful information about the quantum world, researchers must overcome uncertainty. This uncertainty, also known as quantum noise, is one of the great barriers to making accurate measurements. Aephraim Steinberg, a member of CIFAR’s Quantum Information Processing program, has uncovered the limits to reducing this noise. He gives it a little “squeeze” using a system designed to compress light to its fundamental quantum limit.

Light is the driving force behind some of science’s most powerful measuring tools, from microscopes and medical imaging devices to telescopes and X-ray scanners. The smallest particles of light – photons – have quantum properties that endow them with the ability to store and manipulate information. These photons literally shed light on everything from investigations into the nature of the Universe to new means of information processing.

Dr. Steinberg’s experiment combined three separate photons in an optical fibre similar to those used in telecommunications. Together, these photons are called a triphoton, and when they come together they form what’s called a polarization state – a property that describes its electric field. Dr. Steinberg says that the uncertainty of a polarization measurement can be thought of as a continent floating on the surface of a sphere.

To glean the quantum information encoded in its electric field, Dr. Steinberg squeezed the triphoton, thereby squeezing the uncertainty. Squeezing increases certainty in a measurement, but it does so at a cost: By squeezing the certainty of one property that is of particular interest, the uncertainty of another complementary property inevitably grows.

According to quantum theory, it is possible to squeeze the uncertainty continent in one direction and see it expand in another direction, as if the continent was made of dough. But Dr. Steinberg discovered that at a certain point – the fundamental quantum limit – the continent lengthens so much that it begins to wrap around the surface of the sphere.

This finding illustrates how the world of polarization – like the Earth – is not flat. The spherical nature of polarization limits how much squeezing is possible. It contrasts all previous work, which assumed that one could squeeze polarization indefinitely in one direction, simply tolerating more uncertainty in the other.

The next step in this work is to squeeze larger, more complex quantum systems to their fundamental limit. This feat will bring researchers even closer to the highly accurate and reproducible measurements that are required for breakthroughs in the quantum world.

This story relates to our research program: Quantum Information Processing