Super resolution phase measurements -- without entanglement

Jun 15, 2007 By Miranda Marquit feature

“People have been trying to make entangled states of various physical systems, and this is hard to do,” Kevin Resch tells PhysOrg.com. “But if you can get the same result from measuring entanglement rather than preparing it, then it can make things much easier.”

What Resch, a scientist at the University of Waterloo in Ontario, Canada, is proposing is a time-reversal scheme for measuring the oscillations associated with photon entanglement. “Rather than preparing the state and then measuring,” he continues, “we measured the state that others were trying to prepare.”

Work on this experiment, which demonstrated super resolution without creating entangled states, was performed by an international team of scientists, with the actual experiment done in Andrew White’s Quantum Technology Laboratory at the University of Queensland in Brisbane, Australia. Participants include scientists from Imperial College in London, University of Bristol in the U.K., the University of Vienna in Austria and Griffith University in Brisbane, Australia. The results are published in Physical Review Letters, in a paper titled, “Time-Reversal and Super-Resolving Phase Measurements.”

“Quantum mechanics,” Resch explains, “is largely a tool for predicting probabilities. To find these, there are essentially three steps: preparation of an initial state, the evolution of that state, and the detection, or measurement, of a final state. The time reversal we refer to swaps the roles of the preparation and measurement steps.”

Resch says that in addition to being easier than trying to prepare an entangled state, starting with the measuring step also provides the ability to use more photons in metrology: “Reversing the roles of preparation and measurement have allowed us to see phase super-resolution of up to six photons.” He explains that this technique was developed mainly to aid in quantum metrology, the ability to measure to super-precise levels. Prior to this work, only four photons had been used for quantum metrology. “Six photons have been entangled for other purposes,” Resch allows, “but for phase super-resolution, previous experiments had demonstrated up to four.”

Another benefit to come from this demonstration was a further demarcation between phase super-resolution and phase super-sensitivity. “In metrology,” Resch explains, “there are two effects: resolution and sensitivity. For the most part, people have assumed that they are mainly equivalent.” Resch says that phase super-resolution describes rapid oscillations in an interference pattern, while phase super-sensitivity deals reducing phase uncertainty. “We have drawn a line between the two,” he continues. “We have derived separate criteria that have to be satisfied in order to claim super-sensitivity from super-resolution.

In order to get the super-resolution phase measurements, Resch and his colleagues created a device that was designed to measure six individual photons. “Rather than preparing the photons in an entangled state, we performed an entangling measurement. We used six photon detectors and recorded the events when all of them fired at the same time. We found that the oscillations are the same as those that follow a time-forward method of preparing entanglement.”

The time-reversal approach, however, is very general. Resch explains that though it appears to work well with metrology, “it remains an interesting open question which other quantum protocols can be similarly simplified through a time-reversal approach.”

Resch hopes that this technique will be able to make better measurements of very small features. “Many of the most sensitive measurements in quantum mechanics have to do with measuring fringes, looking for the applications where measuring more rapid oscillations gives better measurement.” He compares these fringes to markings on a ruler, pointing out that a meter stick with no markings would be inefficient for measuring objects much smaller than a meter. Add additional markings, though, and the meter stick becomes a more precise tool.

Resch continues, “One needs a good ruler to measure small length changes. This might be a useful technique in metrology, with many future applications.”

Copyright 2007 PhysOrg.com.
All rights reserved. This material may not be published, broadcast, rewritten or redistributed in whole or part without the express written permission of PhysOrg.com.

Explore further: New insights found in black hole collisions

add to favorites email to friend print save as pdf

Related Stories

Physicists Demonstrate Qubit-Qutrit Entanglement

Feb 26, 2008

For the first time, physicists have entangled a qubit with a “qutrit” – the 3D version of the 2D qubit. Qubit-qutrit entanglement could lead to advantages in quantum computing, such as increased security and more efficient ...

Recommended for you

New insights found in black hole collisions

Mar 27, 2015

New research provides revelations about the most energetic event in the universe—the merging of two spinning, orbiting black holes into a much larger black hole.

X-rays probe LHC for cause of short circuit

Mar 27, 2015

The LHC has now transitioned from powering tests to the machine checkout phase. This phase involves the full-scale tests of all systems in preparation for beam. Early last Saturday morning, during the ramp-down, ...

Swimming algae offer insights into living fluid dynamics

Mar 27, 2015

None of us would be alive if sperm cells didn't know how to swim, or if the cilia in our lungs couldn't prevent fluid buildup. But we know very little about the dynamics of so-called "living fluids," those ...

Fluctuation X-ray scattering

Mar 26, 2015

In biology, materials science and the energy sciences, structural information provides important insights into the understanding of matter. The link between a structure and its properties can suggest new ...

Hydrodynamics approaches to granular matter

Mar 26, 2015

Sand, rocks, grains, salt or sugar are what physicists call granular media. A better understanding of granular media is important - particularly when mixed with water and air, as it forms the foundations of houses and off-shore ...

User comments : 0

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.