Quantum physics 'rules' -- Australian scientists create world's most accurate 'ruler'

NEVER try telling a quantum physicist that near enough is good enough – Australian researchers have invented a technique that, for the first time, measures lengths as accurately as the laws of physics allow.

In a paper published in leading international science journal Nature the research team revealed a system that uses individual photons – single particles of light – as a ruler for microscopic distances. The experiment was performed in the laboratory of Dr Geoff Pryde at Griffith University's Centre for Quantum Dynamics, by him and his PhD student Brendon Higgins. Centre Director, Professor Howard Wiseman, developed the theory, together with University of Sydney's Dr Stephen Bartlett and Macquarie University's Dr Dominic Berry.

Pryde’s apparatus used single photons, with each photon making a number of passes through the sample being measured.

Using just 36 photons making a total of 378 passes, the team was able to measure length differences less than one ten thousandth of the width of a human hair.

“This is an incredibly small amount of light.” said Pryde. “For comparison, scanning a barcode uses quadrillions of photons. Even the dim standby light on your DVD player shines out many trillions of photons a second.”

"Similar schemes based on interferometry – a technique using waves of electromagnetic radiation such as light – have been used for centuries to measure length, and other properties of objects, with great accuracy." said Wiseman.

"The key difference is we have done it in a way that gets as much information out of each pass of a photon through the sample as is allowed by Heisenberg's uncertainty principle. In this sense, it’s the best measurement possible – something that’s never been done before."

"The obvious applications are in making accurate measurements using less light than was previously thought possible. This is particularly important in fields such as medical research, as passing light through a biological sample can damage it," said Pryde.

So why do we need to measure with such accuracy?

"Measurement underpins all science, and through history we've seen that advances in precision measurement lead to unexpected scientific discoveries, which in turn lead to new technologies and applications," Pryde said.

"Old-style interferometers taught us that the Earth wasn’t moving through a mysterious substance called 'aether', but actually through a vacuum. This ultimately led to Einstein’s theory of relativity. We don’t know yet where this new technique will lead us.”

The team's next goal is to use more single photons and passes to get an even finer measurement.

"In principle this is possible, but there are some technological hurdles to overcome first!” said Pryde.

In the strange world of quantum physics, a photon can simultaneously take two different paths to the same destination. Where the photon goes next depends on the difference between the lengths of the two paths. If the length of one path is known, this allows scientists to measure the length of the other path with great accuracy. The novel feature of the Griffith experiment is to make the photon retrace the paths many times before detecting it. This amplifies the effect of the difference in path lengths. The same idea can also be applied to measuring other quantities like speed, frequency, and time.

The technique combined concepts from research in quantum computers (through Australia’s Centre for Quantum Computer Technology), in quantum control, and in quantum communication. “This is a great example of ideas coming together from different directions to make something new and exciting,” said Bartlett.

Source: Research Australia

Related stories:

Single photon detectors for telecommunications wavelengths
Practically speaking, single photon detection has not been something pursued very heavily at the wavelengths used for telecommunication signals. Part of the problem is that performance of single photon detectors are rather constrained at such long wavelengths. But, says Robert Thew, a scientist at the University of Geneva, the time is coming when single photon detectors may be needed in telecommunications.
Physicists Seek Answers to Quantum Correlations
After performing multiple tests on two entangled photons, physicists have yet again found that the photons seem to be communicating faster than the speed of light - at least 100,000 times faster. The researchers hope that their results might encourage theorists to come up with new explanations for the strange quantum mechanical effect.
Researchers make milestone discovery in quantum mechanics
Researchers at UC Santa Barbara have recently reached what they are calling a milestone in experimental quantum mechanics.
Getting many quantum states from one experimental setup
(PhysOrg.com) -- “In the traditional approach to entanglement with linear optics, one designs a new setup for each single state that you want,” Witlef Wieczorek tells PhysOrg.com. “What we’ve done is to make a single setup that allows you to observe lots of different quantum states.”
World's Largest Quantum Bell Test Spans Three Swiss Towns
In an attempt to rule out any kind of communication between entangled particles, physicists from the University of Geneva have sent two entangled photons traveling to different towns located 18 km apart – the longest distance for this type of quantum measurement. The distance enabled the physicists to completely finish performing their quantum measurements at each detector before any information could have time to travel between the two towns.
Carbon Nanotubes as a Single-Photon Source
Carbon nanotubes, as true multi-purpose materials, have potential applications in everything from electrical circuits and drug delivery to golf clubs and space elevators. Recently, physicists have investigated single-walled carbon nanotubes (CNTs) for one more use: as a single-photon source, where they could help make quantum communication networks extremely secure and efficient.
Two for One: New Design Enables More Cost-Effective Quantum Key Distribution
Researchers at the National Institute of Standards and Technology have demonstrated a simpler and potentially lower-cost method for distributing strings of digits, or “keys,” for use in quantum cryptography, the most secure method of transmitting data. The new “quantum key distribution” (QKD) method, outlined in an upcoming paper, minimizes the required number of detectors, by far the most costly components in quantum cryptography.
New superlattice structure enables high performance infrared imaging
Scientists at the Center for Quantum Devices (CQD) in the McCormick School of Engineering at Northwestern University have demonstrated for the first time a high-performance infrared imager, based on a Type II superlattice, which looks at wavelengths 20 times longer than visible light.

News discussion:

Physics news