Boffins measure 27 quantum states of light

Just how dead is Schrödinger's cat?

Top three mobile application threats

One of the key tenets of quantum mechanics, that it's impossible to measure a quantum state without collapsing the wave function, has taken an arrow to the knee, courtesy of a University of Rochester experiment.

Building on work carried out in 2011 at the National Research Council of Canada, the UoR researchers say they've directly measured a 27-dimensional quantum state without destroying it.

The UoR researchers worked with the orbital angular momentum state of a light beam. OAM refers to the “twistedness” of the beam, and since it exists as a quantum state, its various possible states exist simultaneously in superposition.

The traditional view of quantum mechanics, expressed in the famous (but famously misunderstood) “Schrödinger's cat” thought experiment, is that observing the quantum states will collapse the wave function. You can only view the cat as alive or dead, not both. In the case of the light beam, taking a “strong” measurement of the beam will collapse the wave function and yield a single value for its orbital angular momentum.

University of Rochester's quantum characterisation experiment

Schematic of the University of Rochester quantum state characterisation experiment

Image: M. Malik

To characterise all of the states that existed before they collapse, the university explains, would usually require a tedious process called quantum tomography (if you don't mind Wikipedia, it's explained here). The researchers describe quantum tomography as being like creating a 3D image using a large number of 2D images.

In their “direct measurement” experiment, the UoR researchers say they've sped up the process of characterising the quantum system of the light beam. They do this by taking “weak” measurements followed by “strong” measurements.

The weak measurements disturb the quantum system without making it collapse: each weak measurement is of low enough quality that it only yields partial information about the quantum state. After enough weak measurements, the researchers say, they can build up a statement of the probability of the system's different states – in other words, is the cat in Schrödinger's box more likely to be alive or dead?

The strong measurement then collapses the wave function of a second variable.

In other words, in The Register's understanding (the author will accept correction on this point), the experiment essentially builds a dataset based on repeated weak-then-strong measurements, that tabulates:

  • What were the possible values for this quantity in the weak measurement, before the wave function collapsed?
  • What value was observed in the strong measurement?

This yields a plot from which the superpositions can be inferred much more quickly than using quantum tomography. To quote the university press release:

“This sequence of weak and strong measurements is then repeated for multiple identically prepared quantum systems, until the wave function is known with the required precision.”

The previous Canadian research performed direct measurement on the polarisation state of light, but in the Rochester work, the researchers claim they're the first to use direct measurement at such a high level of dimensionality.

The researchers say that fast characterisation of high-dimensional quantum states can help improve quantum-based secure communications systems. ®

High performance access to file storage

More from The Register

next story
KILLER SPONGES menacing California coastline
Surfers are safe, crustaceans less so
Opportunity selfie: Martian winds have given the spunky ol' rover a spring cleaning
Power levels up 70 per cent as the rover keeps on truckin'
LOHAN and the amazing technicolor spaceplane
Our Vulture 2 livery is wrapped, and it's les noix du mutt
Liftoff! SpaceX Falcon 9 lifts Dragon on third resupply mission to ISS
SpaceX snaps smartly into one-second launch window
Americans optimistic about technology despite being afraid of EVERYTHING
R.I.P. LADEE: Probe smashes into lunar surface at 3,600mph
Swan dive signs off successful science mission
Discovery time for 200m WONDER MATERIALS shaved from 4 MILLENNIA... to 4 years
Alloy, Alloy: Boffins in speed-classification breakthrough
prev story


SANS - Survey on application security programs
In this whitepaper learn about the state of application security programs and practices of 488 surveyed respondents, and discover how mature and effective these programs are.
Combat fraud and increase customer satisfaction
Based on their experience using HP ArcSight Enterprise Security Manager for IT security operations, Finansbank moved to HP ArcSight ESM for fraud management.
The benefits of software based PBX
Why you should break free from your proprietary PBX and how to leverage your existing server hardware.
Top three mobile application threats
Learn about three of the top mobile application security threats facing businesses today and recommendations on how to mitigate the risk.
3 Big data security analytics techniques
Applying these Big Data security analytics techniques can help you make your business safer by detecting attacks early, before significant damage is done.