Scientists score one more victory over uncertainty in quantum physics measurements

Feb 26, 2012 by John Toon
Michael Chapman, a professor in the School of Physics at Georgia Tech, poses with optical equipment in his laboratory. Chapman’s research team is exploring squeezed states using atoms of Bose-Einstein condensates. (Credit: Gary Meek)

(PhysOrg.com) -- Most people attempt to reduce the little uncertainties of life by carrying umbrellas on cloudy days, purchasing automobile insurance or hiring inspectors to evaluate homes they might consider purchasing. For scientists, reducing uncertainty is a no less important goal, though in the weird realm of quantum physics, the term has a more specific meaning.

For scientists working in , the says that of properties such as the momentum of an object and its exact position cannot be simultaneously specified with arbitrary . As a result, there must be some uncertainty in either the exact position of the object, or its exact momentum. The amount of uncertainty can be determined, and is often represented graphically by a circle showing the area within which the measurement actually lies.

Over the past few decades, scientists have learned to cheat a bit on the Uncertainty Principle through a process called "squeezing," which has the effect of changing how the uncertainty is shown graphically. Changing the circle to an ellipse and ultimately to almost a line allows one component of the complementary measurements – the momentum or the position, in the case of an object – to be specified more precisely than would otherwise be possible. The actual area of uncertainty remains unchanged, but is represented by a different shape that serves to improve accuracy in measuring one property.

This squeezing has been done in measuring properties of photons and atoms, and can be important to certain high-precision measurements needed by atomic clocks and the magnetometers used to create magnetic resonance imaging views of structures deep inside the body. For the military, squeezing more accuracy could improve the detection of enemy submarines attempting to hide underwater or improve the accuracy of atom-based inertial guidance instruments.

Now physicists at the Georgia Institute of Technology have added another measurement to the list of those that can be squeezed. In a paper appearing online February 26 in the journal Nature Physics, they report squeezing a property called the nematic tensor, which is used to describe the rubidium atoms in Bose-Einstein condensates, a unique form of matter in which all atoms have the same quantum state. The research was sponsored by the National Science Foundation (NSF).

"What is new about our work is that we have probably achieved the highest level of atom squeezing reported so far, and the more squeezing you get, the better," said Michael Chapman, a professor in Georgia Tech's School of Physics. "We are also squeezing something other than what people have squeezed before."

have been squeezing the spin states of atoms for 15 years, but only for atoms that have just two relevant quantum states – known as spin ½ systems. In collections of those atoms, the spin states of the individual atoms can be added together to get a collective angular that describes the entire system of atoms.

In the Bose-Einstein condensate atoms being studied by Chapman's group, the atoms have three quantum states, and their collective spin totals zero – not very helpful for describing systems. So Chapman and graduate students Chris Hamley, Corey Gerving, Thai Hoang and Eva Bookjans learned to squeeze a more complex measure that describes their system of spin 1 atoms: nematic tensor, also known as quadrupole.

Nematicity is a measure of alignment that is important in describing liquid crystals, exotic magnetic materials and some high temperature superconductors.

"We don't have a spin vector pointing in a particular direction, but there is still some residual information in where this collection of atoms is pointing," Chapman explained. "That next higher-order description is the quadrupole, or nematic tensor. Squeezing this actually works quite well, and we get a large degree of improvement, so we think it is relatively promising."

Experimentally, the squeezing is created by entangling some of the atoms, which takes away their independence. Chapman's group accomplishes this by colliding atoms in their ensemble of some 40,000 rubidium atoms.

"After they collide, the state of one atom is connected to that of the other atom, so they have been entangled in that way," he said. "This entanglement creates the squeezing."

Reducing uncertainty in measuring atoms could have important implications for precise magnetic measurements. The next step will be to determine experimentally if the technique can improve the measurement of magnetic field, which could have important applications.

"In principle, this should be a straightforward experiment, but it turns out that the biggest challenge is that magnetic fields in the laboratory fluctuate due to environmental factors such as the effects of devices such as computer monitors," Chapman said. "If we had a noiseless laboratory, we could measure the magnetic field both with and without squeezed states to demonstrate the enhanced precision. But in our current lab environment, our measurements would be affected by outside noise, not the limitations of the atomic sensors we are using."

The new squeezed property could also have application to quantum information systems, which can store information in the spin of and their nematic tensor.

"There are a lot of things you can do with quantum entanglement, and improving the accuracy of measurements is one of them," Chapman added. "We still have to obey Heisenberg's Uncertainty Principle, but we do have the ability to manipulate it."

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sjm
5 / 5 (7) Feb 26, 2012
One does not manipulate Heisenberg's uncertainty principle. The experiment fixed certain parameters but did not manipulate the Uncertainty Principle.
thermodynamics
5 / 5 (7) Feb 26, 2012
sjm: I agree completely. I assume this was just an error in reporting because those in the field don't look at this process as defeating the uncertainty principle. It is interesting that they reported it almost correctly in noting that the area of the uncertainty remained the same while they crushed it down into an ellipse. That is exactly as the principle is normally taught. We don't reduce the uncertainty, we just pay more for it in one dimension or the other as we reduce the uncertainty in the orthogonal vector.
Deathclock
3 / 5 (9) Feb 26, 2012
The HUP is misleading... the problem is things at the quantum scale are not "objects"... The idea that these things are like little billiard balls is false, there is no such thing as "particles" and there is no such thing as "solid". Solidity is a description of a type of human perception and it has to do with the fundamental forces. The only reason we perceive things as solid is because the nuclear and electromagnetic forces prevent something like our hand from going right through something like a wall, that is what solidity is, a manifestation of the physical forces. Nothing is actually "solid", nothing is actually a "particle"... everything is energy. They are measuring energy and expecting it to behave as if it were a tiny billiard ball. That is why we have things like the HUP that seem very counter-intuitive, because people completely misinterpret that which they relate to.
vacuum-mechanics
1 / 5 (2) Feb 26, 2012
By the way, even we are familiar with Heisenbergs uncertainty principle which state that It is not possible to determine both the position and the momentum of a particle with unlimited precision. The only question is why it is so, or what is its mechanism? May be this new unconventional idea could give some hint

http://www.vacuum...id=19=en
tigger
1.5 / 5 (2) Feb 27, 2012
Heisenbergs uncertainty principle seems like the most obvious thing in the world to me... it's so obvious, you can't measure something without 'touching' it... and when you 'touch' it you've changed it, so your measurement doesn't reflect actual.

BUT, the acceptance that the objects there really are hanging out in multiple places at the same time seems like an odd assertion to me... almost coming from a place of arrogance, that unless we can know it then it can't be. See we would never be happy with the idea that things are really really really actually in a place and time it's just that we can't know it because we're part of the system too... we'd never be happy with that idea would we? Sigh. Always obsessed with being the observer.
Irene_Bea
2 / 5 (1) Feb 27, 2012
Dear vacuum-mechanic
Information and Kinetic energy and mass are all energy. in order for a particle to have an exact position and momentum it would have to have infinite energy in its momentum or mass. Consider a photon with a frequency f, a wavelength w, and position x in space. Its position measurement error and its frequency measurement error are subject to uncertainty like a particle containing mass. Taking its w to the Plank length limits f and its energy as well as its information. See Maxwell's Demon for the relationship between kinetic energy and information.
The relationship between Uncertainty and Information is more apparent when you consider hf~energy, knowing that the information carrying capacity of EMF is directly related to its Frequency.
It is interesting that defining anythings center in space to be 0.00000000000... would take an infinite line of numerals.
Irene_Bea
1.3 / 5 (4) Feb 27, 2012
Uncertainty assures Free Will, and destroys any deterministic system.
Irene_Bea
not rated yet Feb 27, 2012
Uncertainty is not because of just the measurement effecting the state of the particle, but is an intrinsic property of any entity that contains energy.
A more recent study of Uncertainty is posted below.

http://www.physor...nty.html
Deathclock
1.6 / 5 (7) Feb 27, 2012
Uncertainty assures Free Will, and destroys any deterministic system.


Uncertainty destroys the scientific method... Uncertainty says that things can occur for no reason and that cannot be explained, which is the definition of magic. Uncertainty validates magic, it validates miracles and ghosts and tarot card readings and psychics and all that stupid bullshit... because it validates magic.

I'm not so hung up on free will that I have to accept the existence of magic to preserve it. Frankly I don't think free will exists whether or not the universe is deterministic. Non-determinism due to true randomness does not suggest free will, will is not random, will is the OPPOSITE of random...
antialias_physorg
3 / 5 (2) Feb 27, 2012
Uncertainty destroys the scientific method

Not quite. The scientific method works for probabilistic statements as well as for hard 'if-then' statements (quantum physics is nothing BUT probabilistic statements - and I dare say it's well based on the scientific method).

Uncertainty assures Free Will,

In the sense of 'free' as in 'not ultimately predictable' NOT in the sense of 'free' as in 'unbounded'.
Deathclock
1.7 / 5 (6) Feb 27, 2012
Uncertainty assures Free Will, and destroys any deterministic system.


No, it doesn't. Unless you consider randomness to be "will"... but if you look up the definition of will you will find it means anything but "random".

If you want to assert free will you have to explain how my personal wishes can affect physical reality. You would have to posit a fifth fundamental force that is controlled by all intelligent creatures, but only locally in their own brains. My brain must be able to affect the physical processes involved in itself in order to assert my will. Otherwise my thoughts and actions are subject to the 4 fundamental forces that I DO NOT have control over.

I'll tell you what free will is. It is an illusion brought about by our ability to recognize alternatives. We can easily say "instead of going to work this morning I could have hopped a plane to Fiji". But is that true, or is it only that we can imagine it as an alternative?
Deathclock
2.3 / 5 (7) Feb 27, 2012
Uncertainty destroys the scientific method

Not quite. The scientific method works for probabilistic statements as well as for hard 'if-then' statements (quantum physics is nothing BUT probabilistic statements - and I dare say it's well based on the scientific method).


The scientific method requires there to be a reason for an observation. The entire point is to determine the reason for the observational evidence we gather. Non-determinism says that things can occur for NO reason, that things can occur which cannot be explained. That destroys the scientific method at that level... certainly up until that level the scientific method works fine, as you mentioned, but as soon as you insist that something HAS NO explanation then the scientific method fails.
vacuum-mechanics
1 / 5 (1) Feb 27, 2012
Dear vacuum-mechanic
Information and Kinetic energy and mass are all energy. in order for a particle to have an exact position and momentum it would have to have infinite energy in its momentum or mass. ......

Why?
Callippo
1 / 5 (1) Feb 27, 2012
but as soon as you insist that something HAS NO explanation then the scientific method fails
You're right, this is a deity concept as applied in physics. Which is why Einstein has been so unsatisfied with it ("The God doesn't play dice with the world"). But who cares about if some stuff is "scientific" or not? The important is, it's realistic. Inside of nested density fluctuations of aether the portion of energy really disappears. One fluctuation can never interact with other completely. After all, when we observe a star, we should always face the fact, some information about far side of star is lost.
Deathclock
1 / 5 (1) Feb 27, 2012
But who cares about if some stuff is "scientific" or not? The important is, it's realistic.


As far as I am concerned scientific and realistic are synonyms.
Skultch
not rated yet Feb 28, 2012
Deathclock,

Your argument is compelling, but I would have posed the question you answered differently. I would not ask if some randomness can allow/cause free will, but can SOME (not total) randomness break hard determinism? (Don't think of a response to that just yet.)

If determinism ceases to be 100% predictive, does this not allow for something else to change the system and manifest into what we consider free will? IOW, determinism breaks down and a new deterministic causal chain begins, and its initial conditions are "determined" by this randomly causaction.

If you want to assert free will you have to explain how my personal wishes can affect physical reality.


Easy. The mind controls the body to make physical changes to the entire system. I think you meant something subtly different there. As long as one random event can break the causal chain, this leaves an opening for something in the system to create a new and previously unpredictable causal chain, no?
Kinedryl
1 / 5 (1) Mar 01, 2012
You cannot beat the uncertainty principle in single measurement, but you can cheat it during repetitive measurements. The quantum wave function always collapses during this, but you can sample it at different interval during next measurement and reconstruct its whole evolution in "stroboscopic" way. This is a principle of so-called weak measurement, used in QM studies recently.

Analogously, at the water surface the energy is always mediated in mixture of transverse and longitudinal waves. The longitudinal waves are dispersive and indeterministic, but you can reconstruct the object with them by taking the picture from many directions at the same moment. In this way, you can get an image of the back side of object, which would otherwise remain unobservable in transverse waves.

In brief, the memo is, the limitations given with quantum mechanics for single measurement can be beaten with combining of the results of multiple correlated observations.
antialias_physorg
not rated yet Mar 01, 2012
Weak measurement does not cheat (or beat) the Uncertainty principle.
http://en.wikiped...surement

Even repeated application of same does not.

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