Physicists propose quantum entanglement for motion of microscopic objects

December 21, 2009

Researchers at the California Institute of Technology (Caltech) have proposed a new paradigm that should allow scientists to observe quantum behavior in small mechanical systems.

Their ideas, described in the latest online issue of the , offer a new means of addressing one of the most fascinating issues in : the nature of quantum superposition and entanglement in progressively larger and more complex systems.

A quantum superposition is a state in which a particle, such as a photon or atom, exists simultaneously in two locations. Entanglement, which Albert Einstein called "spooky action at a distance," allows particles to share information even if they are physically separated.

A key challenge in observing quantum behavior in a small mechanical system is suppressing interactions between the system and its noisy environment — i.e., the surrounding material supporting the system or any other external contact. The random thermal vibrations of the system's surroundings, for example, can be transferred to the mechanical object and destroy its fragile quantum properties. To address this issue, a number of groups worldwide have begun to use cryogenic setups in which the immediate environment is cooled down to a very low temperature to reduce the magnitude of these random vibrations.

The Caltech team suggests a fundamentally different approach: using the forces imparted by intense beams of light to "levitate" the entire mechanical object, thereby freeing it from external contact and material supports. This approach, the researchers show, can dramatically reduce environmental noise, to the point where diverse manifestations of quantum behavior should be observable even when the environment is at room temperature.

Among the scientists involved in the work are Darrick Chang, a postdoctoral scholar at Caltech's Institute for ; Oskar Painter, associate professor of ; and H. Jeff Kimble, Caltech's William L. Valentine Professor and professor of physics.

The idea of using optical forces to trap or levitate small particles is actually well established. It was pioneered by Arthur Ashkin of Bell Laboratories in the 1970s and 1980s, and has since formed the basis for scientific advances such as the development of "optical tweezers" — which are frequently used to control the motion of small biological objects — and the use of lasers to cool atoms and trap them in space. These techniques provide an extremely versatile toolbox for manipulating atoms, and have been employed to demonstrate a variety of quantum phenomena at the atomic level.

In the new work, Chang and his colleagues demonstrate theoretically that similar success can be achieved when an individual atom is replaced by a much more massive — but still nanoscale — mechanical system. A related scheme has been presented simultaneously by a group at the Max Planck Institute of Quantum Optics in Garching, Germany [].

The system proposed by the Caltech team consists of a small sphere made out of a highly transparent material such as fused silica. When the sphere comes into contact with a laser beam, optical forces naturally push the sphere toward the point where the intensity of light is greatest, trapping the sphere at that point. The sphere typically spans about 100 nm in diameter, or roughly a thousandth the width of a human hair. Because of its small size, the sphere's remaining interactions with the environment — any that don't involve direct contact with another material, because the sphere is levitating — are sufficiently weak that should easily emerge.

For such behavior to appear, however, the sphere must also be placed inside an optical cavity, which is formed by two mirrors located on either side of the trapped sphere. The light that bounces back and forth between the mirrors both senses the motion of the sphere and is used to manipulate that motion at a quantum-mechanical level.

The researchers describe how this interaction can be used to remove energy from, or cool, the mechanical motion until it reaches its quantum ground state — the lowest energy allowable by quantum mechanics. A fundamental limit to this process is set by the relative strengths of the optical cooling and the rate at which the environment tends to heat (return energy to) the motion, bringing it back to the ambient temperature.

In principle, the motion of the well-isolated sphere can be cooled starting from room temperature down to a final temperature that is ten million times lower; in that super-cooled state, the center of mass of the sphere moves by only the minimum possible amount set by intrinsic quantum fluctuations.

The researchers also propose a scheme to observe a feature known as entanglement, which lies at the heart of quantum mechanics. Two remotely located systems that are quantum entangled share correlations between them that are stronger than classically allowed. In certain circumstances, entanglement can be a very valuable resource; it forms the basis for proposals to realize improved metrology and more powerful (quantum) computers.

The proposed scheme consists of sending a pair of initially entangled beams of light — the production of which was first accomplished by Kimble's group at Caltech in 1992 —into two separate cavities, each containing a levitated sphere. Through a process known as quantum-state transfer, all of the properties of the light —in particular, the entanglement and its associated correlations — can be mapped onto the motion of the two spheres.

While the sizes of these nanomechanical objects are still very far from those we associate with everyday experience, the Caltech researchers believe that their proposal presents an exciting opportunity to realize and control quantum phenomena at unprecedented scales — in this case, for objects containing approximately 10 million atoms.

Explore further: New method to detect quantum mechanical effects in ordinary objects

More information: PNAS paper: "Cavity optomechanics using an optically levitated nanosphere"

Related Stories

Researchers putting a freeze on oscillator vibrations

June 17, 2009

University of Oregon physicists have successfully landed a one-two punch on a tiny glass sphere, refrigerating it in liquid helium and then dosing its perimeter with a laser beam, to bring its naturally occurring mechanical ...

Physicists move closer to the quantum limit

April 12, 2004

A new experiment in the US has come close to detecting quantum effects in a macroscopic object. Keith Schwab and colleagues from the National Security Agency (NSA) working at the University of Maryland have measured the vibrations ...

Recommended for you

Camera able to capture imagery of an optical Mach cone

January 23, 2017

(—A team of researchers at Washington University in St. Louis has built a camera apparatus capable of capturing moving imagery of an optical Mach cone. In their paper published in the journal Science Advances, ...

Experiment resolves mystery about wind flows on Jupiter

January 23, 2017

One mystery has been whether the jets exist only in the planet's upper atmosphere—much like the Earth's own jet streams—or whether they plunge into Jupiter's gaseous interior. If the latter is true, it could reveal clues ...


Adjust slider to filter visible comments by rank

Display comments: newest first

2.3 / 5 (9) Dec 21, 2009
Sounds like a great proposal but if it doesn't relate to AGW where are they going to get the funding?
2.3 / 5 (3) Dec 21, 2009
To me this really begs the question - is superposition a way to rectify a lack of precision in measurements? I remember reading an article here that quanta only represent averages of states that are measurable.
3.7 / 5 (3) Dec 21, 2009
jonnyboy, what the hell are you talking about? Do you understand that this article is talking about studying quantum effects?

dtxx, what?! Superposition is how we describe the observations we make, nature is expressing that dynamic. If we can demonstrate, or prove through experimentation, that there are other ways to describe the results of affects on systems at this scale then okay, but you don't have such an explanation do you?

I get the impression people think scientists are making things up, from the wrong perspective.
2.6 / 5 (5) Dec 21, 2009
Just as a good telescope can discern a binary star that a crappy telescope sees as a single point of light... We really don't have much precision in terms of measurements at these scales. I'm not a research scientist and I don't have proof, but I bet that extremely rapid oscillation between states can appear as a superposition to an observer with a crude enough instrument.
2.7 / 5 (7) Dec 21, 2009
Dear Bob,

I sent you that entangled photon yesterday, why have you not confirmed the manipulated data. We simply must stop any alternative ideas getting to the science journals. You must believe me, my grant is due for renewal.

love Alice.

Dear Alice,

I have not found the photon you sent but I have used a trick to make it seem statisically likely that I did get the data. I will approach the board and ensure the dismissal of anyone who questions your integrity. I look forward to our next entanglement.

always yours Bob
2.3 / 5 (3) Dec 21, 2009
If that joke is designed to disarm support of entanglement, then you have no idea how those experiments are built or why they work. That joke is a lie, and it exposes your lack of understanding.

dtxx, Same to you.

In entanglement the measured spin of an object is certain for an entangled particle once it's partner has been measured. That's not an accident, or an illusion, or poor execution; it is an actual effect due to an unknown mechanism in nature. Entanglement has been independently verified at universities through out the world, that's not the signature of 'luck' or poor design.

1 / 5 (2) Dec 21, 2009
There is a strange experimental finding by a group led by a HIV Nobel winner Montagnier. A water solution containing human cells infected by bacteria is sterilized by a filtering procedure and healthy cells are added to the filtrate. Within few weeks the infected cells re-appear.


Indeed, there are many explanations possible. RNA could pass 20 nm filter without problem, it could penetrate health cells and to replicate viral proteins inside of them by some horizontal gene transfer mechanism. Or even viral proteins itself could infect health cells, because they could pass microfilter and reconstruct virions in solutions. Or most probably, cells were simply reinfected by virii from air dust, for example.

But there is still possibility, viral particles can infect bacteria via unknown mechanism involving quantum entanglement with water clusters, for example.
1.7 / 5 (6) Dec 22, 2009
Dear SincerelyTwo,

Thank you so much for your support this year. The work you have done in supporting difficult to verify results and the adjustment of hidden variables has been outstanding.

As you know many people doubt the need to worship us Scientists and your constant bigotry has been able to cover for many mistakes, including that awful AGW mess.

Perhaps you could help convince our friend Heisenberg who seems uncertain. Clearly with your inflexible narrow minded belief system even he might join the Church of Science.

We hope that you continue to enjoy assisting in misdirection and obfuscation that enables public awe of Science during the New Year and send our Season Greetings to you.

all the best - Alice and Bob (simultaneously).
3 / 5 (2) Dec 22, 2009

Did you just write that mess? Was that serious? Do you actually understand what you're talking about? Wait, don't answer, I know you don't, obviously. What the hell do you think uncertainty principle talks about? I have got to hear your interpretation of it. I'm convinced you probably think it describes how to make the best fetticini alfredo.

I'm pretty sure 90% of all people are idiots, mostly by choice. That includes the shitty scientists involved in AGW, but using examples of idiot scientists to make an asbolute determination about science itself?! I think you just told me I was 'narrow minded'? I don't know, I think you're doing a perfect job of threatening me with a dildo and then shoving it up your a$$ while asking me 'HOW DO YOU LIKE THAT?!@', well, I like it just fine, very amused at your unimaginably insane ability to fail so hard, at everything you do... or think.
not rated yet Dec 22, 2009
SincerelyTwo -

It's great that you're so well-versed in entanglement and I agree: it is a fundamental mechanism underlying the quantum world as we understand it today. But surely you realize that there have been many such "fundamental" yet poorly understood mechanisms over the course of history, many of which we have later excluded through evidence and observation.

But more to the point: are you seriously chastising random strangers on a comment board for not fully grasping something as non-intuitive and complex as entanglement? Try being a little more gracious toward those who don't share your understanding and perhaps you'll find that people are not simply "idiots by choice", they're just not completely up to speed on quantum mechanics. Not too shocking, really, when you consider that most people also aren't quantum physicists.

You do yourself no credit by behaving incredulous or insulting when questioned by doubters, informed or otherwise.
not rated yet Dec 22, 2009

I'm chastising random strangers on higher level logic, becoming aggressive towards individuals who criticize what they clearly don't understand. A persons world views affects the decisions they make in life. If people truly felt the decisions they make were important to them, they'd respect logical fallacies, quite a lot of people work against themselves without realizing it.

Everyone in the world is becoming louder on the internet, more obnoxious, and it's absurd to me. Every now and I just lash out, because unfortunately I'm afflicted by emotion and carry immeasurable respect for science and maths.

The idea that you shouldn't judge and entire field of study based on a subset of individuals extends to (in contrast) giving the same respect to religion, which I do. Where would society be without sources to realize and push forth morals and ethics? In some situations some attributes are more valuable than others, we shouldn't discredit these fields ... poor strategy. >:]
not rated yet Dec 22, 2009
I think they're both the same person, cleverly impersonating macroscopic systems entanglement. Anyhow I vote for the one with the sense of humor. Other one has a brick up his ass. Hukhuk
not rated yet Dec 26, 2009
Basically every pair of mechanical oscillators coupled by Casimir force could serve as an example of entanglement, observable by naked eye. Such oscillators could be quite large, if we can maintain the separation of planparallel plates.
not rated yet Dec 28, 2009
SincerelyTwo, you often make valid points. However, when you sprinkle all that uncontrolled emotion over your posts it only ends robbing you of credibility.
not rated yet Jan 02, 2010
Nicely done, Sincerely Two - you are showing remarkable restraint faced with a couple that are so hung up on one item they feel they must insert it in everything else.

The quantum entanglement phenomena is fascinating. Those that deify cyber thieves would do well to listen to you.

I wonder if there has been any work on a utilization of this entanglement? The opportunities for instant communications may be the first - but I am sure they go waaaaay beyond that.

"Beam me up - Scotty."

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.