Researchers devise a means to 'gently' measure qubit without destroying superposition

Oct 04, 2012 by Bob Yirka report
Rabi oscillations and feedback. Credit: Nature, 490, 77–80 (04 October 2012) doi:10.1038/nature11505

(Phys.org)—As understood in the field of quantum mechanics, objects are able to exist in more than one state at a time, a property known as superposition. Measuring such states is a challenge however, as doing so causes the superposition property to be destroyed, at least according to classical theory. Now, physicists working at the University of California have found a way to cheat the system, so to speak. They have, as they describe in their paper published in the journal Nature, found a way to take a quick peek at the oscillation of a qubit, without destroying its superposition property.

The whole idea behind was famously explained by Erwin Schrödinger, who suggested it could be thought of as a cat in a box that also contained a radioactive atom . The decay of the atom could not be known without opening the box and checking—an act that would change its state. From a perspective outside the box, the cat was apparently both dead and alive at the same time. In this new work, the researchers suggest that only partially opening the box gives them some insight into the box's inner conditions without disturbing its contents.

To make that happen, the team employed a feedback whereby a superconducting qubit was coupled to a . It was then pushed into a superposition state by cycling its state back and forth between 0 and 1, repeatedly hitting all possible mixtures. Once there, they measured its . This measurement was too weak to destroy the oscillation but strong enough to cause slight change. Using the measurement taken, the team very quickly calculated, and then created, an exact opposite charge which they injected back into the system, causing the oscillation to return to its former frequency: they had managed to measure a tiny part of the system without destroying the superposition state, a feat never before achieved in the lab.

In order to carry out this experiment, the team had to first develop an amplifier that would allow them to inject the charge back into the system without making other changes—no small thing. This development led to the first instance of a qubit being measured, if only partially, without changing its state. The researchers suggest their technique might be used as a means of error control in a quantum system, i.e. computer, by allowing qubits to exist in a superposition state for longer periods of time.

Explore further: Quantum holograms as atomic scale memory keepsake

More information: Stabilizing Rabi oscillations in a superconducting qubit using quantum feedback, Nature, 490, 77–80 (04 October 2012) doi:10.1038/nature11505

Abstract
The act of measurement bridges the quantum and classical worlds by projecting a superposition of possible states into a single (probabilistic) outcome. The timescale of this 'instantaneous' process can be stretched using weak measurements, such that it takes the form of a gradual random walk towards a final state. Remarkably, the interim measurement record is sufficient to continuously track and steer the quantum state using feedback. Here we implement quantum feedback control in a solid-state system, namely a superconducting quantum bit (qubit) coupled to a microwave cavity. A weak measurement of the qubit is implemented by probing the cavity with microwave photons, maintaining its average occupation at less than one photon. These photons are then directed to a high-bandwidth, quantum-noise-limited amplifier, which allows real-time monitoring of the state of the cavity (and, hence, that of the qubit) with high fidelity. We demonstrate quantum feedback control by inhibiting the decay of Rabi oscillations, allowing them to persist indefinitely. Such an ability permits the active suppression of decoherence and enables a method of quantum error correction based on weak continuous measurements. Other applications include quantum state stabilization, entanglement generation using measurement, state purification and adaptive measurements.

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User comments : 20

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eachus
5 / 5 (6) Oct 04, 2012
Um. Qubits are not an ancient measure of length (cubit). Someone needs to update their spell-checker. (To be fair, Q-bits is another spelling for Qubits, but the spell-checker should change one to the other, if at all.)
DarkHorse66
5 / 5 (3) Oct 04, 2012
LOL That someone would be the author of the article: Bob Yirka. He made the same mistake about 2 weeks ago:
http://phys.org/n...nge.html
Considering that it was pointed out in that thread then too, I'm not sure if he actually reads what is subsequently said about his articles (maybe he should), unlike some of the other authors who DO read the comments and then sneak in changes or corrections without warning. I just hope that he DOES read this and makes sure that it is corrected in the next relevant article he writes. Unless, of course, it happens to be an article about Noah's Ark! :D
Cheers, DH66
Ophelia
4 / 5 (4) Oct 04, 2012
"Cubit" is simply embarrassing for a website that supposedly is devoted to providing physics-related news items.

And I doubt it is a spell-checker problem since "qubit" appears elsewhere in the article.

Embarrassing.
El_Nose
not rated yet Oct 04, 2012
Please not that most of the articles are copied from other sites and p-laced here -- the original author is given credit
antialias_physorg
4.3 / 5 (4) Oct 04, 2012
Well, that spells the end of the 'unhackable' information transmission.
The -idea up to now- was that when measuring an entity in a quantum superposition state you'd change it so that the change was noticeable at the other end. With this you can extract a tiny bit of information and return it to its original state.
Claudius
1 / 5 (2) Oct 04, 2012
The -idea up to now- was that when measuring an entity in a quantum superposition state you'd change it so that the change was noticeable at the other end. With this you can extract a tiny bit of information and return it to its original state.


How do you know what it's original state is?

The whole issue of superposition disappears if a many-worlds interpretation is valid. In the many-worlds interpretation, there is no superposition, and measurement does nothing more than to establish which version of the world you are in.
antialias_physorg
not rated yet Oct 04, 2012
How do you know what it's original state is?

You don't. But you know what change you made (as explained in the second paragraph) and how to change it back so that it appears as if nothing happened.

If you do this often enough - each time extracting a minute amount of information - you could, theoretically, get full information about the state of the entity.
Claudius
1 / 5 (2) Oct 04, 2012
How do you know what it's original state is?

You don't. But you know what change you made (as explained in the second paragraph) and how to change it back so that it appears as if nothing happened.

If you do this often enough - each time extracting a minute amount of information - you could, theoretically, get full information about the state of the entity.


And to what observer does it "appear" as if nothing had happened? And how can you change it "back" to a state that you know nothing about?
Lurker2358
1 / 5 (1) Oct 04, 2012
Well, why not use a multi-encryption technique, such as a multi-substitution/transpotion algorithm and then quantum encrypt the results of that?

Surely a "code book" or "nearly code book" level encryption with enough substitutions and transpositions could not be hacked, even with a quantum computer.

I suspect a lot of communications are hacked because they use generic algorithms and don't have large enough keys nor complex enough rotating keys.

In theory, every signal should have a different key for it's encryption algorithm, so that a hacker can never spot a pattern, but to my knowledge, most password features and other encryption features do not work like that.
antialias_physorg
5 / 5 (3) Oct 04, 2012
And how can you change it "back" to a state that you know nothing about?

Read the article. It's explained quite plainly in the second paragraph.
You don't know the state - and you don't need to. But you do know the CHANGE you caused (by the inofrmtaion extracted) and so you just have to undo that change.
Claudius
1 / 5 (2) Oct 04, 2012
And how can you change it "back" to a state that you know nothing about?

Read the article. It's explained quite plainly in the second paragraph.
You don't know the state - and you don't need to. But you do know the CHANGE you caused (by the inofrmtaion extracted) and so you just have to undo that change.


Would you regard this as proof that the many worlds interpretation is false? After all, in the many worlds interpretation, it should not be possible to change the state.

antialias_physorg
5 / 5 (1) Oct 05, 2012
Would you regard this as proof that the many worlds interpretation is false?

No. But neither is it an indication that the many worlds interpretation is real.

The problem with the many worlds interpretation (though intellectually very intriguing, like string theory) is: it currently suffers from the same problem string theory does. It's not testable and so malleable that it would fit any and all observations.
It's certainly something worth working on to find a test. But until someone does it's no use applying it to explain phenomena.
Claudius
not rated yet Oct 05, 2012
Would you regard this as proof that the many worlds interpretation is false?

No. But neither is it an indication that the many worlds interpretation is real.

The problem with the many worlds interpretation (though intellectually very intriguing, like string theory) is: it currently suffers from the same problem string theory does. It's not testable and so malleable that it would fit any and all observations.
It's certainly something worth working on to find a test. But until someone does it's no use applying it to explain phenomena.


David Deutsch outlines what he regards as physical evidence for the many worlds hypothesis in his book "The Fabric of Reality".
antialias_physorg
5 / 5 (1) Oct 05, 2012
Many worlds has a couple of problems I'm not too sure can be readily solved.

1) It requires infinite energy (and an infinitely large explosion of energy content of the many worlds at every instant)
2) It requires that no two worlds can influence each other in any way whatsoever (because by (1) we'd all be instantly put into the most massive black hole you could think of). But since an object supposedly is s superposition within many worlds there has to be a connection.

But maybe there are ways around this. Seriously, though - it's not one of the more useful theories as it doesn't help at all with predictability.
sirchick
not rated yet Oct 06, 2012
Many worlds has a couple of problems I'm not too sure can be readily solved.

1) It requires infinite energy (and an infinitely large explosion of energy content of the many worlds at every instant)
2) It requires that no two worlds can influence each other in any way whatsoever (because by (1) we'd all be instantly put into the most massive black hole you could think of). But since an object supposedly is s superposition within many worlds there has to be a connection.

But maybe there are ways around this. Seriously, though - it's not one of the more useful theories as it doesn't help at all with predictability.


Most proposed ideas have problems that none of us can really answer - the further back we go to the origin of the universe or the smaller we go the problems increase.... the other worlds idea i doubt we could ever do an experiment for anyway - so is it not just more philosophy... unless there is proposed tests I'm not aware of.
Nikstlitselpmur
1 / 5 (2) Oct 07, 2012
How do we know the observation of particles existing in two places simultaneously isn't the result of frame dragging and the Lense–Thirring effect, caused by the black hole at the center of the galaxy?
DarkHorse66
not rated yet Oct 08, 2012
Qubits all the way now...harmony has at last been restored (at least consistency in spelling).
Guys, might I recommend that next time something like this occurs, try the 'send feedback to the editors' hyperlink. I tried it out (for the first time) less than two hours ago and now I can no longer find any 'cubits' mentioned anywhere in the article. It appears that they even corrected the spelling in the article that I linked to in my post near the top of the thread! Many thanks to the editors for fixing this so promptly. Best Regards, DH66
Deathclock
1 / 5 (1) Oct 08, 2012
I hate to say I told you so but... I told you so...

To use bleeding edge physics such as QM as evidence against something is like building a house on sand, our understanding of these phenomenon WILL change over time, because the study of this field is in it's infancy.
Nikstlitselpmur
1 / 5 (2) Oct 09, 2012
Imagine if they had given Einstein a Nobel for Lambda.
Graeme
not rated yet Oct 10, 2012
This process sounds dubious as the amplifier would be adding noise. Also how could it amplify a partial microwave photon? It is only going to be able to detect one photon at a time and then do something with it, such as create two new ones. Each of these absorptions would be destructive of the quantum state. The regeneration would not be guaranteed to be the same as the original.