Physicists 'uncollapse' a partially collapsed qubit

Nov 11, 2013 by Lisa Zyga feature
Physicists ‘uncollapse’ a partially collapsed qubit
Quantum quality control: By peeking at a qubit, physicists can make sure that the qubit hasn’t decayed, though without finding out the qubit’s state. However, the act of peeking often alters the qubit’s state, causing partial qubit collapse. A recovery method like the one demonstrated in the new paper can be used to reverse the effects of the peek on the qubit. As a result, the peek ensures the qubit is OK without changing the qubit’s state. Credit: J. A. Sherman

(Phys.org) —One of the striking features of a qubit is that, unlike a classical bit, it can be in two states at the same time. That is, until a measurement is made on the qubit, causing it to collapse into a single state. This measurement process and the resulting collapse may at first seem irreversible. (Once you open the box to find a dead cat, there's no going back, right?) But recently physicists have been investigating the possibility of "uncollapsing," or recovering the state of, a qubit that has been partially collapsed due to a weak measurement. The results could be used for implementing quality control in quantum systems.

In a new paper published in Physical Review Letters, physicists J. A. Sherman, et al., at the University of Oxford, have experimentally demonstrated a recovery method that can restore the state of a single qubit, in principle perfectly, after it has partially collapsed.

As the physicists explain, a full collapse of a qubit results from a measurement that reveals the qubit's state, while a partial collapse results from a measurement that can be thought of as a "peek" at the qubit because it doesn't reveal the qubit's state, but simply verifies that the qubit hasn't decayed. The problem is that, often the mere act of peeking alters the qubit's state. A recovery method would essentially reverse the effects of peeking on the qubit, thereby allowing peeking to serve as a sort of non-destructive quantum quality control technique.

The concept of a partial collapse can also be imagined in terms of Schrödinger's cat.

"To really torture the cat analogy, imagine the cat could be in three states: happy, sad, or dead," Sherman told Phys.org. "Then the technique is a way to measure just whether the cat is dead or not without learning anything at all about whether the cat is either happy or sad. The precise quantum mixture of happy and sad is actively recoverable even after verifying that the cat isn't dead.

(a)–(d) Observed qubit states on the Bloch sphere during and after partial collapse, and (e)–(h) during and after recovery. The states are accurately recovered, as shown by the similarity between (a) and (h). Credit: J. A. Sherman, et al. ©2013 American Physical Society

"Once we look inside, the cat will be found either dead or alive, constituting a 'full collapse' of its state. There is no recovering a cat found dead, and likewise there is no repairing quantum information after it is found collapsed."

The physicists explain that the qubit recovery method can be thought of as a generalization of a concept called spin echo, which can be considered as a way to "unwind" a spin rotation. The method was proposed in 2002 by L.-A.Wu, et al., and experimentally realized for the first time by N. Katz, et al., in 2008.

Here, the physicists have improved the accuracy of this method by reducing the infidelity by an order of magnitude. The improvements enabled them to achieve substantial recovery of a qubit's state for relatively large partial collapses. For instance, even with an 80% probability of decay, the information content of the qubit is preserved with an accuracy greater than 98%.

However, the recovery method is not perfect. The probability of recovering the qubit's state depends on how much it has collapsed, so that the more collapsed the qubit is, the less likely it is to recover. A fully collapsed qubit has zero probability of recovery.

Still, the recovery method could be very useful for overcoming one of the biggest challenges in developing quantum systems: decoherence, which results in the loss of a system's quantum properties.

"The source of great interest in quantum information is also its biggest weakness: quantum coherence is fragile since all quantum systems are greatly affected by noisy environments and spontaneous decay," Sherman said. "To make sophisticated use of quantum information, we need methods of detecting and correcting these random errors. The 'reversible peek' we describe is universally applicable, and is most helpful in cases where one qubit state decays much faster (or is more sensitive to noise) than the other. For photonic qubits, the 'reversible peek' can be implemented with birefringent optics and polarizers. For qubits formed from superconductors, it can be realized with microwave pulses. For atomic qubits like ours, we employed optical and radio-frequency pulses. The 'reversible peek' may be one of several techniques which promote any of these quantum computation architectures from a laboratory curiosity to a real, widely deployable, useful device."

In the future, the physicists plan to continue working on ways to use the qubit recovery method for different purposes.

"Ideally, the 'reversible peek' could become a standard and widely used part of any quantum mechanic's toolbox," Sherman said. "Consider the spin-echo. Viewed one way, a spin-echo is a correction procedure for an unwanted qubit phase shift. But far from being an academic novelty, the spin-echo finds use in nearly every experiment, to say nothing about its routine role increasing signal levels in every hospital's magnetic resonance imaging (MRI) machine. (I guess about one-third of the loud noises one hears in an MRI machine are caused by spin-echo pulses.) The 'reversible peek' we investigated has a structure very much like a spin-echo, but is designed to detect and correct for a different sort of error: spontaneous decay."

In the future, the Oxford group will continue investigating many methods of making quantum computation with trapped atomic ions more scalable and robust. In this context, the 'reversible peek' technique may be among several innovations which together make a quantum computation system practical, which is the ultimate goal.

Explore further: Quantum computing: Manipulating a single nuclear spin qubit of a laser cooled atom

More information: J. A. Sherman, et al. "Experimental Recovery of a Qubit from Partial Collapse." Physical Review Letters. DOI: 10.1103/PhysRevLett.111.180501

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User comments : 15

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antialias_physorg
5 / 5 (1) Nov 11, 2013
(I guess about one-third of the loud noises one hears in an MRI machine are caused by spin-echo pulses.)

Not quite. The loud noise you hear in an MR stems from putting juice on individual coils and the coils consequently expanding.

Rest of the article is fascinating, though. Peeking at a state without getting any information..sounds weird but it's another demonstration that one can't trick the information content out of a quantum state.
sigfpe
5 / 5 (2) Nov 11, 2013
> It would be perfect for breaking of quantum encryption, which is based on the "collapse" of information, once it gets read by someone.

Fortunately it wouldn't. This isn't a new theory that contradicts the old theory of quantum mechanics that allowed quantum encryption. "Uncollapse" is predicted by the same theory. So the same argument for the security of quantum encryption still stands.
the_walrus
1 / 5 (5) Nov 11, 2013
Can this be used for FTL communication? If I have 2 sets of entangled cubits and I give half of them to another person, then partially collapse set 1 for "1" or set 2 for "0", all the other person has to do is peek at which one has been partially collapsed, and they will be able to tell whether I sent a 1 or a 0. Or am I missing something.
mogmich
1 / 5 (5) Nov 11, 2013
Wouldn't this make it possible to send a message instantly over any distance? If you have two sets of particles that are entangled in pairs, you can send a code by measuring or not measuring on the first set of particles, but only "peeking" on the other set. This would show the code without making a full measurement on the second set of particles. Or, in other words: The RESULT of a measurement on a particle in the first set is irrelevant, because the measurement itself is the information. Example: The binary code for the number "2" can be send by measuring the first particle, and not measuring the second particle (10).

In fact, I expect that something makes this impossible. Maybe the entanglement would be destroyed when you make a "weak measurement"?
mogmich
1 / 5 (2) Nov 11, 2013
Actually I didn't read the comment above, before sending mine! (exactly the same point, as far as I can see).
antialias_physorg
5 / 5 (4) Nov 12, 2013
If you have two sets of particles that are entangled in pairs, you can send a code by measuring or not measuring on the first set of particles, but only "peeking" on the other set.

Problem is: to send a message you have to encode something on the particle (i.e. you have to SET a property into a defined state).
But since you only read what is already there you can't send a message (setting a defined state precludes entanglement).

If you try to set anything after entangling two entities you break entanglement (i.e. your particle may be set to the message you want to send - but the particle at the other end wouldn't react to that change)

No FTL information transmission that way, I'm afraid.
bluehigh
1 / 5 (10) Nov 12, 2013
You don't even know the cat is in the box until you look.

Happy, sad, dead, alive or ...not there.

Measurement realises the existence of physical properties.

MRBlizzard
1 / 5 (10) Nov 12, 2013
Antialias_physorg:

About breaking the entanglement,
Please give a citation for this derivation/calculation. I want to understand this thoroughly.
Thanks
antialias_physorg
5 / 5 (2) Nov 12, 2013
The derivation is this: You entangle two entities (e.g. by spin) but you don't measure them (this is important). If you do measure one you know something about the other (because of conservation laws: E.g one is spin up the other must be spin down). But you can't take one, measure it, flip its spin to a desired state (i.e. encode a message) and expect the other's spin to flip. You'd be breaking the conservation on which the entanglement relied.

Now you CAN use knowledge of two entangled entities for something at 'superluminal speeds': namely encryption. BTW: this is an excellent way of showing that encryption doesn't constitute information. I.e. that an encypted text does not carry more information than the same text in non-encrypted form.
antialias_physorg
5 / 5 (2) Nov 12, 2013
Just found this site:
http://rationalwi...nglement

...which makes the same point almost verbatim (especially in the 'insanity?' section)
DarkHorse66
not rated yet Nov 14, 2013
Just found this site:
http://rationalwi...nglement

...which makes the same point almost verbatim (especially in the 'insanity?' section)

'Insanity?' ? I think that might actually read: 'Instantly?' Was that a Freudian slip? Um, what else were you concentrating on, while reading the article? Woo? ;)
( http://rationalwi...wiki/Woo )
I hadn't come across that variation of wiki before. Nice find.
Cheers, DH66
srikkanth_kn
1 / 5 (3) Nov 18, 2013
> It would be perfect for breaking of quantum encryption, which is based on the "collapse" of information, once it gets read by someone.

Fortunately it wouldn't. This isn't a new theory that contradicts the old theory of quantum mechanics that allowed quantum encryption. "Uncollapse" is predicted by the same theory. So the same argument for the security of quantum encryption still stands.


Any reference for this?
mohammadshafiq_khan_1
1 / 5 (5) Nov 19, 2013
Here is the total collapse of physics. There is a standing open challenge to the adopted paradigm of physics at
http://www.worlds...mp;tab=2
mogmich
1 / 5 (3) Nov 21, 2013
antialias_physorg: But my point is, that in this case it would be possible to send a message WITHOUT encoding something on the particle! The message you send consists exclusively of a sequence of measuring or not measuring. A message consisting of the binary code for the number 5 could be send by 1)Measuring the first out of three qubits 2)Not measuring the second qubit 3)Measuring the third qubit.

If it is possible to only peek on the other set of three (entangled) qubits, and if this tells you which qubits are still in a superposition because they have not been measured, you get the code 101 out of it.

In other words: The measurements themselves are the message! The states of the particles are not changed in order to encode any message. Of course the state of a particle is changed if you measure it, but this change is irrelevant because it has nothing to do with the message.
antialias_physorg
not rated yet Nov 21, 2013
The message you send consists exclusively of a sequence of measuring or not measuring.

Since there is no correlation between Alice and Bob measuring/not measuring there's no information transfer because Bob has no information which bits he should measure and which ones he should disregard.

It's a subtle bit of business in information theory. In order for information to be transmitted you
a) need a priori knowledge of the message
b) need a posteriori knowledge of the sent bits
c) need to show correlation between the two

Using the entangled property as the information carrier does not allow for a). Using the measurement as information carrier does not allow for c)
Ergo: In either case no information is transmitted.

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