More accurate than Heisenberg allows? Uncertainty in the presence of a quantum memory

Jul 27, 2010

Quantum cryptography is the safest way to encrypt data. It utilizes the fact that transmitted information can only be measured with a strictly limited degree of precision. Scientists at Ludwig-Maximilians-University in Munich and ETH Zurich have now discovered how the use of a quantum memory affects this uncertainty.

A is hard to grasp, because one cannot determine all its properties precisely at the same time. Measurements of certain parameter pairs such as position and momentum remain inaccurate to a degree given by Heisenberg's Uncertainty Principle. This is important for the security of , where information is transmitted in the form of quantum states such as the polarization of particles of light.

A group of scientists from LMU and the ETH in Zurich, including Professor Matthias Christandl, has now shown that position and momentum can be predicted more precisely than Heisenberg's Uncertainty Principle would lead one to expect, if the recipient makes use of a quantum memory that employs ions or atoms. The results show that the magnitude of the uncertainty depends on the degree of correlation ("") between the quantum memory and the quantum particle. "The result not only enhances our understanding of quantum memories, it also provides us with a method for determining the degree of correlation between two quantum particles", says Christandl. "Moreover, the effect we have observed could yield a means of testing the security of quantum cryptographic systems." ( online, July 25, 2010)

Unlike classical computers, quantum computers operate not with bits, but with quantum bits or qubits, quantum mechanical states of particles. The crucial feature of qubits is that they can exist in different states at once, not just 0 or 1, but also as a superposition of 0 and 1. The ability to exploit superposition states is what makes quantum computers potentially so powerful. "The goal of our research is to work out how quantum memories, i.e. memory systems for qubits, might be utilized in the future and how they affect the transmission of quantum bits", explains Christandl, who left LMU Munich in June 2010 to take up a position in the Institute of Theoretical Physics at the ETH in Zurich.

Heisenberg's plays a central role in quantum computing, because it sets a fundamental limit to the accuracy with which a quantum state can be determined. Quantum mechanics also tells us that the measurement of a parameter can itself perturb the state of a particle. If, for example, one were to measure the position of a particle with infinite precision, the particle's momentum would become completely uncertain. Quantum cryptography uses this effect to encrypt data, for instance by entangling two quantum particles in a way that the probability with which the measurement of one particle yields a certain value depends on the state of the other particle. Eavesdropping can thus easily be uncovered, because any measurement will change the state of the particle measured.

The teams at LMU and the ETH Zurich have now shown that the result of a measurement on a quantum particle can be predicted with greater accuracy if information about the particle is available in a quantum memory. Atoms or ions can form the basis for such a quantum memory. The researchers have, for the first time, derived a formula for Heisenberg's Principle, which takes account of the effect of a quantum memory. In the case of so-called entangled particles, whose states are very highly correlated (i.e. to a degree that is greater than that allowed by the laws of classical physics), the uncertainty can disappear. According to Christandl, this can be roughly understood as follows "One might say that the disorder or uncertainty in the state of a particle depends on the information stored in the quantum memory. Imagine having a pile of papers on a table. Often these will appear to be completely disordered -- except to the person who put them there in the first place."

"Our results not only improve our understanding of quantum memories, they also give us a way of measuring entanglement", says Christandl. "The effect could also help us to test the security of quantum cryptographic systems." One can picture the method as a game in which player B transmits a particle to player A. A then performs a measurement on the particle, introducing an uncertainty. A subsequent measurement by B will only yield the value determined by A with an uncertainty given by Heisenberg's Principle. "But if B uses a ", says Christandl, "he can determine the correct value and win the game."

Explore further: New method for non-invasive prostate cancer screening

More information: "The Uncertainty Principle in the Presence of Quantum Memory", M. Berta, M. Christandl, R. Colbeck, J.M. Renes, R. Renner
Nature Physics, 25 July 2010. DOI:10.1038/nphys1734

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User comments : 41

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Parsec
3.6 / 5 (5) Jul 27, 2010
I am not an expert in quantum particles by any means. Reading this article however reminded me of maxwell's demon. For a long time this stood as a thought experiment that cast doubt on the second law. But time demonstrated the flaws in the argument.

I suspect the same problem will arise here. Either quantum memory will maintain the uncertainty, or it will be transferred to the result by reading it.

Until someone actually builds a system like this that actually works, call me dubious.
Jigga
1.8 / 5 (10) Jul 27, 2010
It's a sort of Elitzur–Vaidman problem .

http://en.wikiped...b-tester
Nemo
4 / 5 (4) Jul 27, 2010
I never would have thought that Quantum Mechanics, that arcane science from the 1930's would become a mainstream tool. Looks like that's exactly what's going happen though, and they'll start teaching it in high school so that it will be second nature to adults.
Kedas
1.7 / 5 (6) Jul 27, 2010
It's about time they figured out that Heisenberg was wrong.
I never accepted that rule anyway.

Jigga
1.8 / 5 (11) Jul 27, 2010
It's the same stuff like the observation of superluminal information spreading via entanglement. If we admit, information can spread superluminaly, we can derive, Heisenberg's uncertainty principle can be violated too.

Actually the superluminal information is noncausal one - you can be never sure by source of information in the same way, like during listening of waves at the water surface. If you'll use underwater waves, which are spreading faster, you can get the information faster - but because such information arrived from underwater through extra-dimension, you can be never sure by its direction. Under many situations it doesn't bring a problem - when you're sure, there is no other source of signal, you can still enjoy non-causal communication.

The above example with quantum memory is the same situation. Quantum memory enables averaging of signal in certain time-span, you can avoid uncertainty limit, but you cannot be sure by the original source of information at the price.
shavera
5 / 5 (4) Jul 27, 2010
Jigga, there are no known processes that allow for superluminal message transmission. It's a common misconception of entanglement that somehow one could use entangled particles as a kind of morse code across any distance.
The standard message coding scheme works something like this: A system of 2 particles, each with 2 possible states 0,1, can form 4 total final states. Alice and Bob each have prepared their particles ahead of time to be correlated, but they're in a superposition of states. Alice modifies her particle so that the two are now in one of the 4 final states. All she's done is change the correlation between her particle and Bob's particle. She still has to measure her particle, and tell Bob, via a classical (c or slower) channel what her measurement was, so that Bob can measure his and find out which final state (the final state being the message) Alice had selected. The slowest link in the chain always has to be light speed or slower.
Zed123
not rated yet Jul 27, 2010
Ok, so my understanding is that in a spacially seperated entagled system, Alice can measure her particle's state and instantaneously affect the state of Bob's particle. However, Bob can't use that to determine any information as he can't know at any time whther Alice has measured her particle or not. i.e. at time X Bob's particle may or may not have collapsed to a determined state due to Alice's measurement. If he measures before she does then BOB will be determining the state of the particles. In this sense, the only way to confirm is to measure then ring Alice and check the results are the same.

My question is what if they pre-agree on a set time to measure. Alice measures at 12:00 exactly, Bob measures at 12:01 exactly. Bob knows that Alice has already measured because they previously synchronised their clocks. So he knows the measurmenet is a result of Alice's measurment. But if Bob is 2 light minutes away from Alice, haven't they communciated FTL?

What am I missing here?
Pete83
3.5 / 5 (4) Jul 28, 2010
Zed, as far as I understand it... No, they haven't communicated faster than light. Just because Alice has measured her particle, and therefore Alice knows what result Bob will get when he measures his particle, no transfer of information has taken place. However it does give Alice a 100% accurate description of the future (on the assumption that Bob will always measure his particle).

When it comes down to it we have to stop saying particle and call it a wave-icle.
Zed123
3 / 5 (4) Jul 28, 2010
Pete, thats the point though. If Alice knows exactly what result Bob will get when he measures then she can control the info that Bob see's when he measures his particle. If they had shared a predetermined key then presumably they could communicate information in this way? At specified times Alice could measure her particle and then after that Bob measures his, records its state and uses that against the pre-determined key to understand what Alice is trying to communicate? Scale this up across a number of particles and you could communicate longer messages each time.

I'm no physicist so surely I can't be the first to think of it in this way therefore I'm missing something, I just cant figure out what it is! :)
DamienS
4.2 / 5 (5) Jul 28, 2010
There was no superluminal information transfer in the above linked article by VestaR (et al), which clearly states:

"...the new experiment shows that direct communication between the photons is simply impossible".
Hesperos
not rated yet Jul 28, 2010
Until someone actually builds a system like this that actually works, call me dubious.

The theory needs more testing for sure, but I don't see anything intrinsically contradictory about being able to quantify uncertainty.
KBK
1 / 5 (2) Jul 28, 2010
observation is entanglement or interference and thus entrainment occurs.

simple enough.

Big bang, expanding sphere oscillating and expanding 'outward' thus it's maximal oscillatory 'limits' (in-out- but incapable of being recognized as such as they are all the same 'point'), thus infinite 2-d oscillating stress fields are born. Those intersect and interact with one another, creating our 3-d reality across their interactive torsionally spinning selves. An innie and an outie.

Our 'higher dimension' is merely a quantified artifice that occur ACROSS these torsionally spinning interactives. Inherent Fibonacci, etc, all inclusive in the basics. Everything we know of and speculate on -fits.

Thus infinitely stable spin but a huge release of energy when 'busted apart'. Spooky action at a distance, etc, all of our observed phenomena, both unaccepted and scientifically accepted all occur in a simple easy to understand system that does not really step outside of basic mechanics.
CHollman82
1 / 5 (1) Jul 28, 2010
All she's done is change the correlation between her particle and Bob's particle. She still has to measure her particle, and tell Bob, via a classical (c or slower) channel what her measurement was, so that Bob can measure his and find out which final state (the final state being the message) Alice had selected.


I don't understand this...

If you are treating each entangled particle as a single bit then the state is unimportant, the change is what matters... if a change occurs it can be interpreted as a bit flip, it doesn't matter what the state was before or after, so long as they are different.

Alice and bob each have one of two entangled particles, Alice changes her particle, when bob detects this change the bit represented by that particle is flipped... that's all.

Unless merely examining Bob's particle causes decoupling, if that is the case then it would only work once, but would still be faster than C communication for the one time it worked.
Gawad
not rated yet Jul 28, 2010
All she's done is change the correlation between her particle and Bob's particle. She still has to measure her particle, and tell Bob, via a classical (c or slower) channel what her measurement was, so that Bob can measure his and find out which final state (the final state being the message) Alice had selected.


I don't understand this....


That's because Bob dosn't measure a "change" from, say, spin up or down or visa-versa. All he can do is measure a particle as spin up or spin down at a particular time in his reference frame. That spin at Bob's location may be instantly determined by Alice because of particle entanglement, but Bob has no way of knowing how or when she is going to flip that bit ahead of time except through normal channels at speed c. And if he doesn't wait for Alice to determine the spin state and takes a measurement, then he'll be the one responisble for the spin state at Alice's location.
CHollman82
1 / 5 (1) Jul 28, 2010
And if he doesn't wait for Alice to determine the spin state and takes a measurement, then he'll be the one responisble for the spin state at Alice's location.


Okay, but couldn't you use some type of pre-determined timing scheme to overcome this? Very simply, Alice only changes her particle on odd minutes and whatever his name was only examines his on even minutes?
Gawad
not rated yet Jul 28, 2010
And if he doesn't wait for Alice to determine the spin state and takes a measurement, then he'll be the one responisble for the spin state at Alice's location.


Okay, but couldn't you use some type of pre-determined timing scheme to overcome this? Very simply, Alice only changes her particle on odd minutes and whatever his name was only examines his on even minutes?


You cartainly can set up a scheme where you regularly witness the results of instantaneous EVENTS. But you can't get any meaningful information out of those events unless you set it all up through "regular channels" in the first place, and then you're back to square one.

In practice, Bob can certainly have an agreement with Alice to look for the particle spin state at a certain time, but he can't know what that state is going to be unless Alice has let him know how she is going to flip those bits ahead of time through "regular channels".
Skeptic_Heretic
3.4 / 5 (5) Jul 28, 2010
That still doesn't determine if a flip has occured.

The two entangled particles/photons/wave-icles (awesome term) have no measurement applied. Once one side is measured the other side now has a starting point. We cannot control the spin and prevent wave form collapse. Effectively nothing is transmitted, the connection is simply broken.

In order to use this to relay any sort of information you would need to have an initial known state during entanglement. If this is possible, it is outside of our technological competence and well outside our knowledge of physical reality.

Secondly, any form of entanglement break would have to be correlated with the data on the other side, resulting in the actual data transfer occuring at C or below.
CHollman82
1 / 5 (1) Jul 28, 2010
In practice, Bob can certainly have an agreement with Alice to look for the particle spin state at a certain time, but he can't know what that state is going to be unless Alice has let him know how she is going to flip those bits ahead of time through "regular channels".


I'm still not following you because it doesn't matter what the states are... You aren't looking for a particular state, you are looking for a change from the previous known state.

Maybe my understanding of the topic is insufficient, so let me explain better the way I see this working.

- Start with 2 entangled particles at a distance, the state of which are known.

- At any even minute position A can change the state of their particle, thus changing particle B

- At any odd minute position B can examine their particle, knowing the previous state, to determine if a change has occurred

- Whether or not the examination of particle B effects A (I am unsure) is irrelevant, comm. would work either way.

contd.
CHollman82
1 / 5 (1) Jul 28, 2010
contd.

- If examination of particle B changed A then A would detect this change at even time intervals and would know that the message had been read... this can be used as a read receipt mechanism.

- All that you would need for communication to work is to be able to remember the previous state and examine the current state to detect changes in the state at pre-agreed upon time intervals.

Again, I may simply lack the understanding of how entanglement works to see the problem, but would appreciate if you or anyone could explain it. Particularly, does examining the state change it? And if so, does it change it to the examined state or to some unknown (and therefore unknowable) state?
CHollman82
1 / 5 (1) Jul 28, 2010
But one thing I think some of you who are more informed about the physics of this might be missing is that you don't need to know what state to look for, it is relative, you are looking for a change from the previous NOT a particular state.

As an embedded engineer I have a good deal of experience with binary communication protocols, so I understand that not everyone may understand this.

The 0's and 1's of binary are relative and arbitrary, it doesn't matter which is which when transmitting a message, all you need to be able to do is detect a change... Then, a change in one bit from one time interval to the next can be represented as a binary one and no change in a bit in the same interval can be represented as a binary 0, for example.

The particular spin of the particle is irrelevant, as long as you can tell that the spin has changed from the last examination.
Gawad
5 / 5 (2) Jul 28, 2010
O.k. CHollman82, I think I see the problem you are having. I believe it is this : you (and most people) seem to instinctively believe Bob can either see the particle at a certain moment as either (still) entangled or not (say spin up or spin down) and look for a change in the state. This is incorrect. Niether Alice or Bob can EVER see the particle (or more generally speaking the quantum, property) as entangled (meaning spin up AND down simultaneously). They can only observe an up spin OR a down spin. Even if this doesn't prevent them from entangling their particles in the fist place, they can never observe the quantum state uncollapsed, so the spin can never have "changed from the last examination." And if by "from the last examination" you mean the last particle (pair) examined, well that particle (pair)is not correlated with the next one anyway.

Am I totally misunderstanding you, or does that help at all?
Gawad
5 / 5 (2) Jul 28, 2010
...also, you have to keep in mind that the 0's and 1's you're used to dealing with are classical not quantum. They are not both 0 and 1 simultaneously until someone takes a peek.
CHollman82
1 / 5 (1) Jul 29, 2010
you (and most people) seem to instinctively believe Bob can either see the particle at a certain moment as either entangled or not (say spin up or spin down) and look for a change in the state. This is incorrect.


Yeah this is what I didn't understand... I thought that once entangled the particles remained entangled and the spin could be observed without effecting the entanglement.

You seem to be saying that the quantum superposition is the entanglement, that when the waveform collapses under observation the particles are no longer entangled, I didn't understand that.
Gawad
5 / 5 (2) Jul 29, 2010
Correct. The only I'd clarify is that you can have a superposition of states in other types of cases (famously Schrodinger's cat for example :^) without entanglement of particle pairs...or cats. When you have an entangled particle pair, however, then yes, entanglement implies that at least one quantum property is in a state of undetermined superposition for both particles in the pair. And conservation of quantum numbers requires that when one of the pair is observed to have, say, spin up (if that's the entangled property), then the other's spin state must instantly collapse to spin down, even if light years away.
MarcThibault
not rated yet Jul 31, 2010
For some reason, what everyone is missing is that this involves two attributes of the entangled particles, say color and smell limited by Heisenburg. If you measure one, you can't measure the other.

Both Bob and Alice measure one or the other property at random. Then they send each other messages to say which they measured. It has to be after they've both measured to avoid a man-in-the-middle attack. If they measured different properties, they throw that bit away as useless. If they measured the same property, Bob has the complement of what Alice measured and they have one random bit toward a key to be used to decrypt the actual message when Alice sends it (subluminally).
Slotin
1.3 / 5 (25) Aug 02, 2010
there are no known processes that allow for superluminal message transmission

In 1994 Günter Nimtz and Horst Aichmann carried out a tunneling experiment at the laboratories of Hewlett-Packard after which Nimtz stated that the frequency modulated (FM) carrier wave transported the 40th symphony of Wolfgang Amadeus Mozart 4.7 times faster than light due to the effect of quantum tunneling.
Skeptic_Heretic
3 / 5 (4) Aug 02, 2010
Ah yes, another old faithful among Alizee aliases. Slotin is back.

Provide a link.
Skeptic_Heretic
3 / 5 (4) Aug 02, 2010
Now I know why Slotin is back, VestaR was finally banned.
Slotin
1.2 / 5 (25) Aug 02, 2010
VestaR posts were all deleted from this thread, but I still didn't checked, whether this account was banned already - I'm using it at different computer at school. Actually such policy has no deeper meaning, if it still allows to introduce new accounts - it just increases the number of sock-puppets here.
Skeptic_Heretic
2.3 / 5 (3) Aug 02, 2010
if it still allows to introduce new accounts - it just increases a number of sock-puppets here.
Only if the poster continues to introduce sockpuppets. It's like serial rape. The rapist will never stop unless he is forced to stop. Too bad they can't jsut ban you entirely.
Slotin
1.2 / 5 (24) Aug 02, 2010
It's just attempt for censorship of private ideas, nothing else. I presume, they could ban whoever from here with using of IP address or some header string. In the same way, like they could block deleted users from voting of foreign posts - The voting engine has actually IP address check built in.
MarcThibault
not rated yet Aug 02, 2010
Hello? This thread is about Quantum Entanglement and Heisenberg.

What's with all the whining? Isn't there a separate forum for tantrums?
Gawad
3.7 / 5 (3) Aug 02, 2010
It's just attempt for censorship of private ideas, nothing else. I presume, they could ban whoever from here with using of IP address or some header string. In the same way, like they could block deleted users from voting of foreign posts - The voting engine has actually IP address check built in.


1st off, if you're at a different location on a different machine, how could they know it was "you" they were banning. They can't, so we have to suffer the consequences.

2nd, considering your history on this site, their actions have probably very little to do with the censorship of private ideas and one heck of a lot more to do with trying to keep people like you from HIJACKING the forums on the site. You more than any other poster have contibuted to the pain-in-the-ass hoops all now have to jump through because of your:

* forum FLOODING
* MULTI-PAGE posts
* MULTIPLE-IDENTIIES
* VOTING WARS

Yet still you obsessively-compulsively post around all these restrictions.
Jigga
1.3 / 5 (13) Aug 02, 2010
their actions have probably very little to do with the censorship of private ideas
Actually all posts about aether theory are deleted automatically from here by now. The rest is only consequence of this approach.
..What's with all the whining?..
This is part of strategy of my opponents. They've no matter of fact arguments, so they're collecting ad-hominem remarks. Nothing very new in this context.
croghan27
2.3 / 5 (3) Aug 02, 2010
Now I know why Slotin is back, VestaR was finally banned.


PHEW - thanks for saying that (finally) SH - I kept seeing responses to VestaR with no original posts. Figured I was losing it, I did, I did.
Skeptic_Heretic
3.4 / 5 (5) Aug 03, 2010
PHEW - thanks for saying that (finally) SH - I kept seeing responses to VestaR with no original posts. Figured I was losing it, I did, I did.

Growing list of Alizee aliases can be found in my profile.
Ethelred
2.6 / 5 (5) Aug 03, 2010
This is part of strategy of my opponents. They've no matter of fact arguments,


You simply ignore facts. In particular you ignore the fact that you cheat on the ranking system which is what has so many annoyed with you.

Ethelred
MustaI
1 / 5 (11) Aug 03, 2010
You should forget to some ranking system for future.
Ethelred
3 / 5 (4) Aug 03, 2010
What an unconscionable hypocrite.

I will forget the ranking system when others do.

I have said many times that it SHOULD GO AWAY. As should you as YOU are the worst abuser of the system, bar none. Next worse was the Global Warming deniers who would give ANY post no matter how well thought out and how well supported a one if it so much as hinted that Global Warming just might possibly be real. Even mentioning that Bush had finally figured out reality to a small degree would get you a one.

They and you are the reason my ranking is below four.

Heck YOU alone probably have me down to four.

Mostly for telling the truth about your misbehavior.

Any you have the nerve to claim that YOU are censored via ranking when you engage in that behavior constantly. So much so that I have finally begun a consistent campaign of retaliation.

Ethelred
Ethelred
3 / 5 (4) Aug 03, 2010
I thank you for confirming that you are a total hypocrite by giving me a one via your banned VespaR sockpuppet. It is interesting the way you constantly accuse of others of being horrible for acting the way YOU do.

Rules are only for others and not the MultiNamed Crank.

Except it is people like you that forced others to create those rules.

Ethelred
croghan27
5 / 5 (1) Aug 03, 2010
PHEW - thanks for saying that (finally) SH - I kept seeing responses to VestaR with no original posts. Figured I was losing it, I did, I did.

Growing list of Alizee aliases can be found in my profile.


I see it in another thread .. the one on grumpy people. :)