B Can Entangled Particles Stay Undetermined Despite Decoherence During Separation?

[Joseph Flatt]

If quantum objects’ superpositions decohere rapidly due to exposure to a surrounding environment, why have I heard it said that two entangled particles can be a large distance apart while still having undetermined properties? Wouldn’t decoherence occur while the particles were moving apart?

 

[StevieTNZ]

Smaller systems undergo decoherence slower than bigger systems.

 

[PeterDonis]

If quantum objects’ superpositions decohere rapidly due to exposure to a surrounding environment

They don't always. In particular, in experimental situations where specific measures are taken to isolate systems from the outside environment, quantum coherence can be maintained for quite some time.

 

[Nugatory]

Wouldn’t decoherence occur while the particles were moving apart?

Yes, but if we work hard enough at isolating the particles from the environment (for example, keep our particles inside a vacuum chamber to avoid interactions with the atmosphere, or use photons which don’t easily interact with uncharged matter) they can move quite respectable distances before decoherence gets in the way. Google for “Micius satellite” to see an example of entanglement maintained over more than 1000 kilometers.

 

[vanhees71]

Such experiments are most easy to realize with photons, because they do not interact too strongly with the environment. Another advantage is that they can be produced in entangled states by shooting with a laser on certain birefringent crystals (e.g., Barium Borate) creating entangled two-photon pairs. A lot of experiments have been made, where the photons have been detected at far distant places in the sense that the detection events where space-like separated, ensuring that the measurement on one of the photon cannot have any causal influence on the other photon or its measurement.

 

[Joseph Flatt]

Thank you all for your replies. It’s much appreciated.

 

[EPR]

For this level of question you were given appropriate answers.
Otherwise, it's all one field if we talk about electrons or photons. Everything is entangled with everything else, as interaction also means entanglement. In some situations, interaction also means measurement which this time breaks the entanglement and the field manifests as particles(or objects). Some physicists think there is something truly profound in this finding, others think it's just a peculiarity of Nature. It's not easy to digest or make sense of, esp. since physicists still struggle to weed out when an interaction becomes a measurement.
What they shield the entangled pair in those experiments is from potential measurements and much less from interactions.
Measurement is something on top of interaction and they are seldom synonymous. There are many attempts to make them equal but those attempts aren't very convincing.

 

[StevieTNZ]

Everything is entangled with everything else ...

Indeed; something amazing I learned while reading Michael Esfeld's chapter in https://www.wiley.com/en-us/Entangl...m+Information+and+Computation-p-9783527404704

 

[haushofer]

The entangled photons used in Zeilingers famous experiment on La Palma and Tenerife,

https://www.scientificamerican.com/article/entangled-photons-quantum-spookiness/

could already decohere due to photons from the moonlight.

 

[EPR]

Decoherence on its own cannot explain single outcomes. It could, if measurement was involved on top of decoherence. Or if a new world was created.

Few physicists believe the wavefunction has an ontic status that shifts its ontic phase during interaction with other ontic wavefunctions and thus loses quantum coherence. More interaction means more entanglements. Measurement means end of entanglement.
People confuse interaction with measurement because sometimes , under specific conditions, they are one and the same.

 

[vanhees71]

What do you mean distinguishes a measurement from an interaction of the measured object with the measurement device? I don't see any hint of such a difference. To our present understanding there are universally valid laws of nature describing the interaction of particles, and QT explains how all (known) matter around us is built up by the elementary particles of the standard model, i.e., quarks, leptons, gluons, photons, and W+Z bosons. Decoherence is an emergent phenomenon when reducing the description of macroscopic matter to the macroscopically relevant collective degrees of freedom through coarse graining.

 

[vanhees71]

The entangled photons used in Zeilingers famous experiment on La Palma and Tenerife,

https://www.scientificamerican.com/article/entangled-photons-quantum-spookiness/

could already decohere due to photons from the moonlight.

How do you come to that conclusion? The demonstration of the entanglement of these photons detected at 144 km distance shows that there is no considerable decoherence. The photon-photon interaction ("Delbrück scattering") is so tiny that it is neglible.

It's a different story for charged particles, where indeed even the cosmic background radiation is sufficient to lead to considerable decoherence. That's why it is so much easier to do long-distance entanglement experiments with photons rather than with charged particles.

 

[EPR]

What do you mean distinguishes a measurement from an interaction of the measured object with the measurement device? I don't see any hint of such a difference. To our present understanding there are universally valid laws of nature describing the interaction of particles, and QT explains how all (known) matter around us is built up by the elementary particles of the standard model, i.e., quarks, leptons, gluons, photons, and W+Z bosons. Decoherence is an emergent phenomenon when reducing the description of macroscopic matter to the macroscopically relevant collective degrees of freedom through coarse graining.

This is a case where interaction and measurement can be used interchangeably. Interactions that can't reveal information about the quantum state are generally not measurements. They lead to entanglements. Measurements break the entanglement generated during interactions.

 

[vanhees71]

Measurements must lead to entanglement between the measured object and the measurement device. Otherwise you don't have a measurement of the observable you measure. Then there should also be dissipation in order to store the measurement result irreversibly. All this does not need any "extra rules" for measurements compared to what you call interactions.

 

[EPR]

In your selected scenario interaction and measurement are one and the same - it's measurement during interaction of the measuring device with the measured object.

Measurements break the entanglements created ubiquitously during interactions by collapsing the wavefunction of the entangled particles. How is it consistent to say that interactions create entanglements and also destroy the same entanglements?

 

[vanhees71]

Take the Stern-Gerlach experiment. There you create an entanglement between the spin-component to be measured and the position (or momentum) of the atom by applying an appropriate inhomogeneous magnetic field. The measurement then is to put a plate to register the particle at a position. Then of course the atom is absorbed at the plate and certainly it doesn't make sense to talk about a collapse as if the atom was now in a determined state as a single system. Nevertheless the pointer observable (position of the atom on the plate) provides a measurement of the spin component due to the entanglement between spin component and position of the atom immediately before the atom hitting the plate, where it is absorbed leaving an observable trace and thus an irreversible storage of the measurement result.

 

[rogeragrimes]

One alternate way to define decoherence (as described by many quantum physicists including Dr. Wojciech Zurek) is that we humans cannot easily track the original entangled observed particles (that we intend to experimentally observe) and still be sure that the observed readings are or are not the result of other unwanted entangled particles, each of which imparts its own entangled imprint. In this model, entanglement is not some hard to create and easy to destroy property, but unavoidable and guaranteed for any two quantum particles that meet. So, you start out trying to measure only two particles to see what the future state/result is...but because both of those particles are potentially entangling with other particles after their own entanglement (it's one big ever expanding Schroedinger's with an expanding number of variables), we can't figure out if the properties we are observing later on are due only to the original two entangled objects. It's like trying to find your original two drops of water that then fell into the ocean. The two drops are there, but they are interacting with a lot of other drops impacting them. The "decoherence" isn't a collapse or breaking of the original entanglement, but our inability to follow the original two drops and accurately consider the impacts of all the other drops that happen afterwards.

 

[PeterDonis]

One alternate way to define decoherence (as described by many quantum physicists including Dr. Wojciech Zurek) is that we humans cannot easily track the original entangled observed particles (that we intend to experimentally observe) and still be sure that the observed readings are or are not the result of other unwanted entangled particles, each of which imparts its own entangled imprint.

This is nothing like anything I've read about decoherence, including anything by Zurek. Can you give a reference for where you are getting this from?

 

[rogeragrimes]

It's from many of his lectures, and implied in some of his papers and writings. For example, in . See 14.30-15.00, where he casually says that the interaction - any interaction - between the quantum system S and its environment "generally leads to entanglement". No special conditions or anything - he says it as if we all know and accept it. Simple as that. Philip Ball discusses the theory in his book Beyond Weird, as well. It's the pre-condition of the observer theory...where no one (or any measurement device) can observe any quantum event without now becoming a part of the entangled equation. It's an subplot of the Many Worlds theory as well. After thinking about it for 20 years, it makes the Copenhagen interpretation of a spontaneous, irreversible, quantum collapse to a classical world seem silly and not the other way around. More importantly, there is no scientific evidence that I know of that refutes this interpretation. So, until something shows up that does, it's in the running.

 

[PeterDonis]

the interaction - any interaction - between the quantum system S and its environment "generally leads to entanglement"

That's true, but it's not the same thing as decoherence. Also it's not the same thing as what you were claiming in your previous post.

 

[rogeragrimes]

That's true, but it's not the same thing as decoherence. Also it's not the same thing as what you were claiming in your previous post.

I'm not sur. We may have to disagree. First, Zurek literally says, "What is decoherence?" before explaining that the system naturally entangles with its environment, with new degrees of freedom, as part of the definition. Second, what I wrote is explained by Philip Ball in his book Beyond Weird, and this particular snippet of lecture is part of what Ball uses to support his explanation of what decoherence is in the book. Zurek doesn't say the new entangled states are impossible for humans to follow, but I think it is a natural conclusion because any entanglement experiment we are conducting is trying to prevent and exclude natural entanglement...and each additional unwanted entanglement makes it more difficult to follow. But even outside of Zurek and Ball, no one has presented evidence excluding it as a possibility.

 

[PeterDonis]

We may have to disagree.

It's not a matter of disagreement; it's a matter of properly understanding what the reference you give is saying. The reference you give is indeed saying that interactions create entanglement, but I've already said that's not the issue. The issue is this particular claim of yours, which I quoted before, and which I'll quote again:

One alternate way to define decoherence (as described by many quantum physicists including Dr. Wojciech Zurek) is that we humans cannot easily track the original entangled observed particles (that we intend to experimentally observe) and still be sure that the observed readings are or are not the result of other unwanted entangled particles, each of which imparts its own entangled imprint.

Nothing in the reference you gave says this. If you think it does, you are misunderstanding what it says.

You are saying that, once decoherence happens, we can't tell whether our observation of the result of a quantum measurement is due to the system we measured or to some other "unwanted entangled particles". That's not true, and it's not what the reference you gave says.

What is true, and what the reference you gave says, is that, once decoherence happens--once the measured system (and also the measuring device) has interacted with its environment and spread entanglement to many other degrees of freedom through those interactions--the different alternative outcomes of the measurement can no longer interfere with each other. That doesn't create any uncertainty about where our original measurement result came from: it came from the system we measured and its interaction with the measuring device. Nothing about decoherence changes that.

 

[DrChinese]

See 14.30-15.00, where he casually says that the interaction - any interaction - between the quantum system S and its environment "generally leads to entanglement". No special conditions or anything - he says it as if we all know and accept it. Simple as that.

Despite the words used, this does not mean what you think it does. There are at least 2 glaring issues:

1. A particle cannot be maximally entangled (on some basis, say x-spin) with 1 particle, and also be entangled with another particle. So everything is NOT entangled with everything. Of course, a group (N>2) of particles can be less than maximally entangled.

2. You can always redefine the meaning of "environment" and "interaction", but entangled particles interact all the time with things like fiber, mirrors, filters, etc. without losing their entanglement (i.e. they don't decohere).