What is Decoherence: Definition and 174 Discussions
Quantum decoherence is the loss of quantum coherence. In quantum mechanics, particles such as electrons are described by a wave function, a mathematical representation of the quantum state of a system; a probabilistic interpretation of the wave function is used to explain various quantum effects. As long as there exists a definite phase relation between different states, the system is said to be coherent. A definite phase relationship is necessary to perform quantum computing on quantum information encoded in quantum states. Coherence is preserved under the laws of quantum physics.
If a quantum system were perfectly isolated, it would maintain coherence indefinitely, but it would be impossible to manipulate or investigate it. If it is not perfectly isolated, for example during a measurement, coherence is shared with the environment and appears to be lost with time; a process called quantum decoherence. As a result of this process, quantum behavior is apparently lost, just as energy appears to be lost by friction in classical mechanics.
Decoherence was first introduced in 1970 by the German physicist H. Dieter Zeh and has been a subject of active research since the 1980s. Decoherence has been developed into a complete framework, but there is controversy as to whether it solves the measurement problem, as the founders of decoherence theory admit in their seminal papers.Decoherence can be viewed as the loss of information from a system into the environment (often modeled as a heat bath), since every system is loosely coupled with the energetic state of its surroundings. Viewed in isolation, the system's dynamics are non-unitary (although the combined system plus environment evolves in a unitary fashion). Thus the dynamics of the system alone are irreversible. As with any coupling, entanglements are generated between the system and environment. These have the effect of sharing quantum information with—or transferring it to—the surroundings.
Decoherence has been used to understand the possibility of the collapse of the wave function in quantum mechanics. Decoherence does not generate actual wave-function collapse. It only provides a framework for apparent wave-function collapse, as the quantum nature of the system "leaks" into the environment. That is, components of the wave function are decoupled from a coherent system and acquire phases from their immediate surroundings. A total superposition of the global or universal wavefunction still exists (and remains coherent at the global level), but its ultimate fate remains an interpretational issue. With respect to the measurement problem, decoherence provides an explanation for the transition of the system to a mixture of states that seem to correspond to those states observers perceive. Moreover, our observation tells us that this mixture looks like a proper quantum ensemble in a measurement situation, as we observe that measurements lead to the "realization" of precisely one state in the "ensemble".
Decoherence represents a challenge for the practical realization of quantum computers, since such machines are expected to rely heavily on the undisturbed evolution of quantum coherences. Simply put, they require that the coherence of states be preserved and that decoherence is managed, in order to actually perform quantum computation. The preservation of coherence, and mitigation of decoherence effects, are thus related to the concept of quantum error correction.
Let's say a memory qubit inside a quantum computer is in state
## α \left|1\right>+β\left|0\right> ##
This computer is equipped with a device that emits photons that carry the same qubit as the aforementioned memory location.
Alice and Bob, that are very far from each other, receive and...
This paper about Quantum Darwinism https://arxiv.org/pdf/0903.5082v1.pdf starts with statement:
"The quantum principle of superposition implies that any combination of quantum states is also a legal state. This seems to be in conflict with everyday reality: States we encounter are localized...
Layman question(s), but I hope not too stoopid -- many thanks to anyone with the patience to read and attempt even part of an answer, or share a possibly relevant link! I've got time today to follow and read links...
1) Saw a recent 'popular' article discussing that darn Cat as if still a...
A question about page 6 of https://arxiv.org/pdf/quant-ph/0101077v1.pdf
"The second unanswered question in the Everett picture was more subtle but equally important: what physical mechanism picks out the classical states — face up and face down for the card — as special? The problem was that...
After decoherence, is there something wrong if the branch chosen would be selected by consciousness?
So there is ambient consciousness that select default branches and human consciousness can bias which branches chosen? Is there any experiment that has excluded this possibility?
This thread was posted and discussed on Physics Overflow, I am re-posting it here to hear other opinions.
http://www.physicsoverflow.org/36063/given-decoherence-still-random-quantum-jumps-interpretations?
Environmental decoherence explains how the wavefunction of a quantum system q, as a result...
Suppose we have a pair of entangled particles. Suppose the first particle of the pair interacts with a macro-object and decoheres. Does the macro object get entangled with the second particle?
So I whilst understanding basics of some quantum phenomena like superposition, tunnelling, fluctuations etc I happened to watch the movie "Coherence" where there's a scientifically unsatisfactory reference to quantum decoherence. What exactly is this concept?
I've seen a couple of lectures by Penrose where he describes an experiment to test superposition of physical location of a very small, but macroscopic object.
I can't find a reference to it online, but the experiment involved sending a photon through a half-mirror, and depending on the route...
Is this statement true?
>>... over the past several decades we’ve come to understand that the classical and quantum worlds don’t exactly operate by “different” rules. Rather, the classical world emerges from the quantum in a comprehensible way: you might say that classical physics is simply...
I learned that the moment a wavefunction collapse takes place is a matter of interpretation. So, I suppose the phenomenon 'wavefuntion collapse' is something that has to be witnessed by observation at some point to be able to establish it at all! So my question is: if collapse doesn't actually...
Just taking an advance in what I want to learn someday:
I understand that decoherence and entanglement are more or less equivalent. So, I take it decoherence is in principle a process of entanglement.
Consider two particles A and B who are entangled. if A decoheres by interacting with particle...
Hi.
The Poynting vector is a 3-tuple of real or complex numbers (depending on the respective formulation of electrodynamics) times a unit. It may be pictured as an arrow with some length and direction in IR^3 or IC^3. But is it a "vector" in the strict mathematical sense, i.e. an element of a...
Modelling the onset of decoherence in a subspace as a transition from this subspace
http://www.ba.infn.it/~pascazio/publications/sudarshan_seven_quests.pdf
(Section 10 is relevant)
I am currently reading papers discussing the Zeno Effect. The linked paper discusses modelling a transition out of...
I am currently reading papers discussing the Zeno Effect, which discuss how measuring a system at high frequencies can almost freeze the state of a system, or keep the system in a specific subspace of states. This can be easily seen using the projection postulate. Often the topic of decoherence...
In this paper Caldeira gives a model for the Stern Gerlach device in the vacuum.
the incoming particle is described by the tensor product of a space term and a spin term a |u> + b |d>
the SG is in the vacuum (no air around). Under the effect of the spatial variation of the magnetic field, there...
In the never ending quest to understand a complicated idea for which I completely lack the academic requirements to do so, I'd like to ask a few basic questions that I hope will allow simple yes/no responses (to ease the frustration of the physicist/mathematician contributors). Sadly, the...
In another thread, stevendaryl and I were trying to understand whether MWI can be formulated in the Heisenberg picture. Since neither of us really understands MWI, I tried to retreat to safer ground by asking:
Can decoherence be formulated in the Heisenberg picture?
Hi guys,
just a quick question. This is an idea that came up to my mind while thinking about decoherence and stability of macroscopic objects in the world. As we know all superpositions and interference effects are destroyed in a enormously tiny timescale, and as Mfb mentioned it is...
Can anybody explain what is the difference between apparent collapse and collapse? I've read that decoherence leads to apparent collapse but what does that mean, is there an eigenstate after decoherence but its not known why did it occur. I don't understand it all.
Can anybody explain this quote from stanford encyclopedia:
Indeed, while it is well-known that localised states of macroscopic objects spread very slowly with time under the free Schrödinger evolution (i.e., if there are no interactions), the situation turns out to be different if they are in...
The concept of environmental decoherence has been a perpetual thorn in my brain despite an embarrassing amount of time contemplating the idea. At this point, I'm not even sure if I understand what it is purported to resolve. In many ways, it seems to me to be primarily a theory about logical set...
I was wondering about decoherence:
Suppose a measuring apparatus measures a single quantum property (say, spin). After the actual measurement of the property has taken place, the result 'decoheres' into the apparatus until the apparatus shows the measurement result, after which it decoheres...
For a state |\Psi(t)\rangle = \sum_{k}c_k e^{-iE_kt/\hbar}|E_k\rangle , the density matrix elements in the energy basis are
\rho_{ab}(t) = c_a c^*_be^{-it(E_a -E_b)/\hbar}
How is it that in the long time limit, this reduces to \rho_{ab}(t) \approx |c_a|^2 \delta_{ab} ?
Is there some...
Hi,
I am an amateur enthusiast and this is a direct question (not a proposal, I'm not nearly qualified to offer a proposal). I would like to throw to experts and people who are more qualified, because it's got me a bit stumped. Could I ask for an answer which is as simple as I am please, lols -...
If yes, then how is it any different from other facts of causality, and why is there a puzzle? Can one not simply view quantum particle changes as the causal interaction between subatomic and macroscopic objects, yielding certain processes of behavior under physical law, depending on the...
Despite the best efforts of some of PF's finest, I continue to struggle with the general concept of spontaneous quantum state reduction by means of environmentally induced decoherence.
I think that much of my confusion lies in the confounding degree of ambiguity in the delineation between the...
2 basic questions from a non-physicist (sigh!). Is decoherence essentially a measurement? And if so is the system of quantum particles set into the classical world forever? Or is there a mechanism by which it will evolve again into a superimposed state? Secondly, if the particle (as in the 2...
Hi,
I've been reading some (very) basic texts on decoherence and have some questions about it:
1. What is the difference between decoherence and dephasing?
2. Assuming decoherence, can we completely do away with collapse theories (i.e. theories in which wavefunction collapse must be...
Does anyone know of QM notes (or a review article) that covers entanglement, the measurement problem, Bell inequalities, decoherence, or the delayed choice experiment (or the more recent mesoscopic experiments). So to speak the more modern and the exciting aspects of QM. I think closest to that...
If I say that decoherence is an irreversible loss of information, then in the double-slit with erasure when initially decoherence has occurred with the measurement, and then after erasure of the information the state is again as it was at the beginning , how can this occur if the initial...
I keep hearing about things like "quantum decoherence" and the notion that measurement doesn't need a conscious observer. However, I haven't really seen these topics discussed in any of the textbooks I've used (mostly on the level of Griffiths and higher). I haven't even seen a reference to...
It says here that time dilation due to gravity can cause decoherence:
http://www.newscientist.com/article/dn27735-Earth's-gravity-may-force-us-to-do-quantum-experiments-in-space.html
But.. wouldn't the effect of time dilation be more like a modification of the expected unitary evolution, so...
Many discussions get around the subject of how / why does decoherence happen. Interaction with the environment, getting to macroscopic scale, irreversible change of information... Somehow they seem to imply that the natural state of things in the Universe is quantum coherence and then they ask...
When photons reflect from a mirror coated with, say, an aluminium film... the reflection is presumably down to their interactions with free electrons in the metal (and each electron is in an unknown state before and after).
Yet the interference does not go away if a mirror is inserted in one...
Hi fellow PFers, long time reader here. I have a query that was motivated
by a comment on another thread (“Decoherence question”).
It is about the quantum properties of decohered macroscopic systems.
In my (incomplete and perhaps misapprehended) understanding, a macroscopic
object, say a chair...
I would like to understand why is decoherence needed in QM.
As I understand decoherence models irreversibility.
But if two quantum systems interact and resulting states are going in different directions they can't produce interference, right? Even if they do not interact with environment. So...
So I understand that as the number of entangled particles increases, observable quantum mechanical properties decrease to the extent that the mass of particles collectively loses its wave-particle character and behaves classically.
In other words, the particles' collective position-space...
It is stated that classical states are robust against decoherence.. what would happen if classical states can decohere too? Or how do you imagine it for example occurring to a table.. How would the table look like if it suffers decoherence too? Would you fall down if you sit on one?
From time to time I hear that Quantum Decoherence can address the measurement problem only when accompanied by deBroglie-Bohm theory(dBB) (or any hidden variable theory? or MWI?). I want to know why is that?
Also, I saw some papers recently(e.g. this) that prove dBB is incompatible with QM...
When a particle decoheres, or its component states get entangled with the ``environment``, surely this is not a final eigenstate. The particle is interacting ( becoming entangled etc) with other particles and systems constantly. Therefore, isn't decoherence a continuous process?
In quantum mechanics, POVMs can be considered more fundamental than PVMs because PVMs can be considered a special sort of POVM. However, because of the Naimark extension, one can formally consider POVMs to always be derived from PVMs. Accordingly, one could argue that choosing PVMs or POVMs to...
Hi,
I was wondering whether we are sure (I know, strong word) that decoherence is the mechanism that takes us from the quantum world to our classical world. Correct me if I'm wrong, but basically decoherence is a phenomenon where we have a bunch of quantum states that, when piled onto each...
I have a couple of questions about entanglement and decoherence!
1. Sometimes you read that, strictly speaking, all electrons are entangled with one another. But can that be right?! Isn't it at least the case that electrons have to have interacted with one another in the past in order to...
I read it some where that there is very small decoherence for photons. The reason being that photons do not interact with each other (Is that because photons are chargeless, colorless and flavourless particles?) and hence the information that they contain tends to stay with them. They have a...
Decoherence relationship to the "measurement problem"
I have heard the argument that "decoherence" in quantum states causes the quantum collapse to occur, and that this solves the "measurement problem". But I'm still left with a nagging question... Does decoherence only produce quantum collapse...