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.

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  1. Kinker

    B Does decoherence prevent 100% of quantum tunneling of macro object?

    Isn't the quantum tunneling probability of macroscopic objects always zero due to quantum decoherence? It may be possible in the microscopic world, but I always think it is impossible in the macroscopic world due to countless interactions. Isn't this the same in a universe with infinite time...
  2. B

    Decoherence vs Observer Wave Function Collapse & MWI: Questions

    I have a couple of questions about decoherence vs. observer wave function collapse and multi-worlds interpretation. Am I right that Zeh's decoherence theory does not involve an observer, and esp not a conscious observer? Also, am I right that his theory does not involve or align with the...
  3. shunyadragonvv

    I Quantum decoherence and the emergence of space/time and gravity

    Quantum decoherence. and the emergence of continuous space/time and gravity In another forum I have experienced a lot of combative dialogue asserting that continuous time/space is a property of the smallest Quantum scale. My present knowledge indicates this not true, and that the goal of the...
  4. LittleSchwinger

    A Decoherence in the Heisenberg Picture

    When I'm teaching Advanced QM, I like to include how to describe some processes in the Heisenberg picture (e.g. double slit) so that a student's thinking isn't overly attached to the "dynamics of the quantum state", but they can also understand effects involving operator evolution. This is a...
  5. Talisman

    I What does decoherence have to do with phases?

    Here's the simplest example of decoherence I can think of. (I will drop normalizing factors for ease of typing.) Start with a state ##|\psi\rangle = |0\rangle + |1\rangle##. Measure it in the basis: $$|+\rangle = |0\rangle + |1\rangle, |-\rangle = |0\rangle - |1\rangle$$ It will always...
  6. H

    A Is decoherence necessary to observe a quantum event?

    Hi Pfs, I wonder if in quantum mechanics the question "Did this event occur?" has a yes or no answer like in classical phyics or in our daily life. We are accustomed to consider linear superpositions of states like "dead" and "alive" but id "dead" is a state the death of the cat is an event. i...
  7. D

    I Decoherence and Quantum tunnelling

    A macroscopic object has the order of Avogadro’s number of particles. That’s over 10^23. So the probability of all of them tunneling, at the same time, is on the order of that original small probability, to the 10^23 power. And then on top of that, you have to factor in the chances of it...
  8. B

    B Entangled particle decoherence question

    Suppose there are two entangled particles A and B, separated by a few miles or light years. If the spin for particle A is observed, then from my understanding, A will experience quantum decoherence. And now we can be sure that B will have the opposite spin since B is entangled with A. Assuming...
  9. Question69

    The Emergence of Classicality -- Is Decoherence accepted among physicists?

    I have tried to study decoherence theory for a while now, and it does seem to answer why classical distributions arise, instead of quantum ones.Is this fully accepted as an explanation for a classical world out of a quantum one?
  10. M

    I What is the spin-boson model for decoherence in a Penning trap?

    Hello! I read a bit about decoherence lately (I made a post few weeks ago about it and got some reading suggestions) and I would like to try to apply it to a practical situation I need, which is a Penning trap with a single ion at the center. For now I would like to account just for the...
  11. Question69

    B Quantum Decoherence: What Happens to Other Outcomes?

    What happens to the other possible outcomes after decoherence has been finalized? Where do they go?
  12. M

    I How should I think about decoherence?

    Hello! Assume we have a 2 level system that is in a superposition of its 2 states: $$\psi = \frac{|0>+|1>}{\sqrt(2)}$$ If we put it in a chamber at a given temperature (say that the chamber has perfect vacuum) the system will lose its coherence due to the blackbody radiation from the chamber...
  13. entropy1

    I The splitting into worlds in MWI

    If we have binary decoherence at spacetime location X, as far as I understand, we almost instantly have a splitting into two worlds. Now if we have a different location Y, about a lightyear apart from X, then the decoherence hasn't reached Y before a year has passed. So for Y, the split hasn't...
  14. J

    B Can Entangled Particles Stay Undetermined Despite Decoherence During Separation?

    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?
  15. H

    A Is there still a collapse problem after decoherence?

    Hi Pf, I take the case of Alice and Bob in the EPR experiment. Here Bob does not measure spin projections but make a Young's double slit experiment. What Alice does on her side will not be known by Bob. She can decide to let her particles go freely to the left (in the environment of Bob). Bob...
  16. entropy1

    I Decoherence and direction of causality

    If I use the expression ##(|A\rangle+|B\rangle)|M\rangle \rightarrow |A\rangle|M_A\rangle+|B\rangle|M_B\rangle## for decoherence, does that mean that we can infer that, IF the measured value is ##A## that THEN we will measure ##M_A##, OR that IF we measure ##M_A## THEN the measured value must...
  17. O

    I Observing a double-slit experiment

    Do I have the proper understanding of the following three double-slit experiment situations? #1 While a standard double-slit experiment is run via a Mach-Zehnder interferometer apparatus that completely lacks which-way detectors, can an ordinary human experimenter be present and watch that...
  18. Haorong Wu

    I Does decoherence mean the loss of entanglement?

    Hello, there. I am studying a model for decoherence of two entangled photons. The space for the first photon is 2 dimensional, while that for the other one is 6 dimensional. In total, the system will be in a 12 dimensional space. Initially, they are set to one of the Bell states, such as...
  19. M

    I Questions about Adler paper "Why Decoherence has not Solved ...."

    On page 8, I don't understand the last paragraph. It says if ##|A \rangle = U|0\rangle ## and ##|B \rangle = U|0\rangle ## then ##\langle A | B \rangle = 1##. Of course ##U |0\rangle## is a unique state (##A## and ##B## are the same by this definition). So I assumed what he meant was more...
  20. M

    I Decoherence references and a question about the measurement problem

    Couple questions here. 1. What do you think is the best source to read about decoherence, both from an intuitive/high level pov, but also with mathematical details. I'd partially read a recommended paper by Schlosshauer a while back, is that still the bible for this? 2. If probabilities in...
  21. Ali Beladi

    I What does this tricky quantum mechanics equation mean?

    I'm a current high school student and I’m aspiring to become a biochemist. I’m at the moment writing an article about adaptive mutations but there is a lot of tricky quantum mechanics in it which I simply don't get. I have asked everyone and got no answer until someone recommended to ask it in a...
  22. Jamister

    I How is the Born rule deduced from decoherence?

    Weinberg says in his book "Lectures on Quantum Mechanics" that the born rule can be deduced from decoherence, and this solves the measurement problem. I'm looking for reference, book or article, to understand this better. Thank you
  23. entropy1

    B What causes loss of quantum coherence?

    Generally, what physical process causes loss of quantum coherence? Is it known?
  24. allisrelative

    I Does Decoherence get rid of all Quantumness?

    The answer is no and even when decoherence occurs for Wigner's Friend in the lab, quantum coherence remains. Let's start with the paper that illustrates this. Assisted Macroscopic Quantumness CONT. https://arxiv.org/abs/1711.10498 Wow, I recently read this paper and the results are simply...
  25. Q

    B Is sunlight through gaps in leaves during an eclipse decohered?

    Is the reason you can make out the shape of the sun and the moon is because the gaps between the leaves cause the light to be photons instead of waves?
  26. entropy1

    I Can we accurately determine the trajectory of an electron using a SG detector?

    Would this be an accurate portrayal of measuring the spin of an electron with a SG detector?: The electron is in a superposition of spin-up and spin-down; Upon entering the magnetic field of the SG detector, the electron enters a superposition of an upward trajectory and a downward trajectory...
  27. A. Neumaier

    I New review on decoherence by Schlosshauer

    I'd like to draw attention to the recent survey M. Schlosshauer, Quantum decoherence, Physics Reports (2019). Available online 18 October 2019
  28. Danny Boy

    A Correlation functions of quantum Ising model

    I have a single technical question regarding a statement on page 7 of the paper "Dynamical quantum correlations of Ising models on an arbitrary lattice and their resilience to decoherence". The paper up until page 7 defines a general correlation function ##\mathcal{G}## of a basic quantum Ising...
  29. P

    A Can decoherence be associated with heat generation?

    This question is inspired by a comment that @thephystudent made where he said that "The dephasing between the Bragg pulses is not unitary, I believe it can be explicitly written in Lindblad form and generates heat. I believe this Point of view is the same as (among others) the papers of...
  30. J

    I Copenhagen and Decoherence

    Something puzzling and inconsistent. Decoherence was supposed to remove the need for observers in Copenhagen. But how come Lee Smolin didn't discuss anything about decoherence with regards to the Copenhagenists but only the Everettians in his book "Einstein Unfinished Revolution: The Search for...
  31. A

    A How Does Decoherence Affect a Single Two-Level System in Hilbert Space?

    My question is about decoherence, specifically in regards to its interpretation in Hilbert space. In every single textbook I've read, decoherence has always been explained by (1) introduce density matrix and (2) explain that interactions with environment cause off-diagonal terms (coherences)...
  32. Auto-Didact

    A Ballentine: Decoherence doesn't resolve the measurement problem

    This thread is a direct shoot-off from this post in the Insights thread Against "interpretation" - Comments. I am usually not a big fan of Ballentine, but I tend to fully agree with him on the following issue (taken from this paper, credits to @bhobba) Decoherence theory is a pragmatic...
  33. Machine1701

    I Why doesn't decoherence apply to unobserved experiments?

    This is based on the Hugh Everetts interpretation of quantum mechanics, where the waveform never collapses but the observer becomes a part of it, thus experiencing decoherence of the wave form from their point of view. So, let's say someone is doing the two-slit experiment. The particles going...
  34. J

    I Does "Decoherence" really explain anything?

    I just read an excerpt from Philip Ball's new book and was disappointed by its hyperbolic, boosterish tone. He adopts the pose that physicists have made great strides in understanding what happens during measurement. Wave functions decohere, etc. He concludes by saying something like "now...
  35. nomadreid

    I Decoherence: is "other states lost to the environment" part?

    In Wikipedia's outline of the Many Worlds Interpretation of quantum physics https://en.wikipedia.org/wiki/Many-worlds_interpretation, it states "In many-worlds, the subjective appearance of wavefunction collapse is explained by the mechanism of quantum decoherence,...", yet I thought decoherence...
  36. F

    I Decorrelate 2 particles initially entangled

    Hello, I am studying the entanglement in quantum physics, especially the Aspect's experience where 2 photons initially correlated keep this correlation through the measure of their polarity. The Wikipedia article is only available in French, but there is an article in English written by Alain...
  37. Tspirit

    How Does One Derive Equation (8.27) from (8.26) in Gerry's Quantum Optics Text?

    Homework Statement In Gerry's book "Introductory quantum optics", Chapter 8.5 Decoherence, Page 207, there is a calculation which doesn't give the detail derivation, who can give me the calculation from (8.26) to (8.27)? Homework Equations The coherent state is defined by...
  38. T

    B What, in essence, distinguishes light from matter?

    Before quantum mechanics, light was generally seen as a wave and matter as particles (biliards). From e.g. the discovery of the photoelectric effect, one saw that light can also be seen as a particle. From e.g. the double slit experiment, one makes the interpretation that matter can also be seen...
  39. Johny Boy

    A Decoherence of measurement outcomes

    Suppose we have a quantum system ##Q## with an initial state ##\rho^{(Q)}##. The measurement process will involve two additional quantum systems: an apparatus system ##A## and an environment system ##E##, hence giving the initial state of the system ##\rho^{(AEQ)} = \rho_{0}^{(AE)}\otimes...
  40. A

    A Decoherence in the double slit experiment

    I have two questions about the following type of scenario: We have a laser sending photons through the usual double slit apparatus giving us the usual interference pattern, except that now we introduce some physical matter (that are not photons) that the photons will interact with before going...
  41. P

    B Psi-ontic view, macro objects and decoherence

    Good evening! I know, that the questions similar to mine were frequently asked in this forum. Recently I even saw the thread about macro-realism and was satisfied by the answer to it. However, one article made me try to find some clarification. The link to the article...
  42. E

    I Decoherence Pure States Into Mixed States

    According to decoherance. Say there is a pure state initially in state: |ψ⟩=α|0⟩+β|1⟩ After decoherance (interaction with environment), the system will transform into the improper mixed state of: ρ=|α|2|0⟩⟨0|+|β|2|1⟩⟨1| This is the "apparent" collapse that decoherance refers to. With the...
  43. J

    B How decoherence destroys superpositions

    If the particle is undisturbed in the two slit experiment, an interference pattern will show up. The weird quantum behavior here is that the particle can have "negative probabilities" associated with its position and momentum, thus they can cancel each other and form the pattern we see. When...
  44. N

    B Understanding Decoherence: Exploring Bohr and Einstein's Debates

    Hi, I'm in ninth grade, and I can't really grasp an idea of how decoherence works. Also, does anyone know where I can good information on what was discussed in the debates between Bohr and Einstein. It seems really interesting.
  45. J

    I Do we still experience quantum effects in decohered systems?

    As I understand it, decoherence is when the quantum system becomes entangeld with the measuring quantum system. If most of the macroscopic systems we experience are all decohered, then why do we still see quantum effects like in a transistor? Is it position AND momentum that decohere?
  46. Binax011

    I What is the speed of decoherence and its predictions for coherent systems?

    Does anyone know or know of the general predictions of the speed at which of coherent system losses coherence to contact with a decoherence system? Is that speed limit faster than the cosmic speed limit?
  47. Auto-Didact

    A Experiments probing the macroscopic limits of QM

    This thread is to serve as both a compilation and ground of discussion of key experiments, both historical and planned, which attempt to probe possible macroscopic limits of QM, taking into account e.g. some particular gravitational/optical/mechanical/superconducting/etc aspect and/or...
  48. Auto-Didact

    A Weinberg: detecting changes to QM using atomic clocks

    Steven Weinberg has lately been critical of QM. He now also has a technical paper out called 'Lindblad Decoherence in Atomic Clocks', available on arxiv. Here is the abstract: It's a short paper (6 pgs of text), arguing for objective collapse (a la GRW/Diosi-Penrose/etc) instead of...
  49. O

    B Marble Ball as a Subsystem in Quantum Decoherence

    Can a marble ball be considered as a subsystem of an entangled system where it is entangled with the environment? Or is "subsystem" in entanglement/docoherence only reserved for small quantum thing like electrons or photons?
  50. F

    I Decoherence and standard formalism

    In the standard mathematical formalism, the environment were treated classically, this is because observers (being macroscopic recording mechanisms) are treated classically, so the system is isolated. Decoherence is about open system, so how is decoherence compatible with Copenhagen or the...