What is Quantum coherence: Definition and 26 Discussions
In physics, two wave sources are coherent if their frequency and waveform are identical. Coherence is an ideal property of waves that enables stationary (i.e. temporally and spatially constant) interference. It contains several distinct concepts, which are limiting cases that never quite occur in reality but allow an understanding of the physics of waves, and has become a very important concept in quantum physics. More generally, coherence describes all properties of the correlation between physical quantities of a single wave, or between several waves or wave packets.
Interference is the addition, in the mathematical sense, of wave functions. A single wave can interfere with itself, but this is still an addition of two waves (see Young's slits experiment). Constructive or destructive interferences are limit cases, and two waves always interfere, even if the result of the addition is complicated or not remarkable. When interfering, two waves can add together to create a wave of greater amplitude than either one (constructive interference) or subtract from each other to create a wave of lesser amplitude than either one (destructive interference), depending on their relative phase. Two waves are said to be coherent if they have a constant relative phase. The amount of coherence can readily be measured by the interference visibility, which looks at the size of the interference fringes relative to the input waves (as the phase offset is varied); a precise mathematical definition of the degree of coherence is given by means of correlation functions.
Spatial coherence describes the correlation (or predictable relationship) between waves at different points in space, either lateral or longitudinal. Temporal coherence describes the correlation between waves observed at different moments in time. Both are observed in the Michelson–Morley experiment and Young's interference experiment. Once the fringes are obtained in the Michelson interferometer, when one of the mirrors is moved away gradually from the beam-splitter, the time for the beam to travel increases and the fringes become dull and finally disappear, showing temporal coherence. Similarly, in a double-slit experiment, if the space between the two slits is increased, the coherence dies gradually and finally the fringes disappear, showing spatial coherence. In both cases, the fringe amplitude slowly disappears, as the path difference increases past the coherence length.
I have some doubts regarding my personal interpretation that i was contemplating about in the context of Wigner's friend experiment (also tested in the laboratory recently). Could it be that a system is always in a superposition, and when we perform a measurement, we obtain a definite value due...
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...
I am so new to circuit quantum electrodynamics. As far as I know, there are few things I could manipulate, like resonator, qubit resonance frequencies, Hamiltonian, coupling strength, Hilbert-space cutoff, dissipation rate, but they do not make sense to me and I do not how they can relate to my...
What is it of the photon that gets polarized from a quantum mechanical perspective? In the classical perspective it is often thought that it is the oscillating electric field that gets polarized. But in the quantum case: Is it the de Broglie wave function? Or is it the spin and in case it is the...
How we should understand the randomness of quantum events in the context of the significant role that they apparently play in our macroscopic world. Using processes as superconductivity, super-fluidity, and in Bose-Einstein Condensates researchers have been able to produce macroscopic quantum...
I have recently been reading some stuff on quantum information in the physics literature which refers to 'a mechanism by which a measurement in A determines quantum coherences in B', where A and B are subsystems of a larger system.
I am aware of the meaning of the terms 'decoherence' and...
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...
Over what range is the Pauli Exclusion Principle important? As a chemist I take it as an atom, but that seems an arbitrary stipulation. For instance, in metals how far apart must 2 electrons be to have the same 4 quantum numbers? I have seen that it has to do more with quantum coherence field...
The following, regarding quantum measurement, is stated in the paper "Limitation on the amount of accessible information in a quantum channel" :
"Our discussion of measurement will be based on a specific physical model of measurement, to which we now turn. Suppose we have a quantum system ##Q##...
Hi to all who might respond,
Consider the 'peculiar' double-slit setup below.
There is a double-slit configuration such that the two slits are never open at the same time. That is: whenever the top slit is open for a certain interval, Ts, the bottom slit is closed for the same interval and vice...
If I measured the velocity of an electron what happens to its position? Does it collapse into a specific position or is it still uncertain?
Here's a thought experiment... we measure the velocity of an electron and then send it through the double slit at the same time. Does the electron still...
Hi,
I am learning quantum entanglement. I am interested to create an up to date list of all known :
- Photon Quantum States
- Particle Quantum States
- Classically entagled photon states
I guess that there is an organization out there that already have this info.
If someone can point me into...
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?
Hi guys,
as it has been said many times on the forum and outside of it the ability to achieve and maintain coherence is the biggest reason why macroscopic superpositions cannot be measured. The typical examples of macroscopic coherence are superconductors where atoms all behave in a similar...
Am I reading some more recent articles on quantum gravity correctly that seem to be implying that a coherent particle is not subject to gravity?
If you have a coherent C70 buckyball molecule (see Zeilinger - Update: "Matter-wave interferometer for large molecules", Jan. 2002), the force of...
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...
Hey people!
I'm studying the article "The Quantum Theory of Optical Coherence" by Glauber (see eg. http://prola.aps.org/pdf/PR/v130/i6/p2529_1), and I've a couple of questions.
1. Why are the arguments of E(+) and E(-) in equation 3.6 two different sets of coordinates? We obtained E(-) by...
I am presently in a Biophysics lab, and am formulating a project wrt transmembrane protein association in lipid bilayers (based on the 4 stage model of membrane protein folding, i.e. partitioning, folding, insertion, association).
I was wondering if anyone on here has heard of any decent...
It is promised that the quantum coherence will help to improve various measurements.
Is it theoretically possible to improve detection of neutrinos with help of quantum coherence?
There seems to be some debate as to whether laser light is an example of quantum coherence (as posted in this thread: link). I think it deserves its own discussion. I'm no expert in this field, so I mainly go by what I read; I found this interesting Wikipedia reference here:
Comments?
I'm not a physicist, and am trying to better understand quantum coherence. I understand that it's defined by the fact that the entire quantum system can be defined by a single wave function. My question is whether, due to wave-particle duality on the quantum level, it is equally valid to speak...
I have a question about quantum coherence phenomena. I understand (basically) how laser light results from light waves that are in sync. I also see how ferromagnetism results from orderly alignment of electron spin amongst iron atoms (though I've posted in another thread asking whether this is a...
Has anyone performed experiments regarding macroscopic quantum coherence (superposition of and macroscopic object)?
What about testing if macroscopic objects obey macrorealism, or whether QM prevails?
Links to articles or experiments would be apprecited.
Hey folks. I'm giving a poster at a conference in a month or so and I'm supposed to explain my research (optimal control of quantum systems in dissipative environments) to people from many other disciplines (psychology, biology, etc). I'd like to hear any tips those on this board have on how to...
Hi,
I just passed a course of a quantum coherence theory (or quantum optics). However, during the course i found more questions rather than answers.
One particular:
Imagine there's a white light coming from the sun. What is the coherence length of this light, according to the quantum...
Pascal principle with light pressure
Hello,
Supose you have an U tube, in which the inner walls are made absolutelly reflective. Now, put a termical state (temperature T) for the radiation field inside the tube and close it with reflective discs which can slide without friction. Supose...