Every-day electromagnetic radiation, such as radio and TV waves, is also an example of near coherent states (macroscopic quantum coherence). That should “give one pause” regarding the conventional demarcation between quantum and classical.

— Wikipedia on Coherent states

2011.10.09 Sunday ACHK

Coherent states 3

In classical optics light is thought of as electromagnetic waves radiating from a source. Often, coherent laser light is thought of as light that is emitted by many such sources that are in phase. Actually, the picture of one photon being in-phase with another is not valid in quantum theory.

Laser radiation is produced in a resonant cavity where the resonant frequency of the cavity is the same as the frequency associated with the atomic transitions providing energy flow into the field. As energy in the resonant mode builds up, the probability for stimulated emission, in that mode only, increases. That is a positive feedback loop in which the amplitude in the resonant mode increases exponentially until some non-linear effects limit it.

As a counter-example, a light bulb radiates light into a continuum of modes, and there is nothing that selects any one mode over the other. The emission process is highly random in space and time (see thermal light). In a laser, however, light is emitted into a resonant mode, and that mode is highly coherent. Thus, laser light is idealized as a coherent state.

— Wikipedia on Coherent states

2011.10.07 Friday ACHK

Quantum coherence, 4

O’Connell and his colleagues coupled together a mechanical resonator, similar to a tiny springboard, and a qubit, a device that can be in a superposition of two quantum states at the same time. They were able to make the resonator vibrate a small amount and a large amount simultaneously — an effect which would be impossible in classical physics. The mechanical resonator was just large enough to see with the naked eye — about as long as the width of a human hair.

— Wikipedia on Quantum machine

2011.10.04 Tuesday ACHK

Large-scale (macroscopic) quantum coherence leads to novel phenomena. For instance, the laser, superconductivity, and superfluidity are examples of highly coherent quantum systems, whose effects are evident at the macroscopic scale. These examples of quantum coherence are Bose–Einstein condensates. Here, all the particles that make up the condensate are in-phase; they are thus necessarily all described by a single quantum wavefunction.

On the other hand, the Schrodinger’s cat thought experiment, highlights the fact that quantum coherence is not typically seen at the macroscopic scale but has been observed in the motion of a mechanical resonator (see Quantum machine).

— Wikipedia on Quantum coherence

2011.10.02 Sunday ACHK