Trying to understand Quantum Eraser

[Widdekind]

In the Quantum 'eraser' experiment, Quantum-Entangled photons are created; and, then, manipulations applied to one such Entangled photon, evoke "sympathetic metamorphoses", in its Entangled partner. And, these "sympathetic responses" are manifested "Quantum quickly", apparently instantaneously, in zero measured time:

 

Because pairs of photons are entangled, giving one a diagonal polarization (rotating its plane of vibration 45 degrees) will cause a complementary polarization of its entangled pair member. So from this point on, the photons heading down [the other path] toward the double slits will meet the two circular polarizers after having been rotated.

Is the following an accurate picture of the physical phenomena ? In this writer's understanding, all "Quantum quick" -- to wit, instantaneous & non-local -- phenomena occur completely "intra-Wave-Function", within one single WF, through the "tenuous tails" of probability, that always connect the "pieces of probability", of a spatially sundered WF. If so, then the BBO crystal, utilized in the QEE, would seemingly create two mutually co-entangled "half-and-halved" photons, per the proceeding picture:

 

Quantum "Detection", would involve a simultaneous "active measurement", of one "half-photon-cum-full-photon" (which WF "collapsed" into the detector), with a co-occurring "null anti-measurement", of the other "half-photon-now-not-here-and-so-there" photon (whose WF "collapsed" out-and-away-from the detector, i.e., into the "not-here-in-the-detector" state), as this writer currently comprehends the concept (simultaneous "Measurement", of one, and "Null Measurement", of other):

 

Are these pictures appropriate? And, if manipulations applied to one Entangled "photon packet", instantly & non-locally communicate complimentary responses, in the other Entangled "photon packet", then, could you create "steerable lasers", by causing the course of the one to "veer left", and so inducing the other to "veer right", and so on?? (To wit, could you create, if we would allow, "wire-guided, gyro-jet" lasers ??)

[electrician]

The theory of quantum mechanics reveals a fundamental weirdness in the way the world works.atoms and other particles travel around like well-behaved billiard balls on the green baize of reality.

Some more assumptions related to this are: It's not unusual to fumble when trying to recall one's—especially in the absence of integrated electronic health records (EHRs). But those cumbersome forms and recitations help doctors to predict patients' risks for disease later in life, especially for partially heritable family medical history afflictions, such as breast or colorectal cancers.

[imatfaal]

Half-photon? No - spontaeneous parametric down conversion takes a stream of photon and every so often one photon is down converted into two entangled photons (lower energy) . In this case the polarisations are orthogonal. We call one the signal and the other the idler.

 

I don't know where you got sympathetic metamorphosis from - it's not in the wiki article so putting it in quotes is perhaps disingenous. The wiki article isn't great - perhaps you could have look at this explanation that was linked into it http://grad.physics.sunysb.edu/~amarch/ You need to pay particular attention to the way in which the polarization is used via experimental co-incidence counting rather than remote manipulation that the wiki article implies

[keithlucas1947]

I am not a physicist and have met quantum mechanics only as part of an engineering course - but I do have training in field theory and a PhD in maths.

I am familiar with the solution of Maxwells equations in specific cases including circular polarisation.

 

I am trying to understand the Quantum Eraser.

My first question is to understand where classical field theory departs from Quantum theory.

It seems to me that Maxwells equations (viewing light only as a wave) would predict interference fringes in every case for the dual slit experiment.

The Quarter Wave Plates would introduce equal and opposite orthogonal vectors which would cancel out at the target, leaving normal interference fringes. Is that correct?

 

Does the Quantum Eraser indicate that Maxwells equations break down in the case of single photons?

If so - what are the limits of the validity of Maxwells equations?

[imatfaal]

The heavyweights may wish to correct me, but I think the interesting parts of this series of experiments is

• that wave-like behaviour continues to occur at a particle level (ie diffraction with individual photons),
• that if one enforces classical behaviour (ie by ascertaining which slit - quarter wave polarisers) the wave like qualities disappear,
• whilst still ascertaining which slit - if you erase that information (ie the polariser on the idler photon after spontaneous down conversion) then the fringes reappear
• the timing of the erasion of the information 'seems' to violate causality - the light path of the idler to the eraser device can be significantly longer than the path of the singal to the fringe detector

I found the original papers made much more sense than the wikipedia pages:

 

Quantum eraser Walborn

 

Delayed Choice Kim

[Widdekind]

Please ponder (neutral-)pion decay, [math]\pi^0 \rightarrow \gamma + \gamma[/math]. The resulting pair of photons are quantum-Entangled, from the spontaneous pion decay and coupled creation of the photon pair, up until the first quantum-Measurement performed upon the pair (Ford. 101 Quantum Questions). By symmetry, each photon of the pair should (1) do "everything it can", i.e., "go left, go right; spin left, spin right"; and (2) do "everything opposite what the other guy does", i.e., "I go left, when he goes right; I spin left, when he spins right, etc." The latter (2) ensures conservation of (zero) total linear & angular momenta (zero, both before, and after, the spontaneous pion decay).

 

Now, the full, quantum-Entangled, Wave-Function super-poses such "equal-but-opposite" states, with a minus-sign, not a plus-sign. And, that anti-phase addition is quantum-physically important (Ford. 101 Quantum Questions). Cannot one visualize that minus sign, as essentially saying, that the super-posed photon states, are 180-degrees out of phase, w.r.t. the vector-potentials [math]\vec{A}_{1,2}[/math] that generate those photons ? (Recall, the direction of [math]\vec{A}[/math] defines a photon's direction of polarization; and, that vector-potential generates both [math]\vec{E} = -\frac{\partial \vec{A}}{\partial t}[/math] and [math]\vec{B} = \nabla \times \vec{A}[/math].) If so, then (neutral-)pion decay generates two equal-but-opposite photons, broken down into four co-coupled quarters-of-Wave-Functions, two of which are pro-phase ([math]\vec{A}[/math] "zigs then zags"), and the other two of which are anti-phase ([math]\vec{A}[/math] "zags then zigs"). If so, then, making a monetary analogy, (1) each photon "dollar" is generated in four "quarters", two of which go left (and two go right), and two of which spin left (and two spin right), with two "zigging & zagging" and the other two "zagging & zigging", so that each "dollar" has zero net linear, and zero net angular, momentum, and zero net vector potential (zero net "zig plus zag", as it were); (2) each photon "quarter" is cross-coupled, to anti-corresponding "quarters", of the other photon:

 

fig. 1 --

the first photon is symbolized by a F-16; the second photon is symbolized by a Sukoi-35; direction of travel is directly indicated, and denoted with capital letters (L,R); direction of spin is indicated by upright, and inverted, flight, and denoted with lower-case letters (l,r).

Edited August 8, 2011 by Widdekind

[IM Egdall]

My first question is to understand where classical field theory departs from Quantum theory.

It seems to me that Maxwells equations (viewing light only as a wave) would predict interference fringes in every case for the dual slit experiment.

 

As I understand it, when we have lots of light, we get the interference pattern predicted by Maxwell. But what happens when we "dim" the light source. We find that if we dim it enough, we get single points of energy recorded one at a time on the detector screen. This is most certainly not predicted by Maxwell's theory. And over time, these single events bulld up to produce the same interference pattern produced when we had lots of light.

 

Take a look at the simulation: http://phet.colorado...ve-interference

 

Now set the simulation to "double-slits",

the screen to "hits"

the gun control to the left

and fire the laser

 

What you see is the screen filling slowly with points (photons). And there is no way to predict where an individual photon shows up on the screen. This is the probabilistic nature of light as predicted by quantum mechanics. Again, over time, we see the interference pattern.

 

So it seems that an individual photon goes through both slits and interferes with itself! Quantum field theory says that the "photon" travels like a wave (the wave function), passes through both slits, interferes, but is detected only locally as a single point of energy.

 

Again Maxwell's equations have no explanation for this behavior.

Edited August 8, 2011 by I ME