al onestone

Sorry for the delayed response but they moved my thread from physics to speculations, and I didn't even know this section existed or I would have originally posted here. Quantropy, I do know exactly why conventional ideas in FTL don't work, and its for the reason that no-signalling theorem states, operations on Bob's system do not disturb the partial state operator of Alice's system (assuming Alice and Bob have members of a downconverted pair). But these are "conventional" ideas. I looked at the link you gave (from 2004) and it does give a relevant thread where a similar idea is presented, but that idea was just a response from someone who had recently read about the delayed choice quantum eraser. The real original paper concerning downconverted pairs and the possibility of FTL is the ZWM paper,(http://quantmag.ppole.ru/Articles/experiment/Mandel(1991).pdf) I highly suggest it. In the ZWM paper the authors show how interference between signals (second order interference) is collected even though the signals are entangled with the idlers that are being collected coincidentally. The ZWM is not capable of FTL because the idler from one downconversion is transmitted through the other downconversion so that the phases are locked. This is how they get coherence and thus interference, as well as for the reason that the two crystals are commonly pumped by one source. This is required for coherence because downconversion is spontaneous and highly improbable. If the pump goes to the one crystal and downconverts, you can't assume that if it had gone to the other crystal it would have downconverted there as well, its one in a billion atleast. What I propose is to separate the crystals all together, and pump them separately. So how do you get coherence between separate emissions of signal photons? The same way you would get coherence between any two seperate sources of photons, the way they did it back in the 60's (see Pfleegor & Mandel 1967,1968), by simply monitoring the relative phase between the two emissions and postselecting the observations that are in phase. For a detailed explaination I suggest taking a closer look at my thought experiment at http://modifiedzwm.webs.com

In the ZWM study of 1991 Mandel et al get coherence between the signal photons of two distinct downconversion sources. This is shown with an interference collection between the two possible sources of signal photons (even though they are entangled with the idler). This is achievable because the two downconversions are pumped by a common source which is beamsplit, and because the idler from one downconversion crystal is transmitted through the other downconversion crystal and superimposed with the second idler (made indistinguishable). Thus the name of his paper "Induced coherence without induced emission". Is it possible to get coherence between two distinct sources of signals (without the transmission and superimposing of the two idlers) if you simply add the two idlers at a beamsplitter to make them indistinguishable (which negates the which-path information)? You would have to look for interference between the two signals in the same way as you would look for interference between any two separate indistinguishable sources of photons (Hull 1949, Pfleegor & Mandel 1967). You can "postselect" the observations that are in phase or you can look for temporal interference between two detectors. Note this is dis-similar to the DCQE. A more thorough explaination of this thought experiment is at http://modifiedzwm.webs.com/

The optical two-source interference effect due to Hull (1949) has largely been overlooked by the history of quantum mechanics. Mandel and Fleegor brushed up the effect by reducing the signal strength of the sources and reducing exposure times for interference collection. In the end this effect was only looked at for its theoretical insights concerning the photon/EM wave. But is there more to this effect? Can it be used for other purposes like spin/polarization recombination of photons? The question is about the use of two-sources for optical effects other than simple indistinguishability interference. Is it possible to produce a coherent state by combining (with a beamsplitter) two sources which have distinguishable states, lets say one horizontally polarized (H) and one vertically polarized (V)? Obviously you cannot get interference from such a combination but one might still get a small portion of the output which is in a coherent state, H + V. Only the portion of the two beams that are "in phase" would project onto a coherent state of 45degree polarization, much like in the conventional two-source interference effects where the "in phase" portion is postselected with an appropriate method. Of course the method of postselection would have to be different but it is possible to filter out the coherent portion of the output if there is a coherent portion. I'm wondering if anyone in the literature has actually tried this type of thing before, or if there is some form of evidence as to why it would not be possible, experimental or theoretical. The basic thought experiment I'm proposing is shown in the attached figure. In this setup only the 45deg polarized portion in a coherent state would produce a double slit interference pattern. It suffices to say that there are other methods to produce an effect if indeed there is a coherent projection onto the 45deg state (H + V) at the beamsplitter.

This does not work. The second paper clarifies the issue. The counting rate at Di does not display interference because it is not a function of phase. For whatever reason, after the math only Ds displays interference.

In the ZWM study of early 1991, Mandel (U of Rochester) achieves "induced coherence withouyt induced emmission" between two optically pumped downconversion crystals (figure provided). The explaination of the experiment goes like this: The pump beam goes to the 50/50 beamsplitter (BS) and then pumps one of two downconversion crystals (DC1 and DC2) which transform the photon into two photons (signal and idler) with each having roughly half the frequency/energy. The idler from DC1 is forced to transmit through the DC2 crystal so that it retains coherence between the output of the two crystals. The two signal outputs from DC1 and DC2 are combined at a detector Ds . There are two types of interference here, first is the interference in the coincidence counter (CC). When one of the optical paths is varied there is interference as a function of the combined phases (φs + φi). The coincidence interference results from indistinguishability between the combined pathways from DC1 to Ds and Di -or- from DC2 to Ds and Di . We cannot tell which way the pump photon went after the beam splitter. The second interference occurs at detector Ds and is a simple function of the phase. The interference that occurs at this detector is a lower order. It is simply the indistinguishability between the optical paths from the pump photon to detector Ds . If the detector has a count you cannot tell if it came from DC1 or DC2 depending on which path it took after the beam splitter. The coincidence interference(4th order) requires that all four optical paths to the two detectors be identical (or at least differing by no more than a coherence length). The interference at Ds (2nd order) requires the two optical paths from BS to DC1 to Ds and from BS to DC2 to Ds be identical. By placing the neutral density filter (NDF) between DC1 and DC2 in the idler path i1 , the interference at detector Ds vanishes. Why? Because the paths to Ds from the pump are now distinguishable. How? If the pump photon goes to DC1 and downconverts then the signal will go to Ds but the idler will be attenuated by the filter and you will not get a coincidence count at Di . If the pump photon goes to DC2 and downconverts then the signal will go to Ds and the idler will go to Di and there will be a coincidence count. So the presence or non-presence of a coincidence count at Di is the measurement which will determine whether the pump photon went to DC1 or DC2 to downconvert. So by inserting the NDF into the idler path, Mandel has eliminated the interference at detector Ds . And it is worth noting that the interference effect at Ds can only be negated by a distinguishability measurement which occurs at a second detector Di which is spatially separated from Ds , and that these measurements don’t actually need to be made to negate the interference. Mandel notes that these measurements at Di need only be in principle possible to negate the interference pattern. He got the same results without even making the distinguishing measurements. Absolutely Brilliant! My question is, what is happening at detector Di when there is no NDF placed in the idler path? Is there interference (2nd order) at this detector also? Mandel refferred to this detector as "superfluous" to the idler detector, so he gave no analysis of its counting rate. If there is interference at Di then one should be able to negate this effect by placing a NDF in the signal path s1 which could be done very close to the detector Ds. In this case we would be affecting what measurements are made at Di by performing a change in the preparation at Ds? Relevant papers of the study: X. Y. Zou, L. J. Wang, and L. Mandel, Phys. Rev. Lett. 67, 318 (1991). L. J. Wang, X. Y. Zou, and L. Mandel, Phys. Rev. A 44, 4614 (1991).

I'm trying to understand how a single photon is sifted out of a beam and given a unitary operation in so called "deterministic teleportation". In the teleportation scheme, we perform Bell State Measurement/Analysis on a pair of Alice's particles, of which one is already entangled to Bob's particle. This projects Bob's particle(photon) onto an unentangled pure state in the polarization basis (a qubit state). This particle then has a unitary operation performed on it to transform it to the original state of Alice's second particle. But in a real experimental situation, how does Bob figure out which exact particle to perform the operation on? and how does he separate it from the beam to perform the operation?

I'm trying to get to the bottom of things concerning the shaping of an optical pulse(change of state) with linear optics. Using linear optical apparatus we can shape the pulse of a photon in such methods as cavity dumping. You feed an optical cavity with near monochromatic(short bandwidth) input and allow no output. The light builds up and then you release all the light at once by opening up the cavity, and in the end you have an output with pulse length equal to the inner loop length of the cavity, even if the cavity loop is shorter than the coherence length of the input. So you can shorten the coherence length of the output which consequently widens the bandwidth. This is a change of state. In QM all changes of state can be modelled by a physically real coupling, aka the Von Neumann measurement scheme. My question is, what physically real coupling is taking place when the photon output has its change of state? You can create a mathematical model of the unitary operator which would act on the initial state description to produce the final output state, but what is the physically real meaning of the operation? Is there an answer?

OK, lets assume that we can trap photons in a cavity with the end result of shortening its coherence length/pulse shape. And a consequence of this is the widening of its bandwidth. Let's also assume that we can find a unitary operator which produces the desired effect of changing the state description in the appropriate manner. My first problem with this is that in QM we require that all changes of state can be modelled by a unitary operator in the Von Neumann measurement scheme. If our preparation reduces the pulse width, this is like reducing the uncertainty in position which effectively is a measurement. In Von Neumann measurement we always have a measurement apparatus that is being "coupled to" and this projects the system onto a new basis. In our scheme we reshape the pulse with linear optics apparatii where no photons are coupling to anything physically real. It seems as though photons escape the typical QM Von Neumann description of measurement. Anyway, assuming that our cavity can be idealized in a thought experiment as weve described so far, what if we use as a source an entangled pair of photon beams from downconversion. If we use polarization entangled beams and we subject one to a pulse reshaping preparation, which consequently shortens the pulse shape and widens the bandwidth, what effect does this have on the second entangled partner beam? Does the second beam have a similarly modified bandwidth? or does the second beam become disentangled from the first (as in teleportation, Bell state analysis)? Remember, not just the polarization is entangled in downconversion, the momentum/energy is entangled via wavevector conservation. So if the one beam's bandwidth is widened, then the energies of the second beam cant all match those of the first unless it is similarly modified? Please resolve.

In order to change the eigenvalue spectrum of the description, what would be the mathematical model of a unitary operator which represents a "beam reshaping" process?

Let me get this straight, you're saying that the photons which become trapped, photons with an already established energy bandwidth/uncertainty, have a reduced positional uncertainty and some how expand their bandwidth miraculously? I doubt it. In order for photons to enter a state with a larger bandwidth they have to be absorbed and re-emitted, which would make them differrent photons from the original ones. This is still not resolved.

I'm working on a thought experiment and I have a question about trapping a pulse inside a looped preparation . If a pulsed particle beam is prepared with a definite energy(E), bandwidth(ΔE) and positional uncertainty/coherence length(Δx) is the pulse able to be trapped inside of a loop type preparation(a cavity) if the total length of the loop/cavity is shorter than the positional uncertainty of the particles. Typical cavities in optics all have open input and output ports. I'm talking specifically about a loop which is enabled by a switch which opens and closes the loop. A simple example in optics would use a polarizing beam splitter as the entry point of the loop, only allowing in one polarization. Secondary rigid mirrors could be used to redirect the beam towards the back side of the beam splitter where the beam would be deflected because of its polarization. This would constitute a loop/cavity where the transmitted photons are trapped inside. In order to open the loop to release the photons you simply insert a wave plate which rotates the polarization and allows it to exit through the beam splitter. In total this would constitute a switched loop preparation which can trap photons and release them upon command of the experimenter. My question is, if the loop length is shorter than the uncertainty in position,Δx(the linewidth of the photon distribution) is it still possible for photons to become trapped inside or does the loop act as a forbidden region? If the photons are allowed to become trapped, is the pulse shape shortened to the loop length after release? This would violate Heisenberg uncertainty. If you think that they can become trapped and that it has no effect on the pulse shape, then you have to admit that the photons are still comming out of the loop at a time later than it would require light to traverse one length of the loop. This doesn't make sense. Either they don't become trapped or they do and they all exit within the amount of time needed to traverse one loop length. Please resolve.