A single photon can be in as many as four places at the same time

Vladan Vuletic, “Quantum physics: Entangled quartet,” News & Views, Nature 468: 384–385, 18 November 2010, summarizes the paper K. S. Choi, A. Goban, S. B. Papp, S. J. van Enk & H. J. Kimble, “Entanglement of spin waves among four quantum memories,” Nature 468: 412-416, 18 November 2010.

Particle-type versus wave-type measurements.

“Single photons can be stored in atomic gases. Choi et al. investigate what happens to interference when light is stored simultaneously in as many as four spatially distinct atomic clouds. The authors demonstrate quantum correlations (entanglement) in this composite matter–light system, and study how entanglement ultimately fades away to leave only classical correlations.

Choi et al. have measured quantum entanglement in a composite matter–light system by combining results from particle-type and wave-type measurements. In the particle-type set-up, a photon stored in one box can reach only one detector (D1, D2, D3 or D4). In the wave-type measurement, the photon is placed simultaneously in all four boxes and the light emerging from the boxes is combined through an arrangement of partially reflecting and totally reflecting mirrors such that light from any box can reach any detector.

In a classical world, something is either a particle or a wave, so a physical system will exhibit correlations either in the particle-type or wave-type detection set-up — but not in both. However, in the quantum world that we live in, it is possible to place, for example, a single photon simultaneously in all boxes such that correlations are observed in both detection set-ups. And this is exactly what Choi et al. have done in their experiment.

The authors measured correlations between the different boxes, either in the particle-type detection set-up or in the wave-type set-up. From the combination of these measurements, they extracted the degree of entanglement of the light shared between the four boxes. Using a method previously developed for a single photon travelling simultaneously along four possible paths, they identified quantitative criteria, involving combinations of particle-type and wave-type detection results, that allowed them to distinguish among entanglement between all four boxes, or three, or just two of them. In the presence of noise and other imperfections, they observed a gradual transition from four-party entanglement to no entanglement.”

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7 Responses to A single photon can be in as many as four places at the same time

  1. jerry bylander says:

    I don’t see how a force carrier can connect to two electrons in the way described.

  2. jerry bylander says:

    “a single photon can be at as many as four places at once.” OK, but if so how can it carry the strong force between two particles.

    • emulenews says:

      Jerry, do you mean electromagnetic force instead of strong force? Photons have a double role in physics: virtual photons are the carriers of electromagnetic force between charged particles and “real” photons are free particles corresponding to quantum degrees of freedom of electromagnetic waves.

  3. jerry bylander says:

    Thanks for the heads up. I have had various quantum mech courses since 1955 variously until the 1980’s and I was pleased to just now learn about virtual photons. Am I correct in understanding that exchange of virtual photons between electrons for example leads to repulsion due what used to be known as charge? If so how is the charge on an electron set to a specific value? Thanks.
    Since a virtual photon is unobservable except indirectly through the Casimir effect, is it essentially just a fudge factor? Or just a wiggle on a Feynman diagram? (? by an experimentalist)

    • emulenews says:

      Jerry, experimentally, it seems that electric charge is quantized, the electric charges of all the known elementary particles are integer multiples (to within experimental errors) of the d-quark charge. However, inside the standard model of elementary particles there is no explanation to this fact. Nor is there any explanation for the exact value of electric charge.

      Fudge factor? Wiggle? Maybe, but the process of production of a photon in QED is exactly the same for virtual photons and “real” ones. The only difference is that sometimes the photons violate some conservation laws (constrained by the Heisenberg’s uncertainty principle) so they are not observable (i.e. they are virtual photons) and other times they do not violate them so they are observable (i.e. “real” photons). There is not enough reason to consider that virtual and “real” photons are two different kind of particles. Everything works fine considering that both are exactly the same particle.

      Casimir effect is the result of nonlocal quantum effects in the zero-point electromagnetic energy of the vacuum due to the geometry and topology of the boundary conditions. Since QED is a local (Lorentz invarint) theory, the majority of phisicits uses virtual particles (violating Lorentz invariance) in the vacuum as the preferred explanation. However, (nonrelativistic) quantum mechanics shows subtle nonlocal effects which can be used to understand Casimir effect without virtual photons.

      Virtual particles are very difficult to understand conceptually, although the mathematics (f.ex. Feynman diagrams) is very clear and easy to apply in practical problems.

  4. jerry bylander says:

    thanks. I suspect I have raised questions only grad students voiced at the time…but I can see why some physicist were unhappy with the theory. Back in the day when I was doing solid state, I also read the various particle physics articles in Phys rev letters and wondered what was special about kaons etc. I am now reading “Quantum Man” which is fascinating and bringing out some of the fine points. Only I have to try to replace the words with the theory!! And I just gave away all my books prior to moving.

  5. Pingback: God Literally Being Fate? - Page 3 - Religious Education Forum

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