Because the pulse has deposited a tiny bit of its energy in the crystal, one of the outbound photons is of lower energy, and hence longer wavelength, than the photons of the incoming pulse. Walmsley and his colleagues set up an experiment that would attempt to entangle two different diamonds using phonons. They used two squares of synthetically produced diamond, each three millimeters across. A laser pulse, bisected by a beam splitter, passes through the diamonds; any photons that scatter off of the diamond to generate a phonon are funneled into a photon detector.
One such photon reaching the detector signals the presence of a phonon in the diamonds.
Entangled: The series | Institute for Quantum Computing | University of Waterloo
But because of the experimental design, there is no way of knowing which diamond is vibrating. Instead the two diamonds enter an entangled state in which they share one phonon between them. To verify the presence of entanglement, the researchers carried out a test to check that the diamonds were not acting independently. In the absence of entanglement, after all, half the laser pulses could set the left-hand diamond vibrating and the other half could act on the right-hand diamond, with no quantum correlation between the two objects.
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If that were the case, then the phonon would be fully confined to one diamond. If, on the other hand, the phonon were indeed shared by the two entangled diamonds, then any detectable effect of the phonon could bear the imprint of both objects. So the researchers fired a second optical pulse into the diamonds, with the intent of de-exciting the vibration and producing a signal photon that indicates that the phonon has been removed from the system.
The phonon's vibrational energy gives the optical pulse a boost, producing a photon with higher energy, or shorter wavelength, than the incoming photons and eliminating the phonon in the process.
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Once again, there is no way of knowing which diamond produced the photon, because the paths leading from each diamond to the detectors are merged, so there is no way of knowing where the phonon was. But the researchers found that each of the photon paths leading from the diamonds to the detectors had an interfering effect on the other—adjusting how the two paths were joined affected the photon counts in the detectors.
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In essence, a single photon reaching the detectors carried information about both paths. So it cannot be said to have traveled down one path from one diamond: the photon, as with the vibrational phonon that produced it, came from both diamonds. After running the experiment over and over again to gather statistically significant results, the researchers concluded with confidence that entanglement had indeed been achieved.
Entangled diamonds vibrate together
The catch to using phonons for macroscopic entanglement is that they do not last long—only seven picoseconds, or seven trillionths of a second, in diamond. So the experimenters had to rely on extremely fast optical pulses to carry out their experiment, creating entangled states with phonons and then damping the phonons with the second pulse to test that entanglement just 0. Because of this brevity, such entanglement schemes may not take over for more established techniques using photons or single atoms, but Walmsley hopes that researchers will consider the possibilities of using fairly ordinary, room-temperature materials in quantum technologies.
Indeed, the new study is just the latest to show how quantum mechanics applies in real-world, macroscopic systems. Electron spin is a quantum concept that is almost impossible to visualise but, since spin information is preserved when electrons are transmitted, this quantum spin could provide a basis for quantum computers and information processing.
The SET allows only one electron to be added at a time, leaving the second electron of the pair free in the superconductor. Still entangled? The second part of the project aims to prove that the separated pair of electrons does not lose their entanglement — and to see how far, and for how long, they can be separated while remaining entangled.
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