Two different teams have shown, for the first time, that it’s possible to create entangled pairs of photons and electrons using quantum dots. The advance could help pave the way to practical quantum communication networks, which promise to be far more secure than today’s telecommunication networks.
Entangling light and matter is nothing new – it’s been done before in systems of trapped ions and atoms, for example, as well as in diamonds. But quantum dots, which can be made from nanometer-scale patches of semiconducting materials like InAs, are a promising system for communications. They, too, can be used to construct quantum bits (in this case, using electron spins). But they are also relatively compact, spit out photons at a good clip, and could be built on conventional or future, hybrid electronic-photonic chips.
So how do you create entangled electron-photon pairs? The teams, both using the same basic set-up, outlined their approaches in a pair of papers published last week in Nature. A short laser pulse was used to excite an electron in a quantum dot into a higher energy state. When the electron relaxed to a lower energy, its spin became a superposition of two states: spin up and spin down (à la Schrödinger’s cat). The process also results in the emission of a photon, whose frequency and polarization are linked to the electron (the entire system is a superposition of a blue, vertically polarized photon paired with a spin-up electron and a red, horizontally-polarized photon with a spin-down electron, Sophia Economou of the Naval Research Laboratory explains in an accompanying commentary).
The correlation between frequency and polarization made the resulting photons difficult to work with—it wouldn’t take much to disrupt their delicate quantum states. So to simplify matters, each team scrambled one of the two properties. One, which included Kristiaan De Greve, now a post-doc at Harvard University, and researchers at Stanford University, Wurzburg University in Germany, and Heriot-Watt University in Scotland, chose to preserve photon polarization while lowering the frequency so that all the photons were in the infrared range, a useful range for telecommunications. Atac Imamoglu’s group at the Institute of Quantum Electronics at ETH Zurich in Switzerland, opted instead to give the photons the same, anticlockwise circular polarization, which helped them distinguish the entangled photons from background laser light.
Demonstrating this sort of photon-electron entanglement is just a first step for both teams. The ultimate goal is to find a way to transfer information from one dot to another. That would allow researchers to create systems of quantum dots that can act as repeaters in a quantum network, storing and relaying information as needed, De Greve says. Such quantum repeaters could help improve the robustness of quantum information transmissions, which degrade over long distances, whether they’re sent through air or a telecommunications fiber.
Of course, transferring information from one dot to another is (as you might imagine) a bit tricky. Quantum information can’t be perfectly copied, or cloned. That’s the whole point behind un-counterfeitable quantum cash. But physicists have found that it is possible to transmit the links between particles through a process called “entanglement swapping”.
It would work roughly as follows: Dot A emits an entangled photon A, and dot B emits an entangled photon B. Under the right circumstances, a simultaneous observation of photon A with photon B (perhaps after they have been sent through a beam splitter so that they interfere with one another) will transfer entanglement. The result is that dot A and dot B end up entangled with one another, even if they’ve never interacted before. These dots could be linked to others in turn, creating a chain of dots that could allow you to transfer quantum information over a long distances.
Entanglement swapping has already been demonstrated in atoms and in ions, De Greve says. But he notes that these systems are slower and may be more difficult to scale in size than quantum dots. And he adds that, while he and his colleagues have been able to convert the entangled photons to useful telecommunications frequencies, the photon frequencies used in the atom and ion experiments are a bit too high in frequency to be transmitted over fibers for long-distances: “We should have an edge there.”