Computer hardware is almost always realized via electrons, e.g. good old fashioned electricity. Apply some voltage to a conductive material and now we're scooting around electric charges, which can represent binary information. Add some switches and gates and that information can be manipulated in useful ways. Voila: a computer.
The times are a changing, however. While electrons will likely persist as the fundamental information carriers of computing in the quantum age, they'll become much less suitable for future networking and security tasks such as quantum key distribution, distributed quantum computation, quantum clock synchronization, and probably a bunch of stuff we haven't thought of yet. The looming "quantum internet" (communication via quantum phenomena) will likely require different particle messengers: photons, the units of light itself.
So, we have two different sorts of particle and two different sorts of phenomena (from a high-level perspective): electricity and light. In a paper out this week in Nature Communications, engineers at Stanford offer a way of unifying the two in a communication scheme based on quantum entanglement, e.g. the capability of particles to share the same quantum state over infinitely large distances. Here, electrons confined to atoms in one location can be entangled with electrons confined to another, with photons acting as a bridge or relay system.
To entangle particles, they need to start out in the same location, where they can made to share some single state corresponding to one of several particle properties, such as spin ("up" or "down"), momentum, angular momentum, position, or really any other feature that can be properly measured. Once the unifying state is prepared, the particles become essentially one particle with respect to the entangled property and can then be seperated while that unity is preserved, assuming things are handled very, very carefully.
If you were wanting to entangle some electrons in this computer here with electrons in a computer somewhere far away, it's not like you can just fax the other particle a quantum state. But that's sort of what the Stanford method implies: entangling electrons in different locations via photon messengers. The group managed this at a distance of 1.2 miles. A global quantum internet it is not, but it's a pretty good start.
A significant problem with such a photon-based scheme is that photons tend to depolarize as they move along a fiber-optic channel. Beyond 15 miles or so the photons "decohere," which, for our purposes, means that they lose track of the information they're supposed to be transporting.
"Although spin-photon entanglement has been demonstrated in atomic and solid-state qubit systems, the produced single photons at short wavelengths and with polarization encoding are not suitable for long-distance communication," the Stanford group writes, "because they suffer from high propagation loss and depolarization in optical fibres."
The physicists got around this by employing another particle property, which is its time-stamp or when it arrives at a certain destination relative to another particle. This is called time-bin encoding.
And so the spin property of an electron from one computer—which is confined to a quantum dot entangled with a photon, and that photon is fired down a fiber-optic line. In the middle it meets another photon from the other direction. Normally they would just ignore each other, but thanks to a a technique known as quantum down-conversion, it's possible to match the two wavelengths of the two particles. The second photon has now taken on the quantum state of the first, and so the two are indistinguishable.
If you were to keep doing this on down the line, the originally entangled state can be assured of arriving at its destination intact. At the other end, it can be entangled with a recipient electron, which will now be paired with the original electron some large distance away.
"We have demonstrated correlation between the [quantum dot] spin and the photon arrival time, and mediated two-photon interference at kilometre scales," the Stanford team concludes. "These quantum-networking technologies, together with wavelength conversion and quantum erasure, will enable practical quantum communication between solid-state spin qubits across long distances."