Physicists Craft Single-Photon Quantum Logic Gates

Physicists from the University of Toronto have succeeded in constructing logic gates from single particles of pure light, according to research published in this week's Nature Physics. It's an accomplishment that not only offers insight into the still rather mysterious world of light particles, but it may have implications for future quantum computers, which depend far more on interactions between individual particles (of light, usually) than our primitive electric current-based conventional computers.

A logic gate is the fundamental building block of any computing machine. In conventional computing schemes, information is served to these gates as high and low currents, representing 1s and 0s. A gate's job is to take that information and spit out a 1 or 0 in response, again in the form of high and low currents. This is the foundation of everything a computer does: memory, arithmetic, I/O, etc.

The situation in a quantum computer is different. Rather than bits, which represent information as either a 1 or a 0, we have qubits. Qubits offer the possibility of having values that are simultaneous combinations of 1s and 0s, where the two possibilities exist together in quantum superpositions. This offers an enormous leap in computing power, but managing this sort of information isn't easy.

For one thing, we're no longer dealing with information represented by bulk collections of particles, e.g. electric current. Information in a quantum computer is instead represented at the level of individual particles. This means that we need to consider some pretty fundamental changes to computing hardware.

"Thanks to modern technologies, it is now quite straightforward to put a single quantum particle like a photon in a superposition of two different states," Aephraim M. Steinberg, a physicist at the University of Toronto and a study co-author, told me. "But putting a beam of many photons into such a superposition&mash;in which, say, either every single photon is horizontally polarized or every single photon is vertically polarized, but no one in the universe knows which is the case—is precisely analogous to Schrödinger's famous cat."

With some many particles at once representing a single qubit, it's exceedingly likely that one particle will be interfered with in some way, which, in a quantum system, has the effect of "opening the box" and wiping out the superposition and, thus, the qubit.

"If you have a single photon, it can travel nearly 100 kilometers in optical fibre, for instance, before anything happens to it—no one has any information about what state it's in," Steinberg explained. "But if there were a million photons, within about 100 millimeters, at least one of them would probably get absorbed—that single event would be enough to destroy the delicate superposition state quantum logic relies upon." No more superposition, no more information.

And so it's much more desirable to work with single photons. This has its own difficulties, however, as getting single-photon beams to interact is very, very difficult. Photonic logic gates, such as optical transistors, have until now dealt instead with pulses of light consisting of several hundred photons together, at least. For the reasons above, this is hardly ideal.

The Toronto group's setup consisted of a collection of rubidium atoms cooled to just a millionth of a degree above absolute zero. Single photons are fired through this medium, which experiences a phase shift and a small change in its refractive index. This change is observed using a second "probe" beam, and the result is a coupling between the individual photons and the probe beam via the rubidium "atomic vapor." This is accomplished via what's known as electromagnetically induced transparency (EIT), where the degree of interaction between light and some material (rubidium atoms) can be manually tweaked or tuned.

So, is that it? Not really, and, as Steinberg explained, the challenge of an optical quantum computer remains the fundamental problem of light being light. That is, it carries no electrical charge and so it tends to just bounce around without interacting with other light. This isn't so much a problem with electrons, where a logic gate can be easily manipulated via charge-based interactions.

"Developing the right media—perhaps using nanostructured fibres, or special high-reflectivity optical cavities, or just atoms in 'EIT' states such as we've used or the 'Rydberg' states people are beginning to study now—has been the big challenge," Steinberg said. "This is why no one knows yet what a full-scale quantum computer will look like when we finally have them."