Two by Two

For close to a decade now, two of the hottest buzzwords in technology have been “Quantum Computing”—the promise of storing a information by manipulating the spin of a single electron, and the associated prospect of harnessing quantum entanglement for faster computation has captured the imaginations of physicists and computer scientists alike. As exciting as the theoretical possibilities are, much of the nitty-gritty work of constructing a functional quantum computer has yet to be done.

We’re a few steps closer to that goal today though, thanks to a collaboration of European physicists working with the Nanoelectronics Group at the University of Basel in Switzerland. Two papers from the group were recently accepted for publication in Physical Review Letters, each exploring a different aspect of electron pair behavior in superconductors.

In one of these experiments, a junction was created between a superconductor and a sample of normal metal, connected by a single “quantum dot”—a crystal so small that it can carry only one excess electron at a time. This arrangement is at odds with the nature of superconductors, in which electrons must travel in “Cooper pairs”—bound states of electrons with opposite spins. Theoretically, charge shouldn’t be able to flow from the normal conductor to the superconductor at low energies, because they cross the quantum dot one at a time—unpaired, by necessity. However, in an exemplary display of the weirdness that occurs at the quantum scale, some electrons managed to make it across the junction, in a process called Andreev Tunneling.

As an electron crosses from a normal conductor (N) into a
superconductor (S), it creates an electron hole in the normal material,
carrying the corresponding electron with it into the superconductor. 

Ordinarily, when a single electron crosses into a superconductor, it “steals” a partner from the material it’s leaving, creating an electron hole in its wake, as pictured at right. In Gramich et al.’s
paper, when an electron crosses into the quantum dot, it remains bound to an electron of opposite spin in the normal material. As that electron crosses from the quantum dot into the superconductor, its partner enters the dot to take its place, and then the superconductor after it. The electrons’ surprising willingness to line up “single file” to make their way across the gap prompted one of the researchers to remark, “electrons always find a way”.

An illustration of Andreev Tunneling, assisted by a boson (blue) from the environment, such as a vibration in the atomic lattice of the conductors.
Image Credit: Gramich et al.

The second experiment from the Nanoelectronics group also probes the behavior of Cooper pairs, specifically their potential usefulness in creating spin-entangled pairs of electrons. Ideally, you’d be able to split a Cooper pair and send its constituent electrons to different places without destroying the correlation between their spins (which, again, are always opposite in a Cooper pair). This kind of technique would allow information to be sent to two places at once, and enable complex qubit computations. While successful maintenance of entanglement during Cooper pair splitting hasn’t yet been achieved, the group’s results makes it look plausible in the future.

It remains to be seen how much of the recent buzz about quantum computing is just “hype”, but for now, the field seems to be advancing steadily, if slowly—bit by qubit.

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