Quantum computers are sometimes referred to as the Holy Grail of computing, or maybe the Philosopher’s Stone of computing might be another appropriate medieval reference to a nearly unattainable quest. In any case, while some outfits have claimed they have achieved fairly significant quantum computer prototypes despite being met with skepticism, creating a quantum computer that can calculate something beyond what a kid in elementary school can factor has proven difficult.
One of the fundamental issues researchers have faced in developing quantum computers has been the problem of getting the computers to maintain more than a few quantum bits (or qubits). One of the more promising ways of getting beyond a mere seven qubits has been the use of quantum dots.
Now researchers at the University of Illinois led by Nadya Mason have brought a new wrinkle into this field of research. The research, which was initially published in the journal Nature Physics, was looking at what happens when a normal conducting material like a metal or graphene is sandwiched between two superconducting materials and observing the interface of the materials.
While it has been observed previously that normal metals in these instances take on the characteristics of the superconductor material when current is passed through it (namely, that it too will pass electron pairs through it rather than a single stream of electrons), the Illinois researchers by working with graphene quantum dots were able to better understand the fundamental physics at play: Andreev bound states (ABS).
To date, ABS have proven to difficult to both measure and observe. At is at this point that the researchers developed a novel method to isolate individual ABS by connecting probes to quantum dots made from graphene. As quantum dots do they confined the confined ABS into discrete energy states, which permitted the researchers to not only measure the ABS but to manipulate them.
"Before this, it wasn't really possible to understand the fundamentals of what is transporting the current," Mason said. "Watching an individual bound state allows you to change one parameter and see how one mode changes. You can really get at a systematic understanding. It also allows you to manipulate ABS to use them for different things that just couldn't be done before."
The concurrence of the two nanomaterials, graphene and quantum dots, along with the superconducting material made the breakthrough possible.
"This is a unique case where we found something that we couldn't have discovered without using all of these different elements – without the graphene, or the superconductor, or the quantum dot, it wouldn't have worked. All of these are really necessary to see this unusual state," Mason said.
Dexter Johnson is a contributing editor at IEEE Spectrum, with a focus on nanotechnology.