Graphene Doubles Up on Quantum Dots' Promise in Quantum Computing

Researchers discover that graphene-based quantum dots can maintain four simultaneous quantum states as opposed to just two

Florian Libisch, explaining the structure of graphene
Photo: TU Wien

Quantum dots made from semiconductor materials, like silicon, are beginning to transform the display market. While it is their optoelectronic properties that are being leveraged in displays, the peculiar property of quantum dots that allows their electrons to be forced into discrete quantum states has long held out the promise of enabling quantum computing.

As it turns out, if you really want to exploit quantum dots for quantum computing, you’d have better luck setting aside the semiconductor variety and turning to a pure conductor, like graphene, to do the trick.

Researchers at Technische Universität Wien (TU Wein, or the Vienna University of Technology), along with colleagues from the University of Manchester in the United Kingdom and Rheinisch-Westfälische Technische Hochschule Aachen (RWTH Aachen University), in Germany, have managed to produce quantum dots out of graphene. And according to the multinational team, these dots offer a bold new promise for quantum computing.

So, what’s new? The researchers discovered that quantum dots made from graphene possess four quantum states at a given energy level, unlike semiconductor quantum dots, which have only two.

“In conventional semiconductors, there is only the spin of the electrons,” explained Florian Libisch, the assistant professor at TU Wein who led the research, in an e-mail interview with IEEE Spectrum. “With graphene, there is a second conserved quantity called ‘pseudospin.’ Both of these symmetries together yield 2 x 2 = 4 quantum states.”

These additional quantum states could be a boon to quantum computing, according to the researchers. Quantum computing relies on well-controlled, coherent interactions between quantum bits (qubits). A qubit is a coherent superposition of two quantum states.

The major obstacle towards realization of a working quantum computer is decoherence,which involves the loss of the quantum properties due to interactions with the environment. These interactions collapse the qubit.

“Using our graphene quantum dots, you could think of storing two qubits in the four-fold near-degenerate states—which would make a coherent interaction between these two qubits much more well controlled than the interaction of two two-fold degenerate states,” Libisch told Spectrum. Or in English: “Four localized electron states with the same energy allow for switching between different quantum states to store information,” said Joachim Burgdörfer in a press release. 

The key to the new research described in the journal Nano Letters was to produce the graphene quantum dots without losing these four quantum states. In order to produce a quantum dot, you need to isolate electrons in a small bit of material. The simplest method for doing this is by cutting off tiny flakes from a thin layer of material where the electrons are trapped. While this general technique works for graphene, it breaks the symmetry of the 2D carbon material at the edge of the flakes. That yields quantum dots with only two quantum states.

So, instead of making tiny graphene flakes, the researchers used a combination of electrical and magnetic fields to trap the electrons in the graphene. First, a trap is set for the electrons on the graphene surface by applying an electrical current that attracts and holds onto the low-energy electrons. Then, a magnetic field is applied that forces those trapped electrons into tiny orbits where they stay.

In these tiny orbits, the electrons maintain arbitrary superpositions for a long time, which is an attractive property for quantum computers.

In addition to the four quantum states, graphene quantum dots offer scalability. According to the researchers, it should be possible to fit many graphene quantum dots on a small chip for use in quantum computing.

The advantages don’t end there. “Graphene features other nice properties for applications, including mechanical flexibility (think of flexible displays), and exceptional thinness (less than one nanometer), which potentially allow for higher storage density,” said Libisch.

Now that they have produced these four quantum states, the next step will be tuning them.

Libisch added, “We want to demonstrate how to tune their interactions, which is the next step towards using them for quantum information.”

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IEEE Spectrum’s nanotechnology blog, featuring news and analysis about the development, applications, and future of science and technology at the nanoscale.