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Boffins manage to keep graphene qubits 'quantum coherent' for all of 55... nanoseconds

Doesn't sound very long, but it could have big implications for quantum computing

Physicists have formed qubits – quantum bits – from graphene for the first time, according to research published in Nature Nanotechnology.

The applications of quantum computing are still a little handwavy at the moment, but it all boils down to the creation of qubits.

Traditional binary computers carry out operations using transistors and capacitors, where bits of data can be stored in two states: a 0 or 1. Quantum computers, however, use qubits, which all interact with one another and can exist in a variety of states, a superposition of 0 and 1. The more qubits, the more possible states a system can have.

Qubits are fiddly, and it's difficult to keep them all in quantum superposition. They have to be carefully isolated, as even slight changes in temperature can disrupt the qubits' states. The team led by researchers from the Massachusetts Institute of Technology (MIT) carefully cooled a quantum system made out of two qubits to 10 millikelvin (-273.14°C), just a smidgen above absolute zero.

The qubits were fashioned from graphene-based superconducting circuits. A sheet of graphene, a lattice arrangement of carbon atoms, was sandwiched between two layers of hexagonal boron nitride. The two qubits are connected using an aluminium electrode, and operate like transistors.

Applying a voltage to the system creates current loops that drive tiny magnetic fields that causes the electrons in graphene to jump between the superconducting boron nitride layers, allowing the qubits to switch states between 0 and 1. The electrons are 0 in their ground state, and 1 when they're excited and in superposition. The researchers managed to keep the superconducting qubits in superposition, a state described as "temporally quantum coherent", for about 55 nanoseconds.

"Our motivation is to use the unique properties of graphene to improve the performance of superconducting qubits," said Joel I-Jan Wang, first author of the paper and a postdoctoral researcher in electronics at MIT.

"In this work, we show for the first time that a superconducting qubit made from graphene is temporally quantum coherent, a key requisite for building more sophisticated quantum circuits. Ours is the first device to show a measurable coherence time — a primary metric of a qubit — that's long enough for humans to control."

Being able to control qubits with voltage will help scientists increase the number that can be squeezed onto a single chip. At the moment, only 1,000 qubits can be used and the researchers hope to create systems with more that can be controlled for longer times. ®

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