Practical quantum computers may be another step closer to reality – and like so many technologies, we have graphene to thank for it. The bits of information in quantum computers (qubits) can famously exist in two states at once, and now researchers from MIT and other institutions have managed to record just how long that superposition state can last in a qubit made of graphene.
The idea of a quantum superposition is most famously illustrated by Schrödinger's Cat. For this iconic thought experiment, imagine a cat in a box, whose life depends on whether a radioactive atom decays or not. Theoretically, that cat exists in a superposition of both alive and dead at the same time, and it's only when you open the box and take a look that it collapses into one state or the other.
Quantum computers use that principle of superposition to their advantage. Traditional computers store and process information in binary states, where each bit of data is represented as either a 0 or a 1. Quantum computers on the other hand can shift into a superposition of both 0 and 1 at the same time, which allows them to drastically outperform traditional computers at certain tasks. The longer the qubits can stay in that state – known as their "coherence time" – the more powerful the quantum computer.
The coherence time of graphene-based qubits hadn't been recorded before, so researchers on the new study set out to measure it – and make sure these devices could pull it off at all. Sure enough, the graphene qubits were clocked at 55 nanoseconds, after which they return to their "ground" state of 0.
"Our motivation is to use the unique properties of graphene to improve the performance of superconducting qubits," says Joel I-Jan Wang, first author of the study. "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."
If 55 nanoseconds doesn't sound like a lot … well it isn't, really. Other qubit designs have coherence times that are hundreds of times greater than that, meaning they're much more powerful as quantum computers. But the graphene qubits have other advantages.
For one, graphene has a strange quirk where it takes on the superconductive properties of adjacent superconducting materials. In this case, the MIT team placed a sheet of graphene in between two layers of hexagonal boron nitride (hBN). Sandwiched between those superconducting layers means the qubit can be made to switch between states by applying a voltage, rather than a magnetic field like other designs.
The advantage of that is that the qubit can function more like a traditional transistor, allowing more of them to be crammed into a single chip. If they were still running off magnetic fields, current loops need to be incorporated into the chip as well, which can take up valuable space and interfere with nearby qubits, leading to computing errors. Plus it's more efficient, because the two outer layers of hBN protect the graphene from defects that could throw off traveling electrons.
Those advantages could really help get practical quantum computers up and running, and determining the coherence time of graphene qubits is an important step. It may be short, but the researchers say they're aiming to solve that issue by modifying the structure of the qubits, as well as investigating how the electrons move through them.
The research was published in the journal Nature Nanotechnology.
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