The full power of quantum computing remains out of reach for now, but we're getting closer: physicists just packed 10 qubits on to one superconducting circuit, marking a new record in the march towards next-generation computing.
While researchers have previously linked up to 51 qubits in the same computer, this is a new record for the number of entangled qubits on one superconductor. Advances like this are necessary for making quantum computers more robust, opening them up to more powerful applications.
Researchers from China achieved the feat by forging qubits from tiny pieces of aluminium, placed on a sapphire substrate, and connected to each other around a central bus resonator. The previous record for a system like this was one that held nine qubits.
Qubits are the building blocks of quantum computing, capable of superposition – being in two states at the same time, rather than the computer bits of today, which are either a 1 or a 0 at any given moment.
One of the problems tackled in this study was decoherence, where the quantum computing environment essentially collapses back into a classical computing environment.
This can create errors that make quantum computers less reliable. One way to help make the process more robust is to entangle the qubits.
Entanglement is where quantum states can't be described independently, but only in relation to the others, making for a more solid system.
The only problem - entangled qubits are also prone to the decoherence we mentioned earlier.
To stop this from happening, the superconducting circuits were cooled to incredibly low temperatures to keep the qubits coherent for longer. In this case the central bus was able to create entanglement between two qubits, between multiple pairs, or between all 10 qubits on the board with a single interaction. All without collapsing.
The bus handles the very delicate job of getting the selected qubits oscillating at the same frequency so that they can interact, and to enable this to happen so that it allows the qubits to transfer energy without using up any energy itself.
The team used a measurement technique called quantum tomography to prove that they had indeed entangled their qubits – but we're still a long way from getting this inside a computer. The next step is to prove this can be scaled up further.
To put together a quantum computer that can take pretty much any task we throw at it, like a conventional computer does, we'd need hundreds, if not millions of qubits.
That would be enough to simulate the behaviour of small molecules and other quantum systems.
Multiple entangled qubits on a number of processors will help squeeze the most from the system.
The experiment was praised as "nicely done" by John Martinis from the University of California, Santa Barbara, one of the scientists responsible for the nine-qubit circuit that is no longer the record holder.
Martinis says the speedy entangling and single-qubit operation achieved by the researchers is particularly notable, but the crucial test comes in trying to expand the system without it falling down like a house of cards.
"The hard thing is to scale up with good gate fidelity," says Martinis.
The research has been published in Physical Review Letters.