A group of physicists in China has taken the lead in the race to couple together increasing numbers of superconducting qubits. The researchers have shown that they can entangle 10 qubits connected to one another via a central resonator – so beating the previous record by one qubit – and say that their result paves the way to quantum simulators that can calculate the behaviour of small molecules and other quantum-mechanical systems much more efficiently than even the most powerful conventional computers.
Superconducting circuits create qubits by superimposing two electrical currents, and hold the promise of being able to fabricate many qubits on a single chip through the exploitation of silicon-based manufacturing technology. In the latest work, a multi-institutional group led by Jian-Wei Pan of the University of Science and Technology of China in Hefei, built a circuit consisting of 10 qubits, each half a millimetre across and made from slivers of aluminium laid on to a sapphire substrate. The qubits, which act as non-linear LC oscillators, are arranged in a circle around a component known as a bus resonator.
Initially, the qubits are put into a superposition state of two oscillating currents with different amplitudes by supplying each of them with a very low-energy microwave pulse. To avoid interference at this stage, each qubit is set to a different oscillation frequency. However, for the qubits to interact with one another, they need to have the same frequency. This is where the bus comes in. It allows qubits to transfer energy from one another, but does not absorb any of that energy itself.
Physics World: Ten superconducting qubits entangled by physicists in China