Superconducting capacitors are a new proposed solution for quantum computers

A superconducting quantum tunnelling capacitor is expected to solve a key problem for large-scale quantum computing.

The problem is ensuring that the flow of quantum information is in the right direction. This is also necessary for a conventional computer, with the electricity flowing around the circuits in one direction. A quantum computer’s information is processed using what is called a qubit. A qubit can be a one and a zero at the same time and it is this superposition quality that allows the simultaneous calculations that will outperform conventional machines. Millions of qubits are needed to achieve this performance and they all require precise control and measurement signals and the solution for this is a superconducting quantum tunnelling capacitor, a new type of a circulator.

“For a quantum computer to be built, [with our architecture] you will need a lot of these circulators,” said Clemens Mueller, a theoretical physicist at Eidgenössische Technische Hochschule (ETH) Zürich's department of physics and circulator research team member. “For a quantum computer, at the moment, there is competition between several different architectures in how you would build a quantum computer.”

Circulators already exist, but they are not practical for large scale integration. This is because they are about two centimetres across and each of the one, two or three million qubits require a circulator. The quantum computer would be too large. In stark contrast, today’s quantum computers have tens of qubits. The few million qubits are also necessary to achieve the error correction needed to ensure confidence in the calculations; just as error correction is used in conventional computers.

So, Mueller and his colleagues’ at the University of Queensland, Melbourne’s RMIT University and ETH Zurich, are developing a circulator that uses a superconducting capacitor, far smaller than existing circulators. Super conductance requires extremely low temperatures, just above absolute zero. This capacitor works as a circulator by using the quantum tunnelling of the magnetic flux around it, for that precise control of the signal direction. For Mueller’s quantum computer the signals are microwaves.

The three pointed circulator schematic shows how control signals are routed only to the qubit, quantum signals go directly to the measurement amplifier and the amplified signal is directed to the readout electronics.

Quantum tunnelling is when an electron wave passed through an electric field, which would normally be expected to reflect it. This tunnelling allows for the flow of microwave energy in one direction. The use of superconducting low temperatures for the capacitor is not a problem as many quantum computer architectures need superconductors to remove any resistance to the flow of energy. The entire quantum computer will be chilled to just above absolute zero anyway.

The Swiss, Australian team has two circuit designs for their circulator and one has a three-pointed star like layout. Mueller’s colleagues compare this design with the so-called flux capacitor device seen in the 1985 film Back to the future, that powers the story’s time traveling car.

The better-quality microwave signals that could be achieved with the circulator could also help radar improve its signal to noise ratio and therefore its performance. It could also give mobile phone networks more capacity. The next step for the Swiss, Australian team is to improve the circulator design for greater bandwidth and overcoming a problem of reduced signal direction control when the signal power increases.

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