Researchers extend life of superconducting quantum bits

The power of quantum computers depends on keeping them in a fragile quantum mechanical state, and the new technique could be used to measure the physical characteristics of qubits that knock them out of superposition in the first place. This paves the way to better qubit designs. Teams from MIT, MIT Lincoln Laboratory, Japan's Institute of Physical and Chemical Research and NEC will publish a paper in Nature Physics. To realise qubits, the researchers used a superconducing circuit made from several layers of aluminium deposited on a silicon wafer and cooled to a fraction of a degree above absolute zero. Because of the quantum effects, current flow through the circuit can be in superposition, in effect flowing clockwise and counterclockwise at once. The previous record for keeping a superconducting qubit in superposition was less than 10µs, but by firing the qubit with microwave radiation, the team was able to keep it alive for 23µs. This is much closer to the threshold qubits need to cross in order to perform useful computations. By carefully controlling the rate at which the microwaves were fired, the researchers filtered out noise that occurs outside a narrow frequency band, preventing the qubit from falling out of superposition. By then adjusting the rate, they could also measure exactly how much noise the qubit experiences within any given frequency band. The microwave pulses not only extend the qubit's life, but in a quantum computer could also instruct the qubits in the execution of their own error correcting code – somewhere between 1000 and 10,000 separate instructions.