Quantum pulse filters measure system noise

Filtering methods developed to shield quantum mechanical systems, which are microscopic systems governed by the rules of quantum mechanics, from disruption by environmental noise are now being used to characterize the noise itself. The novel technique, described in Nature Physics, will help physicists identify and block out noise in certain systems, and could improve the prospects of quantum computers. This strategy could also lead to clearer magnetic resonance images, which are used in medicine.

Quantum mechanical properties are extremely fragile and easily disrupted if the system interacts with the external environment. "This is a huge problem when trying to build a quantum computer because the quantumness is often disrupted too quickly to carry out a computation," says Jonas Bylander, a physicist at the Massachusetts Institute of Technology (MIT) who led the study.

Physicists working on magnetic resonance often shield quantum systems using a series of short laser pulses. If the pulses are applied at a rate that is faster than the timescale over which the noise varies, then the pulses act as a filter, reducing any disruption. The researchers, including one from Saudi Arabia, used this strategy to build a filter, using microwave pulses, to protect a superconducting circuit often used in quantum computation research, known as a 'flux qubit'. In the flux qubit, current flows around the circuit in both a clockwise and counterclockwise direction, at the same time.

The team then turned the pulse filters into a noise spectrum analyzer, by noting that noise leaks in through specific narrow frequency ranges. They measured how quickly the qubit was disturbed across different ranges, allowing them to quantify the noise at various frequencies.

"The method can be used to analyze the noise in a wide range of quantum system, to figure out its source and get rid of it, or to tailor pulse sequences to target the noise more effectively," says Bylander.

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