Researchers at the Niels Bohr Institute have significantly increased how quickly changes in delicate quantum states can be detected inside a qubit. By combining commercially available hardware with new adaptive measurement techniques, the team can now observe rapid shifts in qubit behavior that were previously impossible to see.
Qubits are the fundamental units of quantum computers, which scientists hope will one day outperform today's most powerful machines. But qubits are extremely sensitive. The materials used to build them often contain tiny defects that scientists still do not fully understand. These microscopic imperfections can shift position hundreds of times per second. As they move, they alter how quickly a qubit loses energy and with it valuable quantum information.
Until recently, standard testing methods took up to a minute to measure qubit performance. That was far too slow to capture these rapid fluctuations. Instead, researchers could only determine an average energy loss rate, masking the true and often unstable behavior of the qubit.
It is somewhat like asking a strong workhorse to pull a plow while obstacles constantly appear in its path faster than anyone can react. The animal may be capable, but unpredictable disruptions make the job much harder.
FPGA Powered Real Time Qubit Control
A research team from the Niels Bohr Institute's Center for Quantum Devices and the Novo Nordisk Foundation Quantum Computing Programme, led by postdoctoral researcher Dr. Fabrizio Berritta, developed a real time adaptive measurement system that tracks changes in the qubit energy loss (relaxation) rate as they occur. The project involved collaboration with scientists from the Norwegian University of Science and Technology, Leiden University, and Chalmers University.
The new approach relies on a fast classical controller that updates its estimate of a qubit's relaxation rate within milliseconds. This matches the natural speed of the fluctuations themselves, rather than lagging seconds or minutes behind as older methods did.
To achieve this, the team used a Field Programmable Gate Array (FPGA), a type of classical processor designed for extremely rapid operations. By running the experiment directly on the FPGA, they could quickly generate a "best guess" of how fast the qubit was losing energy using only a few measurements. This eliminated the need for slower data transfers to a conventional computer.
Programming FPGAs for such specialized tasks can be challenging. Even so, the researchers succeeded in updating the controller's internal Bayesian model after every single qubit measurement. That allowed the system to continually refine its understanding of the qubit's condition in real time.
As a result, the controller now keeps pace with the qubit's changing environment. Measurements and adjustments happen on nearly the same timescale as the fluctuations themselves, making the system roughly one hundred times faster than previously demonstrated.
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