New Method Makes Quantum Sensors More Resistant to Noise

Researchers in China have developed a new method for building quantum sensors — devices that use quantum-mechanical effects to detect extremely weak magnetic, gravitational or electromagnetic signals — that can adapt to the environmental noise disrupting their precision. The work, from teams at the Quantum Science Center of the Guangdong-Hong Kong-Macao Greater Bay Area, Southern University of Science and Technology and Shenzhen University, was published in Physical Review Letters.

Quantum sensors can outperform classical instruments by exploiting entanglement, a phenomenon in which distant particles become so strongly linked that the state of one dictates the state of the others. But these delicate quantum states are highly sensitive to noise — small environmental disturbances and hardware imperfections that can degrade a sensor's precision. The new approach uses a variational quantum circuit, a sequence of adjustable quantum operations, to search for a probe state that performs best under a sensor's actual, real-world noise conditions, rather than assuming that a theoretically ideal entangled state is always optimal.

"The noise-adaptive quantum metrology scheme comprises a variational quantum circuit for preparing candidate probe states, a parameter encoding stage, and an experimentally measured [out-of-time-order correlator] for efficiently evaluating their sensing performance," said Xiaodong Yang, co-first and co-corresponding author of the paper. The out-of-time-order correlator, or OTOC, tracks how a disturbance spreads through a quantum system over time and can be measured experimentally on existing quantum hardware, making it a practical stand-in for gauging a probe's sensing performance.

The team tested the scheme on a seven-qubit nuclear-spin sensor, repeatedly evaluating candidate probe states and adjusting the quantum circuit to improve precision. The optimal probe states identified this way improved precision by up to 0.698 decibels compared with a standard GHZ state, a common type of entangled state, when sensing a fixed magnetic field. The researchers plan to test the approach on larger multi-qubit systems, including quantum hardware with hundreds of qubits.