This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Quantum sensors, devices that collect measurements by exploiting quantum-mechanical phenomena, could potentially detect extremely weak magnetic, gravitational and electromagnetic signals with greater sensitivity than classical sensors. Some quantum sensors leverage entanglement, a phenomenon that prompts distant particles to become so strongly linked that the physical state of one particle dictates the state of the others.
Despite their potential, these sensors are often very sensitive to noise (i.e., external disturbances originating from the surrounding environment). This means that even small environmental disturbances and hardware imperfections can disrupt delicate quantum states and reduce the devices' sensing precision. Researchers at the Quantum Science Center of Guangdong-Hong Kong-Macao Greater Bay Area, Southern University of Science and Technology and Shenzhen University recently introduced a new method to develop noise-adaptive sensors that can collect more precise measurements.
Their approach, outlined in a paper published in Physical Review Letters, relies on a variational quantum circuit, a sequence of quantum operations that can be adjusted to search for a state that performs well on specific measurement tasks. "A previous study established a direct connection between the out-of-time-order correlator (OTOC) and the quantum Fisher information (QFI)," Xiaodong Yang, co-first and co-corresponding author of the paper, told Phys.org. "Since OTOCs have been experimentally measured in a variety of scalable quantum platforms and the QFI provides a theoretical bound on parameter estimation uncertainty, this connection motivates us to employ the OTOC as an efficient figure of merit for evaluating sensing performance.
By integrating OTOC-based evaluation with parameterized quantum circuits, we thus developed the noise-adaptive quantum metrology scheme presented in this work." An OTOC is a quantity that can be used to track how strongly a disturbance spreads through a quantum system over time. QFI, on the other hand, measures how much information a quantum state contains about an unknown quantity one is trying to measure. The researchers developed a new quantum metrology scheme that relies on these two quantities.
The researchers wanted to devise a method to identify and implement optimal probe states for quantum metrology on large-scale quantum hardware under realistic conditions. The scheme they developed works by identifying a probe state that enables precise measurements under specific noise conditions. "The noise-adaptive quantum metrology scheme comprises a variational quantum circuit for preparing candidate probe states, a parameter encoding stage, and an experimentally measured OTOC for efficiently evaluating their sensing performance," explained Yang.
"Being model-free, hardware-efficient, and scalable, the scheme provides a practical route toward noise-resilient quantum metrology on near-term quantum devices." To demonstrate their scheme's potential, the researchers applied it to a seven-qubit nuclear-spin sensor. Instead of assuming that a maximally entangled state is optimal, they used a circuit to search for a probe state that performed best under the sensor's actual noisy conditions. By repeatedly evaluating candidate states and adjusting the quantum circuit accordingly, the team could improve a sensor's precision and performance.
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