Professor Peng Xinhua's research group at the University of Science and Technology of China (USTC) and Professor Liu Renbao's research group at the Chinese University of Hong Kong have made significant progress in the high-precision measurement of open quantum many-body system temporal correlations. They proposed a theoretical scheme to selectively measure any type of temporal correlation in open quantum many-body systems using quantum channels synthesized through controllable physical processes. For the first time, they successfully detected fourth-order quantum temporal correlations in a nuclear spin system. The research findings were published online in Physical Review Letters on May 16.
Correlations between physical quantities are crucial for understanding many-body systems and the development of quantum technologies. To fully describe the dynamics of a physical system, all temporal correlation information within the system is needed, forming a complete set of dynamical temporal correlations. For quantum many-body systems, the non-commutativity of physical quantities leads to various complex inequivalent forms of quantum correlations. However, current measurement schemes can only extract a few specific forms of temporal correlation information, and so far, there has been no systematic and feasible method to extract all types of temporal correlations in a complete dynamical set.
In 2019, Liu Renbao proposed a scheme for arbitrary quantum temporal correlation measurements based on continuous weak measurements. However, the experimental implementation of this scheme was very challenging.
To address this challenge, the collaboration between Peng Xinhua's and Liu Renbao's research groups innovatively proposed a protocol for selective measurement of arbitrary quantum temporal correlations based on controllable physical processes synthesizing quantum channels. This protocol not only significantly improves the measurement signal-to-noise ratio for high-order quantum correlations and reduces the experimental implementation difficulty but also is applicable to a wider range of experimental systems, including single spins and ensemble quantum systems. Using high-precision quantum control in nuclear magnetic resonance, this work experimentally verified the feasibility of the measurement protocol in multi-spin systems and successfully measured fourth-order quantum temporal correlations in quantum many-body systems.
Furthermore, the study applied the experimentally obtained high-order quantum correlation information to high-precision quantum optimization control tasks. Numerical simulation results show that for single-spin quantum gates, incorporating fourth-order quantum correlation corrections in optimization control, as opposed to previous methods utilizing only second-order quantum correlation information, can increase the gate fidelity from 99.987% to 99.99996%.
The researchers highlight the potential application value of this work in the fields of quantum information and quantum many-body physics. On one hand, providing a complete characterization of all quantum temporal correlation information in a quantum thermal reservoir offers a method to fully represent quantum noise, which is crucial for quantum information technologies such as quantum control and quantum precision measurements. On the other hand, since the dynamical behavior of quantum many-body systems can be decomposed into superpositions of various quantum temporal correlations, precise measurement of arbitrary quantum temporal correlations helps people better characterize and understand the non-equilibrium properties of quantum many-body systems.
Reviewers highly praised the work, stating, "This is an important addition to the field of quantum measurements, providing insights into the dynamics of quantum systems."
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