image: Schematic diagram and experimental setup for optimal orienteering with parallel and antiparallel spins.
The CAS key lab of quantum information makes a significant progress in quantum orienteering. Under the lead of Prof. GUO Guangcan, LI Chuanfeng, XIANG Guoyong and collaborators enhanced the performance of quantum orienteering with entangling measurements via photonic quantum walks. These results were published online by Physical Review Letters on February 13th.
Thanks to quantum entanglement, quantum information processing is much more efficient than its classical counterpart in many tasks, like quantum computation, quantum communication, quantum metrology, and so on. Quantum entanglement can manifest itself in both quantum states and quantum measurements. On contrast to the extensive research of entangling states, there are few experimental studies of entangling measurements because entangling measurements are difficult to realize. In recent years, Prof. Guo-Yong Xiang et. al. developed a method of realizing quantum entangling measurements via photonic quantum walks, and their method not only has high fidelity but also is deterministic. They have used this technology to achieve unprecedented efficiency in quantum state tomography [Nature Communications 9,1414(2018)], and reduce the back action of quantum measurements in quantum thermodynamics [Science Advances 5,3(2019)]. Recently, they used their technology to enhance quantum orienteering.
In quantum orienteering, Alice wants to use quantum resources to communicate a random space direction to Bob, which is of military importance. A simple scheme is that Alice can polarize a spin along the direction and sends it to Bob. As early as 1999, Nicolas Gisin from University of Geneva found that the antiparallel spins were more efficient than parallel spins in quantum orienteering. That is different from the classical counterpart, in which the efficiencies of the two direction encoding schemes are the same. There is no entanglement in quantum states on Alice's side, thus it is the entanglement in quantum measurements on Bob's that boosts the efficiency. As optimal entangling measurements on parallel and antiparallel spins are difficult to realize, there has been no convincing experimental implementation for more than 20 years. Prof. Guo-Yong Xiang et. al. successfully realized such optimal entangling measurements via quantum walks. The experimental results clearly demonstrate that entangling measurements can extract more direction information than local measurements, and the fidelity of antiparallel spins has an improvement of 3.9% than parallel spins in orienteering.
Their work demonstrated a truly nonclassical phenomenon that is owing to entanglement in quantum measurements instead of quantum states. Meanwhile, it offers an effective recipe to realizing entangling measurements in photonic systems. These results are of interest not only to foundational studies of quantum entanglement and quantum measurements, but also to many applications in quantum information processing.