Prof. LI Chuanfeng, TANG Jianshun and their collaborators from CAS Key Lab of Quantum Information directly measure the degree of quantum coherence using interference fringes. This work is published in Physical Review Letters on January 13th.
Quantum coherence, also known as quantum superposition, is the most essential property that distinguishes quantum mechanics from classical theory. It is the fundamental resource for quantum information processing. The study of quantum coherence may date back to the double-slit interference experiment. Nevertheless, the rigorous and systematic resource-theoretic framework of coherence has just been developed recently, and several coherence measures are proposed. Experimentally, the usual method to measure coherence is to perform state tomography and use mathematical expressions.

Prof. LI’s group alternatively develops a method to measure coherence directly using its most essential behavior—the interference fringes. The ancilla states are mixed into the target state with various ratios, and the minimal ratio that makes the interference fringes of the “mixed state” vanish is taken as the quantity of coherence. They also use the witness observable to witness coherence, and the optimal witness constitutes another direct method to measure coherence. For comparison, they perform tomography and calculate quantum coherence, which coincides with the results of their two methods. Their methods are explicit and robust, providing a nice alternative to the tomographic technique.