Over the past 100 years, human society has benefited from quantum mechanics, which was considered as a pillar of modern civilization, where wavefunctions played a key role. What most people did in the past century was 'Shut up and calculate', and wavefunctions always provided a correct probability list of measurement outcomes.
However, the discussion on the following deeper philosophical problem behind it still continues: whether wavefunctions describe the reality of existence and dynamic trajectory of quantum entities. In different delayed-choice experiments that are dedicated to this problem, Copenhagen interpretation denied the reality of wavefunctions for preventing the paradox of a choice made in the present in order to alter a photon's past behavior. However, determinists debate that the past of photons must be realistic and deterministic before the detection, as the famous question of Einstein states: Do you really believe the moon exists only when you look at it?
The cartoon of 'Quantum Twisted Loong' (Cover). Credit: Science China Press
In order to address this long-standing problem, Bao-Sen Shi's research team from the University of Science and Technology of China recently teamed up with Professor Zhi-Han Zhu from Harbin University of Science and Technology and another colleague to propose and show a quantum twisted double-slit experiment, where photonics orbital angular momentum (OAM) and its group velocity slowing-down features are used to extract propagation history of photons after detections.
At first, the state or wavefunction of signal photons diffracted from the twisted double slits is changed into a superposition state between Gaussian mode and different OAM modes; second, the photons are observed in Gaussian mode (choosing particle behavior); and finally, a Hong-Ou-Mandel (HOM) interference between reference and signal photons provides the arrival times of signal photons that enables one to study the propagation history of photons.
The results achieved from the experiment reveal that the nature of the photon before the measurement is pre-existing, as shown by wavefunction. Thus, the nonlocality and physical reality of wavefunctions are confirmed. This finding explains the long-held misunderstanding of the wavefunctions’ role and their collapses in quantum entities’ evolution. Additionally, they present a cartoon titled "Quantum Twisted Loong" to explain photons’ propagation behavior in a double-slit apparatus. Unlike the cartoon titled "Great Smoky Dragon" created in 1983 by Miller and Wheeler, where the dragon’s body is unknown and smoky, the body of Loong is deterministic, that is, the two bodies simultaneously coexist (representing wave nature) prior to their collapse.