Written by AZoQuantumJan 8 2018

**Quantum mechanics is one of the strengths of modern science and technology, and has been advantageous to mankind for nearly 100 years. **

The wave function, or the quantum state, is the representation of a quantum object and has a significant role in quantum mechanics. In spite of this significance, the wave function’s nature is still debatable. To date, different interpretations of the wave function—such as the De Broglie’s pilot wave interpretation, the Copenhagen interpretation and the many-word interpretation—have been put forward. Of these, the Copenhagen interpretation is a conventional and influential one. An interpretation like this considers the wave function simply as a complex probability amplitude, used to compute the probability of spotting the quantum object at a specified place. The wave function, in this case, is a pure mathematical tool and hence is presumed to only provide the understanding of phenomena. However, the Copenhagen interpretation does not describe the real occurrence of the quantum object. Therefore, investigating the nature of the wave function is of utmost significance for revealing the unearthed quantum realm.

In a latest research, Gui-Lu Long, a researcher from the Department of Physics, Tsinghua University, Beijing, China, put forward a realistic interpretation (REIN) for the wave function. The REIN describes a quantum object’s wave function as a real existence in contrast to just mathematical description, or, the quantum object in space occurs in the form of the wave function. In order to exhibit this, Gui-Lu Long and his colleagues, Wei Qin, Zhe Yang, and Jun-Lin Li, also from the Department of Physics, Tsinghua University, devised an encounter-delayed-choice experiment to experimentally execute the scheme. The research, titled “Realistic Interpretation of Quantum Mechanics and Encounter-Delayed-Choice Experiment,” has been reported in *Science China Physics, Mechanics and Astronomy*.

In that study in particular, the team demonstrated that a microscopic or quantum object is extended either in space or, in specific instances, even in disjointed regions of space, with phase and amplitude. The quantum object’s spatial distribution is represented by the square of the modulus of the wave function. Upon evaluation, the space-filling quantum object will obey the quantum mechanics measurement postulate and disintegrate immediately. In this case, the object functions like a particle. Due to the occurrence of a phase, two coherent wave functions might interfere when they encounter each other. As a result, the ensuing wave function will be modified variedly at varied locations: a specific proportion is strengthened by constructive interference, yet others are canceled by destructive interference. Put differently, this alters the quantum object’s spatial distribution. In this case, the object functions similar to a wave.

A good validation of the delayed-choice experiment is provided by the Mach-Zehnder interferometer (MZI), which is a two-path interferometer. Our analyses are confined to the case in which a single photon is directed toward the MZI after passing through two detectors. In conventional terms, the properties of the single photon inside the MZI are dependent on whether the second BS is in positioned accurately. In the absence of the second BS, the single photon travels along only one arm, revealing the characteristics of the particle. In contrast, when the second BS is inserted, the single photon moves along both the arms, thereby revealing characteristics of the wave. By contrast, in the REIN, the first BS disintegrates the single photon into two sub-waves respectively moving along the two arms, irrespective of insertion of the second BS. Or, the photon inside an MZI is a separated and extended object that occurs at both arms at the same instant. In this interpretation, upon absence of the second BS, the two sub-waves respectively move toward the two detectors, and with a possibility unrelated to their relative phase, the measurement disintegrates them into a click in one of the detectors. This is the single photon’s particle characteristic. Moreover, the existence of the second BS can result in interference between the two sub-waves and, in contrast, two resultant sub-waves respectively move toward to the two detectors. The single photon is in the form of the two resultant sub-waves. As a result, the measurement disintegrates the resultant sub-waves into a click in one of the detectors, at a phase-dependent probability. This is the single photon’s wave characteristic. As against the conventional interpretation, the REIN shows that a single photon in an open MZI and the one in a closed MZI do not differ before arriving at the second BS.

In order to give strength to this concept, the team also implemented an encounter-delayed-choice (EDC) experiment. In this experiment, a decision is made whether or not to insert the second BS when the two sub-waves moving at the same time along the two arms of the MZI encounter each other. It is distinct from earlier, or quantum, delayed-choice experiments in which the decision is taken before the encounter. In the case of EDC, the parts of the two-sub waves, subject to the second BS, will interfere, and their configurations modify with respect to the relative phase. However, the remaining parts that are not subject to the second BS do not interfere, hence their configurations remain unaltered. Therefore, the single photon can be split into two parts, where one exhibits the particle nature and the other exhibits the wave nature. Correspondingly, the sub-waves that leave the MZI can be split into two parts, one exhibiting the particle nature and the other exhibiting the wave nature. The data derived from the experiments performed in the study is in better accordance with that proposed by the REIN, indicating that the REIN concept is firmly supported.

“*This difficulty is pertinent to our stubborn notion of a rigid particle of microscopic object for a quantum object, as the name, ‘quantum particle’, suggests*” the researchers of the study wrote, “*If we adopt the view that the quantum object does exist in the form of the wave function, it is easier to understand this form change*.”

National Natural Science Foundation of China (No.11474181), National Basic Research Program of China (No. 2011CB9216002), and Open Research Fund Program of the State Key Laboratory of Low-Dimensional Quantum Physics, Tsinghua University funded this research.