Exploring the realistic nature of the wave function in quantum mechanics

January 5, 2018, Science China Press
Exploring the realistic nature of the wave function in quantum mechanics
Inserting the second BS when the two sub-waves have an encounter, as in (a), can produce two resultant sub-waves, as in (b), if two-sub waves inside the MZI are in-phase. Credit: Science China Press

Quantum mechanics is a pillar of modern science and technology, and has benefited human society for a century. The wave function, also known as the quantum state, is the description of a quantum object and plays a central role in quantum mechanics. Nonetheless, the nature of the wave function is still debated. So far, there have been several interpretations of the wave function, including the Copenhagen interpretation, the De Broglie's pilot wave interpretation, and the many-world interpretation.

Among them, the Copenhagen dominates. It treats the wave function merely as a complex probability amplitude used to calculate the probability of finding the quantum at a given place. In this case, the wave function is a pure mathematical tool, and thus is supposed only to provide the knowledge of phenomena. However, the Copenhagen interpretation cannot describe the real existence of the quantum object. Hence, exploring the nature of the wave function is of fundamental importance for unlocking the mysterious quantum world.

In a recent study, a realistic interpretation (REIN) for the wave function was proposed by Gui-Lu Long, a researcher at the Department of Physics, Tsinghua University, in the Chinese city of Beijing. The REIN states that the wave function of a quantum object is an actual state rather than a mere mathematical description—in other words, the quantum object in space exists in the form of the wave function. To demonstrate this, Gui-Lu Long and his collaborators, Wei Qin, Zhe Yang and Jun-Lin Li, also from the Department of Physics, Tsinghua University, designed an encounter-delayed-choice experiment and experimentally realized the scheme. This study, titled "Realistic Interpretation of Quantum Mechanics and Encounter-Delayed-Choice Experiment," has been published in Science China Physics, Mechanics and Astronomy.

The researchers showed that a quantum or microscopic object is extended in space or even, in some cases, in disjointed regions of space, with amplitude and phase. The square of the modulus of the wave function represents the of the quantum object. When measured, the space-filling quantum object will, according to the measurement postulate in , collapse instantaneously. In this case, the object behaves as a particle. Owing to the existence of a phase, the interference between two coherent wave functions can occur when they have an encounter. Consequently, the resultant wave will change differently at different locations: Some are strengthened due to constructive interference, whereas some others are canceled due to destructive interference. This changes the spatial distribution of the quantum object. In this case, the object behaves as a wave.

A good demonstration of the delayed-choice experiment is given by a two-path interferometer, the Mach-Zehnder interferometer (MZI). Our discussion is restricted to the case where a single photon is directed to the MZI followed by two detectors. According to the traditional perspective, the nature of the single photon inside the MZI depends on whether or not the second BS is in place. If the second BS is absent, the single photon then travels along just one arm, showing the particle nature.

On the contrary, when the researchers inserted the second BS, the single photon traveled along both arms, exhibiting the wave nature. However, in the REIN, the first BS splits the single photon into two sub-waves traveling along the two arms, whether the second BS is inserted or not. That is, the photon in an MZI is an extended and separated object that exists simultaneously at both arms. In this interpretation, if the second BS is absent, the two sub-waves are directed, respectively, to the two detectors, and with a probability independent of their relative phase, the measurement collapses them into a click in one detector. This is the particle nature of the single photon.

Furthermore, the presence of the second BS can cause the two sub-waves to interfere and, instead, two resultant sub-waves are directed to the two detectors. The single photon exists in the form of the two resultant sub-waves. As a consequence, the measurement collapses the resultant sub-waves into a click in one detector, with a phase-dependent probability. This is the wave nature of the single photon. In contrast to the traditional interpretation, the REIN demonstrates that there is no difference between a single photon in a closed MZI and a photon in an open one before they arrive at the second BS.

To support this idea, the researchers also implement an encounter-delayed-choice (EDC) experiment. In the experiment, the second BS is inserted or not when the two sub-waves traveling simultaneously along the two arms of the MZI have an encounter, as shown in Fig. 1(a). It is different from previous (or quantum) delayed-choice experiments where the decision is made before the encounter occurs. In the EDC case, the parts, subject to the second BS, of the two-sub waves, will interfere and their forms change according to the relative phase. But the remaining parts, not subject to the second BS, will not interfere, leaving their forms unchanged. The single can therefore be divided into two parts, one showing the wave nature and one showing the particle nature. Correspondingly, as depicted in Fig. 1(b), the sub-waves having left from the MZI can be divided into two parts, one from the wave nature and the other from the particle nature. Note that Fig. 1(b) shows a special case where the two-sub waves inside the MZI are in phase. The experimental data in the article is in good agreement with the prediction of the REIN, implying that the REIN idea is strongly supported.

"This difficulty is pertinent to our stubborn notion of a rigid particle of microscopic object for a quantum object, as the name, ' particle', suggests," the researchers write. "If we adopt the view that the does exist in the form of the , it is easier to understand this form change."

Explore further: Quantum experiment verifies Einstein's 'spooky action at a distance'

More information: GuiLu Long et al, Realistic interpretation of quantum mechanics and encounter-delayed-choice experiment, Science China Physics, Mechanics & Astronomy (2017). DOI: 10.1007/s11433-017-9122-2

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RobertKarlStonjek
not rated yet Jan 05, 2018
[1] The researchers claim 'a realistic interpretation (REIN)' and then immediately diverge into an entirely different unrealistic interpretation.

We can be informed by Einstein's two postulates which basically say that the physics of all inertial frames are the same, a restatement of Galileo's observations that there is nothing special in a ship travelling at speed on a dead still ocean. Einstein was specifically thinking about relativity, that even if your spaceship was travelling at great speed relative to a stay-at-home twin or the laboratory frame that you would not experience any time dilation or length contraction even though the laboratory frame does measure these. In fact the rocket traveller measures length contraction in the direction of travel, that is, the outside world undergoes relativistic changes.
RobertKarlStonjek
not rated yet Jan 05, 2018
[2] Using this as a starting point we can assume that an observer commoving with a superposed particle would not experience anything unusual, as Einstein states 'the physics remains the same'. But there would be quantum-like changes in the outside world.

This symmetry of perspectives appears to be entirely missed and actual superposition, uncertainty and entanglement is assumed to be actual manifestations rather than, as in the case for relativistic changes, the phenomena exists **between** frames, not inside them.

Holding to this simple observation entirely extinguishes quantum weirdness.
Whydening Gyre
5 / 5 (2) Jan 05, 2018
direct copy and paste from 1st paragraph of article;
... So far, there have been several interpretations of the wave function, including the Copenhagen interpretation, the De Broglie's pilot wave interpretation, and the ' many-word interpretation. '
The last of which describes a great many attempts to explain quantum mechanics...:-)

Seeker2
not rated yet Jan 06, 2018
Until they demonstrate a diffraction pattern with a single photon I'll stick with Copenhagen thanks.
mackita
not rated yet Jan 06, 2018
Until they demonstrate a diffraction pattern with a single photon I'll stick with Copenhagen thanks
The double slit experiment has been already demonstrated with single electrons (by Akira Tonomura and co-workers at Hitachi in 1989). If we would use sufficiently short-wavelength photons, we could demonstrate it even with single photons.
Seeker2
not rated yet Jan 06, 2018
The double slit experiment has been already demonstrated with single electrons (by Akira Tonomura and co-workers at Hitachi in 1989). If we would use sufficiently short-wavelength photons, we could demonstrate it even with single photons.
Strange considering https://www.physi....652291/
Anonym
not rated yet Jan 06, 2018
Eds: Please fix the typo in the lede paragraph.
Anonym
5 / 5 (1) Jan 06, 2018
The following graf seems to be a cut and paste from the source article, or a bad translation by the original editors. At any rate, what is a "BS"? And who is the self-referential "our"?

"A good demonstration of the delayed-choice experiment is given by a two-path interferometer, the Mach-Zehnder interferometer (MZI). Our discussion is restricted to the case where a single photon is directed to the MZI followed by two detectors. According to the traditional perspective, the nature of the single photon inside the MZI depends on whether or not the second BS is in place. If the second BS is absent, the single photon then travels along just one arm, showing the particle nature."
Ojorf
3.7 / 5 (3) Jan 06, 2018
Until they demonstrate a diffraction pattern with a single photon I'll stick with Copenhagen thanks.


It has been demonstrated.
In a single slit experiment there are places on the detector where single photons hit all the time, but where they don't hit after opening a second slit. It has to be due to it interfering with itself.
Ojorf
3 / 5 (2) Jan 06, 2018
Seeker2
not rated yet Jan 06, 2018
See here: http://hyperphysi....html#c1
This is for coherent light which it seems would be multiple photons
mackita
1 / 5 (1) Jan 06, 2018
Single-particle interference was already observed for macroscopic objects The water surface analogy of double slit experiments shows clearly what happens there: the particles makes "wake wave" of vacuum around itself (so-called pilot wave in deBroglie theory), which interferes with obstacles and it creates a flabelliform areas of less or more undulating vacuum around (including before) particle. The undulating water surface also looks like more dense obstacle for particles and waves spreading along it - simply because it's more deformed than the water surface at rest. The particles therefore just follows the path of least resistance after then, because more undulating vacuum behaves like the foam shaken and it gets more dense along these paths.
domanite
not rated yet Jan 07, 2018
Eds: Please fix the typo in the lede paragraph.

Please don't. Sometimes the typo is better than the original.
RNP
5 / 5 (1) Jan 09, 2018
@Anonym
...What is BS...


BS stands for "beam splitter",

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