Suppose you have a piece of clothing that you like very much, you have worn it for a long time and have a deep affection, but it has been with you for too long and looks old and outdated.
Suddenly, one day the school held a clothing donation event, and you didn't have any other old clothes except this one, so you donated it.
At this time, you realize that your feelings for this dress are complicated, and you have already had the idea of giving up this dress in the past.
Before the clothing donation activity, you may think that you love and are close to the dress, but you are faintly dissatisfied, but the clothing donation activity allows you to find that the original idea of abandonment prevails!
In psychology, the future can change the past and rewrite your past psychological feelings. It is not surprising that current events can affect our feelings and opinions about past experiences. But for physics, a discipline that describes objective facts, it would be a big deal to be able to revise the history of the past in the future.
Quantum delay selection experiments
The famous quantum delay selection experiment has given us a strange picture of future events that can change the past.
In classical physics, particles always have a definite position and propagation path, but this changes in the microscopic quantum world.
When a single photon is hit at a device with two slits one by one, if the light is a classical particle, then the photon will be blocked in the middle of the slit, but the experimental results tell us that we can find photons on the receiving screen, and not only that, but also find that the distribution of photons is regular.
Single-photon double-slit experiment[2]
This phenomenon reminds us of waves, which are physical images that can occupy multiple spatial sites. For example, how do you describe the spatial position of the water waves? Do we need to use many (infinite) spatial sites?
Image source network
Light is an electromagnetic wave, which can also be described in terms of waves.
Lightwave[2]
Due to the use of a physical image of light waves, light can pass through two slits at the same time, and the propagation path is no longer determined, and interference can be generated, and interference fringes appear on the receiving screen.
Double-slit test[2]
It should be noted that the interference fringes are obtained as the result of the experimental superposition of multiple emission of single photons.
Interference fringes obtained from multiple photon data collection[2]
The main idea of the quantum delay selection experiment can be described by the single-photon double-slit experiment.
Consider a simplistic, crude thought experiment.
Change the original single-photon experimental device, remove the receiving screen, and place two detectors to receive photons, if either detector receives photons, the path information and particle nature of light are explained; However, the original device on the receiving screen can see interference fringes, which illustrate the fluctuation of light.
To put or not to put the receiver board[2]
In other words, can the receiver board choose whether the light is a wave or a particle? If light passes through a slit before it is placed on the receiving board, will it be determined whether the light traveled in the form of particles or waves in the past will be placed or not placed on the receiving plate in the future? Why is the selection of light states delayed?
The flaw with this experiment is that the range of all possible positions of the received photon is much larger than the range of the positions of the two detectors, and neither detector may be able to detect the photon. But the main idea of the quantum delay selection experiment is described.
But if you can identify and describe this flaw clearly, then you already have the mode of thinking about quantum mechanics.
The quantum delay choice experiment is derived from the cosmic version of the delayed choice experiment proposed by Wheeler: when distant stars come from the depths of the universe, the gravitational pull of galaxies can attract photons, and if we receive enough photons, we can produce interference fringes on the negative, and when we place the telescope at the end of a certain path, we can explain the path information of the photons, thus destroying the interference pattern.
Wheeler Starlight Delay Experiment (Note: The two paths of starlight in the figure depict the same photon)[2]
If a photon comes from billions of light-years away, does it move along one path like a particle, or both, like a wave?
This choice should have been made a long time ago, but why was it delayed and had to do with how it was detected much later?
In 2007, a couple of physicists actually did this experiment, and overcoming the problem of the thought experiment described above, they split the propagation path of light in two, and now it can contain all the possible landing points of a single photon with just two detectors.
They designed an interferometer, and the first goal of the specific device is that a linearly polarized single photon passes through the beam splitter 1 and becomes spatially separated two linearly polarized light (P light, S light) with perpendicular polarization directions, which enter two paths (A\B) respectively, and finally the single photon is received by two detectors (1\2).
Figure a. Schematic diagram of the installation (source paper on the left)[1]
Beam splitter[2]
As shown in Figure A, each of the two detectors has half the probability of receiving a photon, as shown in Figure e(B): if detector 1 receives a photon, then the photon path is A; If detector 2 receives a photon, the photon path is B, which reflects the particle nature of light.
Figure b. Put on the beam splitter 2
The second goal of the experimental setup is to place beam splitter 2 to observe the behavior of the photons.
In fact, the beam splitter 2 consists of a half-wave plate, a polarizing beamsplitter BS′, an electro-optical modulator (EOM) with two optical axes at 22.5° (Fig. d) and a Wollaston prism WP.
The Wollaston Prism WP spatially separates two polarized lights that are in the direction of polarization perpendicular to each other, and finally receives them separately by the two detectors.
Wollaston Prism WP Separation P-light, S-Light
As shown in Figure c, in a specific experiment, the discharge or non-release of the beam splitter 2 is achieved by randomly varying the output value of the electro-optical modulator (EOM) (half-wave voltage Vπ or 0, the curve is shown in Figure c).
When the EOM output value is 0, which is equivalent to the non-release beam splitter 2, the experimental effect is as described above; When the output value is Vπ, which is equivalent to when the beam splitter 2 is put on, as shown in Figure d, the two polarization photons (P light, S light) produce a 45° phase change, respectively, and the P light and S light are reprojected, so that finally the WP mirror outputs to the polarization state of the photons of detector 1 and detector 2 and the beam splitter 2 that changes and has an interfering effect (D1 two optical components are eliminated, D2 two optical components are mutually beneficial).
Figure c.Specific experimental setup[1]
Figure d.Diagram of the state of photons inside beam splitter 2 in both cases[1]
As shown in Figure e(A) of the experimental results, photons will only appear on detector 1 or detector 2 under certain phase shifts generated by tilting BS' when a half-wave voltage Vπ is applied, which can only be caused by the complete cancellation or growth of the two light waves received by the detector.
Figure e.Statistical results of photons received by two detectors (red|blue) at different phases of the beam splitter 2 [1]
Figure f.The two beams of light incident on detector 2 are interfering with each other; Interference cancellation of two beams of light incident on detector 1 (equivalent effect plot)
How the quantum world describes particles
To understand what is happening in this experiment, we need to take a step forward and enter the door of the quantum world.
In quantum mechanics, the wave function is used to describe microscopic particles, and the wave function has a specific value at different spatial positions, and the square of this value describes the probability of finding an electron at a given time and a given location, so we often have the term probability wave.
Wave Function[2]
We often use wave-particle duality to describe quantum matter, and the wave function of microscopic particles is the perfect fusion of two physical images: waves and particles. For example, in the field of wave function definition, there is a numerical value that describes the probability of finding a particle at any coordinate position; Considering the possibility of multiple positions of particles, wave functions can be superimposed, etc., are typical of waves.
Wave-particle duality[2]
With the wave function as a descriptive tool, microscopic particles are like doppelgangers and can move along multiple paths at the same time.
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In the previous single-photon double-slit experiments, the light also traveled in the form of a wave function, so it was able to pass through two slits; When it falls on the receiving screen, the wave function collapses, and all the doppelgangers revert to the same photon, showing particles. The probability of falling to a point at different locations is different, so receiving a single photon multiple times can give the overall effect of interference fringes.
Simulation of photon wave function at different times during single-photon propagation in double-slit experiment (brightness represents the magnitude of probability)[2]
Quantum mechanics explains the phenomena we see with the wave function, but the cunning thing is that it does not allow you to see the whole picture of the wave function at once, and can only be shown through multiple experiments.
Quantum delay selection experiments
What's going on?
When we consider quantum delay experiments, we still unconsciously fall into the thinking mode of classical physics, using the physical picture of a single wave or particle to analyze the problem, so that there is a big loophole.
We are now analyzing problems within the theoretical framework of quantum mechanics, so we have to follow the rules of the description language of the quantum world, and narrate according to the concept of wave function, and the possibility of changing the past is completely eliminated.
In quantum delay choice experiments, putting or not putting the second beam splitter does not determine whether the photon's past state is a wave or a particle, because the photon's past state is described by the wave function, and we cannot presuppose a photon that is just a wave or just a particle.
Before the light reaches the detector, the photon wave function covers both paths A and B
When the beam splitter 2 is placed, only detector 1 (or 2) receives photons that do not prove that the light is a classical wave, because probability waves can also interfere with cancellation or growth; Detector 1 (or 2) receives a photon without beam splitter 2, nor does it account for a single path of the photon, because the photon propagates as a wave function containing information about the two paths before it is measured, only to collapse into a particle when observed by the detector.
Therefore, whether to release the beam splitter 2 or not has no effect on the past state of the photon, and the effect of the beam splitter 2 or the detector on the photon only occurs at the moment of contact with the photon, and cannot change the wave function describing the past state of light, and the light always evolves over time according to the laws of quantum mechanics.
Correct delusions
Going back to our original story, if you had another old dress at that time, you would probably keep the dress that had been with you for a long time, rather than give it up, and your mood towards it would still be more close and reluctant. In fact, the past has not changed, what has changed is your description and understanding of the past state.
In psychology, our feelings are complex, any description is true and reasonable, and our description of the past always bears traces of the present. But objective facts in physics are always unmodifiable.
Our classical education has always accustomed us to describe the behavior of light in terms of the concept of certainty, that light is either a particle, in order to determine the path forward; Either it is a wave, which can show interference fringes, but the quantum world is often uncertain, for example, the wave function is often diffuse, containing multiple states, which is the normal state of quantum, and is often transformed into a familiar certainty when observed, such as the collapse of the wave function of light into a particle state (a certain eigenstate).
The electrons in different energy states of atoms exhibit diffuse wave functions, and different brightness at different positions represent different probabilities of finding electrons [2]
Just as we can retell the story of the past in psychology based on the current situation, the transformation of quantum matter in the past in the form of waves or particles is just a change in the way we narrate based on the "stereotypes" left by past education; Objectively, quantum matter was in a mixed state described by the wave function in the past state, and was not affected by the way of observation.
Source: Popular Science China