Weak Quantum Measurement, or Resuscitation for Schrödinger's Cat

Today, wandering around the Internet, I came across a description of one experiment by a group of American scientists led by Nadab Katz, who were able to reverse the collapse of the qubit wave function, thereby confirming the theory of Alexander Korotkov and Andrew Jordan.



While the scientific community is very wary of this result; However, if Korotkov-Jordan’s theory is correct (and it seems to be true), then this will lead to a revolution, first in quantum mechanics, then in modern physics, and then simply turn the world around. Exactly.



Now I will try to explain why.

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I think we all know well the thought experiment with Schrödinger's cat. Let me remind you, just in case: a cat is placed in a closed box, a vial of poison and an unstable nucleus, which disintegrates in the next hour with a probability of 50%. In this case, if the nucleus disintegrates - the ampoule with poison will open and the cat will die.



From the point of view of quantum mechanics (more precisely, the Copenhagen interpretation of quantum mechanics), until we opened the lid and checked the state of the cat, it will be in the superposition of both states - both alive and dead at the same time. At the moment of measuring the state of a cat, a so-called wave function collapse - the system will choose one of two possible states, and the cat will either live on or will die forever.



In general, any quantum system is in the superposition of all its possible states until a measurement takes place and it takes one of the possible states.



And so, in 2006, Korotkov and Jordan publish an article with the headline "Uncollapsing the wave function by undoing quantum measurements" , which can be roughly translated into Russian as "Undoing the collapse of the wave function by canceling the quantum measurement."



Its meaning is as follows: well, a direct measurement of the state of the system will lead to the collapse of the wave function and the choice of one of the states. But let's say that we can measure the state of the system not directly, but with the help of the so-called weak measurement ( weak measurment ) - such a measurement, in which the measuring device weakly interacts with the object being measured and the complete collapse of the wave function does not occur. Then, according to Korotkov and Jordan, we can not only measure an indirect condition, but also do it several times; in this case, if the result of the experiment does not suit us, we can return the system back and try again.



If we apply Korotkov-Jordan’s theory to Schrödinger's cat, it will look something like this: we can not open the box, but slightly open it and look at it with one eye; if we see that the cat is dead, we can close the box, wait a while, look again and see the cat alive!



It sounds crazy and fantastic, but in 2008 Nadav Katz and his group managed to conduct an experiment in which they actually performed a similar sequence of actions.



The Kac group has placed the qubit in a state of superposition from its both high and low energy states. Any attempt to directly measure the energy of a qubit would lead to the collapse of the wave function and the transition of the qubit to one of two states.



Further, the following happened: using a weak measurement of magnetic fields, the Kac team received information on the state of the qubit (0 or 1). In this case, the qubit partially collapsed in the direction of the measured state. If this state turned out to be zero, the Korotkov-Jordan process of “canceling the collapse” was carried out: the qubit received a magnetic pulse, which transferred it to the strictly opposite state, after which exactly the same weak measurement of its state was made. The result of these three operations (weak measurement of state 1 - reverse - weak measurement of state 2) was the transfer of the qubit to exactly the same state of uncertainty as it was before all these manipulations (i.e., two measurements actually cancel each other's influence). In other words, scientists measured the state of the “cat,” and, if he was dead, again returned him to a state of uncertainty.



Here is a detailed description of the experiment, if someone does not believe: the original , the article in Russian .



Let's talk now about the consequences to which this could potentially lead.

Einstein-Podolsky-Rosen Paradox

In 1935, Einstein, with a group of like-minded people, attempting to challenge the Copenhagen interpretation of quantum mechanics, formulated a paradox, later called the Einstein-Podolsky-Rosen paradox (ERP paradox).



The Heisenberg uncertainty relation states that it is impossible to measure absolutely exactly both the position and the momentum of a quantum system. Well, said Einstein, and if we take a system of two identical particles, and measure the momentum of one of them, and the position of the other, will we violate the principle of uncertainty?



As it turned out, no. Measuring the state of one of the "entangled" particles led to the collapse of the state of another particle, no matter how far they are from each other. This phenomenon was repeatedly confirmed experimentally - in the strongest experiment, the particles were located at a distance of 144 kilometers from each other.



Thanks to quantum confusion, so-called quantum cryptography. Suppose Anna and Boris have a box of tangled photons. They leave for different countries, each taking half photons with them. Next, Anna measures the state of the first photon. His pair in Boris’s box automatically assumes the same condition. So You can generate an encryption key that only Anna and Boris will know at the same time. Further, the message is encrypted with this key and forwarded.



Although the collapse of tangled particles occurs instantly, the transfer of information, as was believed until recently, is thus impossible, since Anna does not have the ability to influence the choice of state by her photon. But Korotkov and Jordan suggested, and the Katz group experimentally verified that there was such a possibility!



If Anna and Boris had a box of tangled qubits, Anna, using the procedure for canceling the collapse of the wave function, could “get” the first qubit out of the box as many times as he wanted, until he finally accepts the state that Anna needs. In this case, Boris's kubit will take exactly the same state - it turns out that Anna gives Boris information INSTANTLY.



But the instant transfer of information - only the tip of the iceberg. According to Korotkov-Jordan's theory, we can cancel the results of any experiment (for the time being, however, with the proviso “quantum”), if it does not suit us, and try again - yes generally, as many times as we like, until we get an acceptable result.



In fact, if the reasoning described above can indeed be reproduced, then we are standing somewhere on the verge of completely incredible changes in physics, which then will change our life beyond recognition.



UPD . A number of commentators accuse me of ignorance; I confess it easily, because I have never been a theoretical physicist and wrote everything as I understood it. If someone understands the question better than me - I would read the post with great interest with a clarification / deeper description of the issue.