TYPES OF ENTANGLEMENT (आरादुपकारक & संनिपत्योपकारक) – 1. Shri Basudeba Mishra
The 2022 Nobel Prize in Physics was awarded for experiments with entangled photons to facilitating quantum information processing. The “quantum” part of a quantum computer is the use of entanglement principles in information processing, where Bell’s inequality plays a major roll to rule out hidden variables, which may contain information about a particle’s future state and exist imperceptible beneath some subatomic level.
Quantum entanglement stipulates that we can’t describe our joint quantum system (अङ्गी) in terms of local descriptions – one for each system (अङ्ग). Entangled particle pairs are said to affect each other’s “choices” instantaneously, irrespective of the distance between them. Symmetry describes one such entangled pair. According to Quantum Mechanics (QM) the two spin states (a quantum mechanical property) of individual particles are in a “superposition of both states” simultaneously (like the Schrodinger’s cat – Schrödinger himself proposed this as a reductio ad absurdum, though now it is used for the opposite purpose). But when one of the pair is measured, both “collapse” permanently to a fixed complementary spin state – up or down – as if they were telepathically connected to each other – a type of teleportation.
Einstein called it “spooky action at a distance”. His EPR thought experiment examined entanglement that could last for infinite distances. Since he believed that information can’t travel faster than light, the two particles, if placed sufficiently apart, can’t communicate with each other. Thus, there is nothing as “collapse”. In real life experiments, it is found that entanglement lasts only over a finite distance – the maximum being a little over 33 kilometers, though one experiment reported the distance at about 1200 kilometers. Beyond that, like a rubber band stretched enough, it snaps.
In the Young’s double-slit experiment, if one puts a detector at one or the other of the two slits, then an interference pattern does not appear at the detector. It is assumed that it so happens because one has set up an experiment in which one can “know” which slit the particle went through. As long as one can tell this, the particle can’t go through both slits at once and one no longer gets an interference pattern. Even when one is actually not observing, the potential of being known (i.e., “knowability”) is enough to stop the formation of an interference pattern. All ability to detect which path a photon took must be removed to obtain interference. In other words, it was assumed that one must not be able to tell, even in principle, which path the elementary particle took in order for it to “take” both paths and form an interference pattern.
Renewed interest in the double slit experiment was interpreted to mean that the Schrödinger equation describes a wave-splitting process with a “probability” wave in both channels and then an instantaneous “collapse” of a potential existence in a state of “being” to one localized “actual” spot – to either detectors A or B – “becoming”. The radiation “becomes” a particle only after we measure it. Thereafter it is always a particle – changed permanently even though some measurements suggest that the particle has wave-like properties between successive measurements. This can produce some non-classical effects. One such example is the phenomenon known as “quantum beats”. Such debate has led to experiments on the nature of “reality” versus “locality”.
In the macro world, we can affect objects only if we can touch them directly or indirectly through a chain of connections. This is called local. But some physicists say that the question of locality is meaningless as the particles do not have any position at all. The problem is not epistemological (about what we know), but ontological (about what it is). Particles can also be entangled quantum-mechanically. The entangled property can be location, charge, spin, etc. Entanglement is a connection between particles irrespective of where they are, what they are and which forces they exert on one another. Thus, entanglement makes for an intimacy between particles (part of the whole) irrespective of the distance between them – creating action at a distance. This is the concept of non-locality.
Special Relativity, which deals with optics and electrodynamics, tends to mix up space and time in a way that transforms entanglement among distinct physical systems into something along the lines of an entanglement among physical situations at different times (systems have fixed position, but evolve in time). This, in a perfectly concrete way, exceeds or eludes or has nothing to do with any sum of situations at distinct temporal instants. This result, like most theoretical results in quantum mechanics, involves manipulating and analyzing the wave-function. It is from the wave-function that the possibility of entanglement is inferred. Thus, the wave-function lies in the heart of puzzles about the non-local effects of quantum mechanics.
Unless an observation is made and communicated, the questions relating to the state of a system are meaningless. We get the specific result based on our experimental set up. If we are looking for a particular state of a system, we will get results related to that. It cannot describe the influences that led to the particular state. If we measure temperature of a patient, we get a result, from which we infer whether the patient has fever or not. But that cannot explain why or how he got the fever. Without measuring the temperature, the question of whether the person has fever or not is meaningless, but not indeterminate. It cannot be said that taking the temperature created fever or cured fever. The same principle applies in the quantum world also. The results of observation depend on the state as it is at the time of observation, which is at least at the next moment. There is a temporal barrier between the state at t and the time of measurement t’. Whether the measured state is local or non-local can’t be found out from such measurements of state.
A wave does not have a fixed position: it is “spread out” through space. Thus, one can’t find the exact coordinates of a “traveling photon” not because it is information that can’t be obtained, but because there is no such thing as an exact position for it. A photon only has a “position” when it is absorbed, which “collapses” it to the point where it is absorbed. But then, it is no longer traveling, and its “wave” characteristics can’t be measured since it no longer exists. This is why we can’t simultaneously measure both wave-states and particle-states: the two states can never exist simultaneously.
The uncertainty relation is a logical necessity from the nature of quanta, and thus a remarkable validation of logic itself. At the core of logic lie the Law of Identity (a thing is itself – अस्तित्व) and its corollary, the Law of Non-Contradiction (a thing cannot simultaneously be both “X” and “not X” – विपर्यासः). Quantum nature puts a barrier to our normal perception, so that we can visualize it only indirectly (उपायप्रत्ययः). It is said that we can visualize and verbalize its qualities in the mutually contradictory terms of waves and particles. Yet the laws of logic demand that, to the extent that the wave-nature and particle-nature of quanta are true descriptions of their real nature, both cannot exist simultaneously: this would violate the Law of Non-Contradiction. The only solution is all particles move in waves (तिरश्चीनो विततो रश्मिरेषामधः स्विदासी३दुपरि स्विदासी३त्), though it may not be perceptible in the macro world. I can prove it.
The above description brings out three prominent features of reality as follows:
- It must have independent cognizable existence (अस्तित्व) that can be determined directly or indirectly through measurement.
- The results of measurement must be intelligible to a conscious agent (ज्ञेयत्व).
- Such characteristics must be expressible – communicable to other intelligent beings (अभिधेयत्व) to ensure that perceptions of the particular characteristics are universal in its nature.
All experiments till date of the double-slit experiment have been carried out by using light or photons. Some used electrons. But scientists are NOT clear about WHAT an electron is, though they know all about its nature and function. Claims that the double-slit experiment has been conducted using metastable helium or rubidium atoms are highly misguiding. Effectively, they used radiation. Conducting the experiment using photons will yield different results – the same as in the macro-world. I have tested with small boars in multi-channel rivers. Hence, it is necessary to understand fermions and bosons, because their classification based on spin is controversial.
(to be continued)
