3.7 The EPR Conundrum

According to the philosophical view called realism, reality exists and has definite properties irrespective of whether they are observed by some agent. Motivated by this view point, Einstein, Podolsky _and Rosen _[ 55] suggested a classical argument to “show" that quantum mechanics is incomplete. EPR assumed (a) the non-existence of action-at-a-distance, (b) that some of the statistical predictions of quantum mechanics are correct, and (c) a reasonable criterion defining the existence of “an element of physical reality". They considered a system of two spatially separated but quantum mechanically correlated particles. A “mysterious” feature appears: By counterfactual reasoning, quantum mechanical experiments yield outcomes which cannot be predicted by quantum theory; hence the quantum mechanical description of the system is incomplete!

One possibility to complete the quantum mechanical description is to postulate additional “hidden-variables" in the hope that completeness, determinism and causality will be thus restored. But then, another conundrum occurs: Using basically the same postulates as those of EPR, Bell [8, 9] showed that no deterministic local hidden-variables theory can reproduce all statistical predictions of quantum mechanics.

We will present first Mermin’s [97, 98] two simple devices that explain EPR conundrum (see [29] for an automata-theoretic analysis).

Mermin’s EPR device [97] has three “completely unconnected" parts, two detectors (D1) and (D2) and a source (S) emitting particles. The source is placed between the detectors: whenever a button is pushed on (S), shortly thereafter two particles emerge, moving off toward detectors (D1) and (D2). Each detector has a switch that can be set in one of three possible positions – labelled 1,2,3 – and a bulb that can flash a red () or a green () light. The purpose of lights is to “communicate" information to the observer. Each detector flashes either red or green whenever a particle reaches it. Because of the lack of any relevant connections between any parts of the device, the link between the emission of particles by (S), i.e. as a result of pressing a button, and the subsequent flashing of detectors, can only be provided by the passage of particles from (S) to (D1) and (D2). Additional tools can be used to check and confirm the lack of any communication, cf. [97], p. 941.
mermin

Figure 7: Mermin’s experiment
The device is repeatedly operated as follows:
1. the switch of either detector (D1) and (D2) is set randomly to 1 or 2 or 3, i.e. the settings or states 11, 12, 13, 21, 22, 23, 31, 32, 33 are equally likely,
2. pushing a button on (S) determines the emission toward both (D1) and (D2),
3. sometime later, (D1) and (D2) flash one of their lights, or ,
4. every run is recorded in the form , meaning that (D1) was set to state and flashed and (D2) was set to and flashed
For example, the record means “(D1) was set to 3 and flashed and (D2) was set to 1 and flashed ".


Long recorded runs show the following pattern:
(a) For records starting with i.e. 11, 22, 33, both (D1) and (D2) flash the same colours, , with equal frequency; and are never flashed.
(b) For records starting with , i.e. 12, 13, 21, 23, 31, 32, both (D1) and (D2) flash the same colour only 1/4 of the time ( and come with equal frequencies); the other 3/4 of the time, they flash different colours (), occurring again with equal frequencies.
Of course, the above patterns are statistical, that is they are subject to usual fluctuations expected in every statistical prediction: patterns are more and more “visible" as the number of runs becomes larger and larger.


The conundrum posed by the existence of Mermin’s device reveals as soon as we notice that the seemingly simplest physical explanation of the pattern (a) is incompatible with pattern (b). Indeed, as (D1) and (D2) are unconnected there is no way for one detector to “know", at any time, the state of the other detector or which colour the other is flashing. Consequently, it seems plausible to assume that the colour flashed by detectors is determined only by some property, or group of properties of particles, say speed, size, shape, etc. What properties determine the colour does not really matter; only the fact that each particle carries a “program" which determines which colour a detector will flash in some state is important. So, we are led to the following two hypotheses:
H1 Particles are classified into eight categories:




H2 Two particles produced in a given run carry identical programs.
According to H1–H2, if particles produced in a run are of type , then both detectors will flash in states 1 and 3; they will flash if both are in state 2. Detectors flash the same colours when being in the same states because particles carry the same programs.


It is clear that from H1–H2 it follows that programs carried by particles do not depend in any way on the specific states of detectors: they are properties of particles not of detectors. Consequently, both particles carry the same program whether or not detectors (D1) and (D2) are in the same states.



We are ready to argue that
[L] _For each type of particle, in runs of type (b) both detectors flash the same colour at least one third of the time. _
If both particles are of types or , then detectors will flash the same colour all the time. For particles carrying programs containing one colour appearing once and the other colour appearing twice, only in two cases out of six possible combinations both detectors will flash the same light. For example, for particles of type , both detectors will flash if (D1) is in state 1 and (D2) is in state 3 and vice versa. In all remaining cases detectors will flash different lights. The argument remains the same for all combinations as the conclusion was solely based on the fact that one colour appears once and the other twice. So, the lights are the same one third of the time.

The conundrum reveals as a significant difference appears between the data dictated by particle programs (colours agree at least one third of the time) and the quantum mechanical prediction (colours agree only one quarter of the time):
under H1–H2, the observed pattern (b) is incompatible with [L].
Based on Greenberg, Horne and Zeilinger’s [64] version of EPR experiment, Mermin [98] imagined a new device, let us call it GHZ, to show quantum nonlocality. The device has a source and three widely separated detectors (A), (B), (C), each of which has only two switch settings, 1 and 2. Any detector, when triggered, flashes red () or green (). Again, detectors are supposed to be far away from the source and there are no connections between the source and detectors (except those induced by a group of particles flying from the source to each detector).

The experiment runs as follows. Each detector is in a randomly chosen state (1 or 2) and then by pressing a button at the source a trio of particles are released towards detectors; each particle will reach a detector and, consequently, each detector will flash a light, green or red. There are eight possible states, but for the argument we need to take into consideration only those for which the number of 1’s is odd, i.e. 111, 122, 212, 221.

According to [64], (a) if one detector is set to 1 (and the others to 2), then an
odd number of red lights always flash, i.e. , and they are equally likely, (b) if all detectors are set to 1, then an odd number of red lights is never flashed:

It is immediate that in case (a) knowing the colour flashed by two detectors, say (A) and (B),
determines uniquely the colour flashed by the third detector, (C). The explanation can come only because particles are emitted by the same source (there are no connections between detectors). A similar conclusion as in the case of EPR device reveals: particles carry programs instructing their detectors what colour to flash. Any particle carries a program of the form telling its detector to flash colour if in state 1 and colour if in state 2. There are four types of programs: . A run in which programs carried by the trio of particles are of types () will result in if the states were 122, in if the states were 212, and in if the states were 221. This is an illegal set of programs as the number of ’s is not odd (in , for example). A legal set of programs is as it produces on 122, 212, 221. There are eight legal programs,

out of 64 possible programs.

The conundrum reveals again as none of the above programs respects (b), i.e. it is compatible with the case 111.
A single 111 run suffices to prove inconsistency! Particle programs require an odd number of ’s to be flashed on 111, but quantum mechanics prohibits this in every 111 run.

Next we are going to present a probabilistic automaton simulating Mermin’s experiment [29]. The states of the automaton are all combinations of states of detectors (D1) and (D2), the input alphabet models the lights, red and green, , the output alphabet captures all combinations of lights flashed by (D1) and (D2), and the output function modeling all combinations of green/red lights flashed by (D1) and (D2) in all their possible states, is probabilistically defined by:

 























For example, with probability 1/2, with probability 0, with probability 0, with probability 1/2, with probability 1/8, with probability 3/8, with probability 3/8, with probability 1/8, etc.

The automaton transition is
not specified. In fact, varying all transition functions we get a class of Mermin EPR automata:
,


where .

Are there two identical, spatially separated, probabilistic automata with identical initial states, whose direct product “simulates" a Mermin’s EPR automaton? More formally, are there two probabilistic automata

such that their direct product is isomorphic to a Mermin’s automaton , i.e.,

, and , for all ?

The answer is
negative. In fact, a stronger result is true:
no single state of any Mermin’s EPR probabilistic automaton can be simulated by the product of the corresponding states of any probabilistic automata .
Indeed, . For a state we get the following contradictory relations:

For a state with we, again, get two contradictory relations:

Every Mermin’s EPR probabilistic automaton has strong correlations preventing it from being decomposed as a direct product of two independent probabilistic automata, no matter what transitions and output functions.



If, without in any way disturbing a system, we can predict with certainty (i.e. with probability equal to unity) the value of a physical quantity, then there exists an element of physical reality corresponding to this physica quantity. See [55], p. 777.
There are no relevant connections, neither mechanical nor electromagnetic.
10A particle of type XYZ will cause a detector in state 1 to flash X; a detector in state 2 will flash Y and a detector in state 3 will flash Z.
The emitting source (S) has no knowledge about the states of (D1) and (D2) and there is no communication among any parts of the device.