User:MRFS/PMBT
The poor man's Bell test
[edit]Needless to say the conduct of a Bell test is a very costly business indeed. But the good news is that it's possible to run an accurate simulation in the comfort of one's own home at minimal expense. The only equipment required consists of two coins and an icosahedral die, commonly known as a D20 because it has 20 faces rather than the usual 6. The reason for using a D20 is that it can mimic the quantum probabilities to 2 decimal places, and thereby provide a reasonably faithful representation of the real Bell test.
So what are these probabilities and how is the test to be done? Detectors can only record electron spin as ±1. Given a pair of entangled electrons it's possible to set up detectors in two separate labs so that they never agree. One will measure its electron as +1 whilst the other will record -1 for its one. In this case the angle between the detectors is nominally 0°. Now rotate one of the detectors through 180° and the detectors will always agree. More generally, if the nominal angle between the detectors is γ then, according to quantum theory, the probability they will agree is sin2(γ/2).
The test itself involves four detectors positioned at what are known as the Bell test angles. Alice has Q at 135° and R at 225° whilst Bob has S at 0° and T at 90°. Thus the probabilities of mutual agreement are as follows:-
P(Q and S agree) = sin2(67½°) = 0.85 which is an 85% chance of agreement
P(Q and T agree) = sin2(22½°) = 0.15 which is a 15% chance of agreement
P(R and S agree) = sin2(112½°) = 0.85 which is an 85% chance of agreement
P(R and T agree) = sin2(67½°) = 0.85 which is an 85% chance of agreement
The D20 can exactly emulate these probabilities. 85% of the time it will roll a number from 1 to 17 inclusive. 15% of the time it will roll 18, 19 or 20.
As for the coins, instead of heads and tails one is marked Q and R for Alice's detectors. The other bears S and T to show Bob's possible choices. A 'trial' will consist of tossing the coins and rolling the D20. First decide on the number of trials and create a blank table like the one below to accommodate the results. The results of 100 trials are displayed below.
Once the preliminaries are over the simulation can begin. Simply run a series of trials and for each trial record the value shown by the D20 in the appropriate cell.
| Q and S | 4 | 14 | 5 | 2 | 4 | 19 | 8 | 17 | 17 | 12 | 9 | 7 | 18 | 4 | 20 | 13 | 6 | 10 | 1 | 2 | 17 | 19 | 11 | 1 | |||||
| Q and T | 14 | 17 | 3 | 2 | 19 | 9 | 5 | 7 | 3 | 12 | 3 | 18 | 15 | 4 | 3 | 5 | 17 | 11 | 4 | 4 | 12 | 11 | 18 | 11 | 20 | ||||
| R and S | 12 | 15 | 15 | 19 | 16 | 8 | 6 | 3 | 14 | 9 | 15 | 16 | 9 | 5 | 18 | 2 | 2 | 8 | 13 | 16 | 8 | 20 | 3 | 14 | |||||
| R and T | 11 | 7 | 2 | 7 | 3 | 12 | 6 | 1 | 18 | 1 | 13 | 14 | 17 | 7 | 19 | 7 | 20 | 10 | 1 | 20 | 8 | 12 | 15 | 18 | 8 | 11 | 6 |
The next stage is to replicate the table changing every entry to either an 'A' or a 'D' according to its value. In rows 1, 3 and 4 the probability of agreement is 85% so values from 1 to 17 inclusive qualify for an 'A'; values 18, 19, 20 are given a 'D'. But in row 2 the probability of agreement is only 15% so here 18, 19, 20 receive an 'A' and 1 - 17 get a 'D'.
| Q and S | A | A | A | A | A | D | A | A | A | A | A | A | D | A | D | A | A | A | A | A | A | D | A | A | E(QS) = 16/24 = 0.67 | ||||
| Q and T | D | D | D | D | A | D | D | D | D | D | D | A | D | D | D | D | D | D | D | D | D | D | A | D | A | E(QT) = − 17/25 = − 0.68 | |||
| R and S | A | A | A | D | A | A | A | A | A | A | A | A | A | A | D | A | A | A | A | A | A | D | A | A | E(RS) = 18/24 = 0.75 | ||||
| R and T | A | A | A | A | A | A | A | A | D | A | A | A | A | A | D | A | D | A | A | D | A | A | A | D | A | A | A | E(RT) = 17/27 = 0.63 |
The final column of each row shows the average (or expectation) of the row. It is given by (NA − ND)/(NA + ND) where NA and ND respectively are the number of 'A's and the number of 'D's in the row.
It only remains to calculate the Bell signal
S = E(QS) – E(QT) + E(RS) + E(RT) = 0.67 − (−0.68) + 0.75 + 0.63 = 2.73
which shows that the Bell/CHSH inequality has been well and truly violated. What a surprise!
However one rather puzzling aspect of this experiment remains. It has faithfully simulated the actual Bell test and has violated the Bell/CHSH inequality, yet it has somehow managed to avoid any mention of loopholes or hidden variables or nonlocality.
Interested readers are encouraged to run their own simulations and draw their own conclusions.