Mermin's device

Each detector (A and B) has a switch with three configurations labeled (1,2 and 3) and a red and a green light bulb. Either the green or the red light will turn on (never both) when a particle enters the device after a given period of time. The light bulbs only emit light in the direction of the observer working on the device.

Each detector (A and B) has a switch with three configurations labeled (1,2 and 3) and a red and a green light bulb. Either the green or the red light will turn on (never both) when a particle enters the device after a given period of time. The light bulbs only emit light in the direction of the observer working on the device.

Additional barriers or instrument can be put in place to check that there is no interference between the three parts (A,B,C), as the parts should remain as independent as possible. Only allowing for a single particle to go from C to A and a single particle from C to B, and nothing else between A and B (no vibrations, no electromagnetic radiation).

Additional barriers or instrument can be put in place to check that there is no interference between the three parts (A,B,C), as the parts should remain as independent as possible. Only allowing for a single particle to go from C to A and a single particle from C to B, and nothing else between A and B (no vibrations, no [[electromagnetic radiation]]).

The experiment runs in the following way. The button of the source C is pushed, particles take some time to travel to the detectors and the detectors flash a light with a color determined by the switch configuration. There are nine total possible configuration of the switches (three for A, three for B).

The experiment runs in the following way. The button of the source C is pushed, particles take some time to travel to the detectors and the detectors flash a light with a color determined by the switch configuration. There are nine total possible configuration of the switches (three for A, three for B).

:<math>|\Psi^-\rangle=\frac{|\uparrow\downarrow\rangle-|\downarrow\uparrow\rangle}{\sqrt{2}}</math>,

:<math>|\Psi^-\rangle=\frac{|\uparrow\downarrow\rangle-|\downarrow\uparrow\rangle}{\sqrt{2}}</math>,

to leave the experiment, where <math>|\uparrow\downarrow\rangle</math> (<math>|\downarrow\uparrow\rangle</math>) is the state where the projection of the spin of particle 1 is aligned (anti-aligned) with a given axis and particle 2 is anti-aligned (aligned) to the same axis. The measurement devices can be replaced with [[Stern–Gerlach experiment|Stern–Gerlach devices]], that measure the spin in a given direction. The three different settings determine whether the detectors are vertical or at ±120° to the vertical in the plane of perpendicular to the line of flight of the particles. Detector A flashes green when the spin of the measured particle is aligned with the detector's magnetic field and flashes red when anti-aligned. Detector B has the opposite color scheme with respect to A. Detector B flashes red when the spin of the measured particle is aligned and flashes green when anti-aligned. Another possibility is to use [[photon]]s that have two possible [[Polarization (physics)|polarizations]], using [[polarizer]]s as detectors, as in [[Aspect's experiment]].

to leave the experiment, where <math>|\uparrow\downarrow\rangle</math> (<math>|\downarrow\uparrow\rangle</math>) is the state where the projection of the spin of particle 1 is aligned (anti-aligned) with a given axis and particle 2 is anti-aligned (aligned) to the same axis. The measurement devices can be replaced with [[Stern–Gerlach experiment|Stern–Gerlach devices]], that measure the spin in a given direction. The three different settings determine whether the detectors are vertical or at ±120° to the vertical in the plane of perpendicular to the line of flight of the particles. Detector A flashes green when the spin of the measured particle is aligned with the detector's [[magnetic field]] and flashes red when anti-aligned. Detector B has the opposite color scheme with respect to A. Detector B flashes red when the spin of the measured particle is aligned and flashes green when anti-aligned. Another possibility is to use [[photon]]s that have two possible [[Polarization (physics)|polarizations]], using [[polarizer]]s as detectors, as in [[Aspect's experiment]].

Quantum mechanics predicts a probability of measuring opposite spin projections given by

Quantum mechanics predicts a probability of measuring opposite spin projections given by