Can Singlets be Prepared with a Preferred Direction?

[anorlunda]

Professor Susskind said that the expectation value of all three sigma operators acting on a singlet pair are zero. Does that mean that a singlet pair can not be prepared in a preferred direction?

 

[The_Duck]

I assume you are talking about the singlet (total L = 0) state of two spin-1/2 particles? This state has no preferred direction: it is spherically symmetric (this is true of any L = 0 state, no matter how complicated).

This causes no particular trouble in preparing a singlet state of two spin-1/2 particles. For example, the ground state of hydrogen (which consists of two spin-1/2 particles, the proton and the neutron) is the spin singlet state, so hydrogen atoms naturally fall into this state. If the singlet state were actually an excited state, though, we could still easily prepare hydrogen atoms in the singlet state by exciting them to the appropriate energy level with e laser.

 

[Meir Achuz]

There is no preferred direction with a singlet spin state.