Editing Singlet state (section)

== Examples ==
The simplest possible angular momentum singlet is a set (bound or unbound) of two [[spin-1/2]] (fermion) particles that are oriented so that their spin directions ("up" and "down") oppose each other; that is, they are antiparallel.

The simplest possible '''bound''' particle pair capable of exhibiting the singlet state is [[positronium]], which consists of an [[electron]] and [[positron]] (antielectron) bound by their opposite electric charges. The electron and positron in positronium can also have identical or parallel spin orientations, which results in an experimentally-distinct form of positronium with a spin&nbsp;1 or triplet state.

An '''unbound''' singlet consists of a pair of entities small enough to exhibit quantum behavior (e.g. particles, atoms, or small molecules), not necessarily of the same type, for which four conditions hold:
# The spins of the two entities are of equal magnitude.
# The current spin values of both entities originated within a single well-defined quantum event ([[wave function]]) at some earlier location in classical space and time.
# The originating wave function relates the two entities in such a way that their net [[angular momentum]] must be zero, which in turn means that if and when they are detected experimentally, conservation of angular momentum will require their spins to be in full opposition (antiparallel).
# Their spin states have remained unperturbed since the originating quantum event &ndash; which is equivalent to asserting that there exists no classical information (observation) of their status anywhere within the universe.

Any spin value can be used for the pair, but the [[Quantum entanglement|entanglement]] effect will be strongest both mathematically and experimentally if the spin magnitude is as small as possible, with the maximum possible effect occurring for entities with spin-1/2 (such as electrons and positrons). Early thought experiments for unbound singlets usually assumed the use of two antiparallel spin-1/2 electrons. However, actual experiments have tended to focus instead on using pairs of spin&nbsp;1 photons. While the entanglement effect is somewhat less pronounced with such spin&nbsp;1 particles, photons are easier to generate in correlated pairs and (usually) easier to keep in an unperturbed quantum state.