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A quantum state is a state of a system of quantum mechanics.
The precise mathematical notion of state depends on what mathematical formalization of quantum mechanics is used.
In the simple formulation over a Hilbert space $H$, a pure state is a ray in $H$. Thus, the pure states form the space $(H \setminus \{0\})/\mathbb{C}$, where we mod out by the action of $\mathbb{C}$ on $H \setminus \{0\}$ by scalar multiplication; equivalently, we can use $S(H)/\mathrm{U}(1)$, the unit sphere in $H$ modulo the action of the unitary group $\mathrm{U}(1)$. Often by abuse of language, one calls $H$ the ‘space of states’.
The mixed states are density matrices on $H$. Every pure state may be interpreted as a mixed state; taking a representative normalised vector ${|\psi\rangle}$ from a ray in Hilbert space, the operator ${|\psi\rangle}{\langle\psi|}$ is a density matrix.
In principle, any quantum mechanical system can be treated using Hilbert spaces, by imposing superselection rules that identify only some self-adjoint operators on $H$ as observables. Alternatively, one may take a more abstract approach, as follows.
In AQFT, a quantum mechanical system is given by a $C^*$-algebra $A$, giving the algebra of observables. Then a state on $A$ is
a $\mathbb{C}$-linear function $\rho\colon A \to \mathbb{C}$
such that
it is positive: for every $a \in A$ we have $\rho(a^\ast a) \geq 0 \in \mathbb{R}$;
it is normalized: $\rho(1) = 1$.
See also state in AQFT and operator algebra.
If $H$ is a Hilbert space, then the bounded operators on $H$ form a $C^*$-algebra $\mathcal{B}H$, and states on the Hilbert space correspond directly to states on $\mathcal{B}H$. Classical mechanics can also be formulated in AQFT; the classical space of states $X$ gives rise to a commutative von Neumann algebra $L^\infty(X)$ as the algebra of observables.
Arguably, the correct notion of state to use is that of quasi-state; every state gives rise to a unique quasi-state, but not conversely. However, when either classical mechanics or Hilbert-space quantum mechanics is formulated in AQFT, every quasi-state is a state (at least if the Hilbert space is not of very low dimension, by Gleason's theorem). See also the Idea-section at Bohr topos for a discussion of this point.
In the FQFT formulation of quantum field theory, a physical system is given by a cobordism representation
In this formulation the (n-1)-morphism in $\mathcal{C}$ assigned to an $(n-1)$-dimensional manifold $\Sigma_{n-1}$ is the space of states over that manifold. A state is accordingly a generalized element of this object.
classical mechanics | semiclassical approximation | … | formal deformation quantization | quantum mechanics | |
---|---|---|---|---|---|
order of Planck's constant $\hbar$ | $\mathcal{O}(\hbar^0)$ | $\mathcal{O}(\hbar^1)$ | $\mathcal{O}(\hbar^n)$ | $\mathcal{O}(\hbar^\infty)$ | |
states | classical state | semiclassical state | quantum state | ||
observables | classical observable | quantum observable |