Quantum mechanics is mathematically formulated in Hilbert space or projective Hilbert space. The space of pure states of a quantum system is given by the one-dimensional subspaces of the corresponding Hilbert space (or the "points" of the projective Hilbert space). For a two-dimensional Hilbert space, this is simply the complex projective line ℂℙ1. This is the Bloch sphere.
The Bloch sphere is a unit 2-sphere, with each pair of antipodal points corresponding to mutually orthogonal state vectors. The north and south poles of the Bloch sphere are typically chosen to correspond to the standard basis vectors and , respectively, which in turn might correspond e.g. to the spin-up and spin-down states of an electron. This choice is arbitrary, however. The points on the surface of the sphere correspond to the pure states of the system, whereas the interior points correspond to the mixed states. The Bloch sphere may be generalized to an n-level quantum system but then the visualization is less useful.
Given an orthonormal basis, any pure state of a two-level quantum system can be written as a superposition of the basis vectors and , where the coefficient or amount of each basis vector is a complex number. Since only the relative phase between the coefficients of the two basis vectors has any physical meaning, we can take the coefficient of to be real and non-negative. We also know from quantum mechanics that the total probability of the system has to be one: , or equivalently . Given this constraint, we can write using the following representation:
where and . Except in the case where is one of the ket vectors or , the representation is unique. The parameters and , re-interpreted as spherical coordinates, specify a point
on the unit sphere in .
where is called the Bloch vector of the system. It is this vector that indicates the point within the sphere that corresponds to a given mixed state. The eigenvalues of are given by . As density operators must be positive-semidefinite, we have . For pure states we must have
in accordance with the previous result. Hence the surface of the Bloch sphere represents all the pure states of a two-dimensional quantum system, whereas the interior corresponds to all the mixed states.
The Bloch vector can be represented in the following basis, with reference to the density operator :
Consider an n-level quantum mechanical system. This system is described by an n-dimensional Hilbert space Hn. The pure state space is by definition the set of 1-dimensional rays of Hn.
To prove this fact, note that there is a natural group action of U(n) on the set of states of Hn. This action is continuous and transitive on the pure states. For any state , the isotropy group of , (defined as the set of elements of U(n) such that ) is isomorphic to the product group
In linear algebra terms, this can be justified as follows. Any of U(n) that leaves invariant must have as an eigenvector. Since the corresponding eigenvalue must be a complex number of modulus 1, this gives the U(1) factor of the isotropy group. The other part of the isotropy group is parametrized by the unitary matrices on the orthogonal complement of , which is isomorphic to U(n - 1). From this the assertion of the theorem follows from basic facts about transitive group actions of compact groups.
The important fact to note above is that the unitary group acts transitively on pure states.
Now the (real) dimension of U(n) is n2. This is easy to see since the exponential map
is a local homeomorphism from the space of self-adjoint complex matrices to U(n). The space of self-adjoint complex matrices has real dimension n2.
Corollary. The real dimension of the pure state space of Hn is 2n − 2.
Let us apply this to consider the real dimension of an m qubit quantum register. The corresponding Hilbert space has dimension 2m.
Formulations of quantum mechanics in terms of pure states are adequate for isolated systems; in general quantum mechanical systems need to be described in terms of density operators. However, while the Bloch sphere parametrizes not only pure states but mixed states for 2-level systems, for states of higher dimensions there is difficulty in extending this to mixed states. The topological description is complicated by the fact that the unitary group does not act transitively on density operators. The orbits moreover are extremely diverse as follows from the following observation:
Theorem. Suppose A is a density operator on an n level quantum mechanical system whose distinct eigenvalues are μ1, ..., μk with multiplicities n1, ...,nk. Then the group of unitary operators V such that V A V* = A is isomorphic (as a Lie group) to
In particular the orbit of A is isomorphic to
We note here that, in the literature, one can find other (not Bloch-style) parametrizations of (mixed) states that do generalize to dimensions higher than 2.
- Specific implementations of the Bloch sphere are enumerated under the qubit article.
- Atomic electron transition
- Gyrovector space
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