Understanding the geometry of state space is of fundamental interest in the field of quantum information, in particular to study entanglement. The states of a d dimensional system form a convex set in the d2−1 dimensional Euclidean space Rd2−1. For a two dimensional system this convex set is nothing but a solid unit sphere (Bloch sphere) with centre at the origin. The surface S of this unit sphere consists of all the pure states of the system. Every unitary transformation U ∈ SU(2) of the Hilbert space corresponds to an associated SO(3) rotation of this space. This kind of correspondence becomes much richer when we go to higher dimension. For example, in the d = 3 case the corresponding unitary group is the eight parameter group SU(3) whereas the rotation group in R is the 28 parameter group SO(8). Therefore, not every point on or inside the sphere S in R will correspond to a state of the system. So, not every rotation in R will correspond to a valid unitary transformation of the 3 dimensional Hilbert space. This gives us a fairly complex convex object as the set of all states. In order to gain insight into the structure of this state space, we study its two dimensional and three dimensional cross-sections. In particular, we describe in detail the discrete symmetries of a non-trivial 3-section of the convex set of states (density matrices) for three dimensional systems. This group turns out to be the same as that of a tetrahedron.
Quantum uncertainty Spin I want to start this section with a description of particle spin. In addition to the usual properties of mass, charge, position and velocity, particles have a further parameter which describes their state of rotation. Physicists call this ‘spin’. We have already seen that in quantum physics it is impossible to measure simultaneously, in a precise way, both speed and position (recall the Heisenberg uncertainty relations). The same considerations apply to spin, which cannot be absolutely determined with arbitrary precision. The things that we can in principle find out are the rate of rotation (that is, the number of revolutions per second) and the component of this rotation in a given direction in space (for example, the angle of the axis of rotation relative to the direction of motion). It is important to understand that these two knowable quantities can take only certain quantised values. This is an aspect of quantum physics that we have already encountered when describing wave motion. We can use as an analogy the stable vibrations that can be sustained on a piano string: the basic note is generated when the centre of the string vibrates to its maximum extent while the two ends are (necessarily) fixed.
In this chapter we consider the unit sphere
S2 of the Euclidean space
R3 with its canonical symplectic structure is a phase space. Then coherent states are labelled by points on
S2 and allow us to build a quantization of the two-sphere
S2. They are defined in each finite dimensional space of an irreducible unitary representation of the symmetry rotation group SO(3) (or its covering SU(2)) of
S2 and give a semiclassical interpretation for the spin. As an application we state the Berezin-Lieb inequalities and compute the thermodynamic limit for large spin systems.
The quantum states of a spin 12 (a qubit) are parametrized by the space CP1∼S2, the Bloch sphere. A spin j for a generic j (a 2j+1-state system) is represented instead by a point in a larger space, CP2j. Here we study the state of a single angular momentum/spin in the limit j→∞. A special class of states, |j,n⟩∈CP2j, with spin oriented towards definite spatial directions, n∈S2, i.e., (J^·n)|j,n⟩=j|j,n⟩, are found to behave as classical angular momenta, jn, in this limit. Vice versa, general spin states in CP2j do not become classical, even at a large j. We study these questions by analyzing the Stern–Gerlach processes, the angular momentum composition rule, and the rotation matrix. Our observations help to better clarify how classical mechanics emerges from quantum mechanics in this context (e.g., with the unique trajectories of a particle carrying a large spin in an inhomogeneous magnetic field) and to make the widespread idea that large spins somehow become classical more precise.
Quantum states of a spin
21 (a qubit) are parametrized by the space
CP1∼S2, the Bloch sphere. A spin
j for a generic
j (a
2j+1-state system) is represented instead by a point of a larger space,
CP2j. Here we study the state of a single angular momentum/spin in the limit,
j→∞. The special class of states $ | j, {\mathbf n}\rangle \in {\mathbf {CP}}^{2j} $, with spin oriented towards definite spatial directions
n∈S2, i.e., $({\mathbf J}\cdot {\mathbf n} ) \, | j, {\mathbf n}\rangle = j\, |j, {\mathbf n}\rangle $, are found to behave as classical angular momenta,
jn, in this limit. Vice versa, general spin states in
CP2j do not become classical, even at large
j. We discuss these questions, by analysing the Stern-Gerlach processes, the angular-momentum composition rule, and the rotation matrix. Our observations help to clarify better how classical mechanics emerges from quantum mechanics in this context (e.g., with unique trajectories for a particle carrying a large spin), and to make the widespread idea that large spins somehow become classical, more precise.