*Michael Fowler
11/19/07*

Our treatment here more or less follows that of Sakurai,
beginning with two imagined Stern-Gerlach experiments. In that experiment, a stream of (non-ionized)
silver atoms from an oven is directed through an inhomogeneous vertical
magnetic field, and the stream splits into two.
The silver atoms have nonzero magnetic moments, and a magnetic moment in
an inhomogeneous magnetic field experiences a nonzero force, causing the atom
to veer from its straight line path, the
magnitude of the deflection being proportional to the component of the atom’s
magnetic moment in the vertical (field) direction. The observation of the beam splitting into
two, and no more, means that the vertical component of the magnetic moment, and
therefore the associated angular momentum, can only have *two* different
values. From the basic analysis of
rotation operators and the properties of angular momentum that follow, this
observation forces us to the conclusion that the total angular momentum of a
silver atom is _{}. Ordinary orbital
angular momenta cannot have half-integer values; this experiment was one of the
first indications that the electron has a spin degree of freedom, an angular
momentum that cannot be interpreted as orbital angular momentum of constituent
parts. The silver atom has 47 electrons,
46 of them have total spin and orbital momenta that separately cancel, the 47^{th}
has no orbital angular momentum, and its spin is the entire angular momentum of
the atom.

Here we shall use the
Stern-Gerlach stream as an example of a large collection of quantum systems
(the atoms) to clarify just how to describe such a collection, often called an *ensemble*.
To avoid unnecessary complications, we only consider the *spin* degrees of
freedom. We begin by examining two
different streams:

Suppose experimentalist *A* prepares a stream of silver
atoms such that each atom is in the spin state _{}:

_{}.

Meanwhile, experimentalist *B* prepares a stream of
silver atoms which is a *mixture*: half the atoms are in state _{} and half are in
the state _{}: call this mix *B*.

*Question*: can we distinguish the *A* stream from
the *B* stream?

Evidently, not by measuring the spin in the *z*-direction! Both will give up 50% of the time, down 50%.

But: we *can *distinguish them by measuring the spin in
the *x*-direction: the _{} quantum state is in
fact just that of a spin in the *x*-direction, so it will give “up” in the
*x*-direction every time—from now on we call it _{}, whereas the state _{} (“up” in the *z*-direction)
will yield “up” in the *x*-direction only 50% of the time, as will _{}.

The state _{} is called a ** pure** state, it’s the kind of
quantum state we’ve been studying this whole course.

The stream *B*, in contrast, is in a ** mixed** state: the kind that actually occurs to a greater or
lesser extent in a real life stream of atoms, different pure quantum states
occurring with different probabilities, but with no phase coherence between
them. In other words, these relative
probabilities in

That being said, though, to find the probability of
measuring spin up in some such mixed state, one *first* uses the
classical-type probability for each component state, *then* for each
quantum state in the mix, one finds the probability of spin up *in that state* by the standard quantum
technique.

Theerefore, for a mixed state in which the system is in
state _{} with probability *w _{i}*,

_{}

and we should emphasize that these _{} do *not* need to be
orthogonal (but they are of course normalized): for example one could be _{}, another _{}. (We put the usually omitted *z* in for emphasis.) The reason we put a hat on _{} here is to emphasize
that this is an operator, but the *w _{i}*
are just numbers.

The equation for the expectation value_{} can be written:

_{}

To see exactly how this comes about, recall that for an
operator _{} in a
finite-dimensional vector space with an orthonormal basis set _{}, _{}, where the repeated suffix implies summation of the diagonal
matrix elements of the operator.

Therefore,

_{}

since _{}, the identity.

This _{} is called the *density
matrix*: its matrix form is made explicit by considering states _{} in a finite *N*-dimensional
vector space (such as spins or angular momenta)

_{}

where the _{} are an orthonormal
basis set, and _{}is the *j*^{th} component of a normalized vector
*V _{i}*. It is convenient
to express

_{}

and evidently

_{}

(Since _{} is just a *number*, _{}.)

_{} is *basis-independent*,
the trace of a matrix being unchanged by a unitary transformation, since it
follows from Tr*ABC* = Tr*BCA* that

_{}.

Note that since the vectors *V _{i}* are
normalized,

_{}

(also evident by putting *A* = 1 in the equation for _{}).

For a system in a ** pure** quantum state

_{},

as for all projection operators.

It’s worth spelling out how this differs from the mixed state by looking at the form of the density matrix.

For the pure state _{}, if a basis is chosen so that _{} is a member of the
basis (this can always be done), _{} is a matrix with every
element zero except the one diagonal element corresponding to _{}, which will be unity.
Obviously, _{}. This is less obvious
in a general basis, where_{} will not necessarily be diagonal. But the statement _{} remains true under a
transformation to a new basis.

For a mixed state, let’s say for example a mixture of
orthogonal states _{}, if we choose a basis including both states, the density
matrix will be diagonal with just two entries _{} Both these numbers
must be less than unity, so _{} A mix of
nonorthogonal states is left as an exercise for the reader.

**First, our case A
above (pure state)**: all spins in state

In the standard _{} basis,

_{}

and

_{}

Notice that _{}.

**Now, case B (50-50 mixed up and down)**:

The density matrix is

_{}

This is proportional to the unit matrix, so

_{}

and similarly for *s _{y}* and

**Finally, a 50-50
mixed state relative to the x-axis**:

That is, 50% of the spins in the state _{}, “up” along the *x*-axis, and 50% in _{}, “down” in the *x*-direction.

It is easy to check that

_{}

This is exactly the same density matrix we found for 50% in
the state _{}, 50% _{}!

The reason is that both formulations describe a state about
which we know nothing—we are in a state of *total
ignorance*, the spins are completely random, all directions are equally
likely. The density matrix describing
such a state cannot depend on the direction we choose for our axes.

Another two-state quantum system that can be analyzed in the
same way is the polarization state of a beam of light, the basis states being
polarization in the *x*-direction and polarization in the *y*-direction,
for a beam traveling parallel to the *z*-axis. Ordinary unpolarized light
corresponds to the random mixed state, with the same density matrix as in the
last example above.

In the mixed state, the quantum states evolve independently according to Schrödinger’s equation, so

_{}

Note that this has the opposite sign from the evolution of a Heisenberg operator, not surprising since the density operator is made up of Schrödinger bras and kets.

The equation is the quantum analogue of *Liouville’s theorem* in statistical mechanics. Liouville’s theorem describes the evolution
in time of an ensemble of identical classical systems, such as many boxes each
filled with the same amount of the same gas at the same temperature, but the
positions and momenta of the individual atoms are randomly different in
each. Each box can be classically
described by a single point in a huge dimensional space, a space having six dimensions
for each atom (position and momentum, we ignore possible internal degrees of
freedom). The whole ensemble, then, is a
gas of these points in this huge space, and the rate of change of local density
of this gas, from _{}, the bracket now being a Poisson bracket (see Classical
Mechanics). Anyway, this is the
classical precursor of, and the reason for the name of, the density matrix.

A system in thermal equilibrium is represented in
statistical mechanics by a *canonical ensemble*. If the eigenstate _{} of the Hamiltonian has
energy *E _{i}*, the relative probability of the system being in
that state is

_{}

where

_{}

Notice that in this formulation, apart from the
normalization constant *Z*, the density operator is analogous to the
propagator _{} for an imaginary time _{}. Incidentally, for
interacting quantum fields, the propagator can be constructed as a set of
Feynman diagrams corresponding to all possible sequences of particle
scatterings by interaction. To find the
thermodynamic properties of a field theory at finite temperature, essentially
the same set of diagrams is used to find the free energy: the diagrams now
describe the system propagating for a finite imaginary time, the same
mathematical tools can be used.

At zero temperature (_{}) the probability coefficients _{}are all zero except for the ground state: the system is in a
pure state, and the density matrix has every element zero except for a single
element on the diagonal. At infinite
temperature, all the *w _{i}*