Homework Set 1

Problem 1: Pure vs. mixed quantum states

A researcher in spintronics is investigated two devices in order to generate spin-polarized currents. One of those devices has spins comprising the current described by the density matrix:

${\hat {\rho }}_{1}={\frac {|\uparrow \rangle \langle \uparrow |+|\downarrow \rangle \langle \downarrow |}{2}}$,

while the spins comprising the current in the other device are described by the density matrix

${\hat {\rho }}_{2}=|u\rangle \langle u|$ , where $\ |u\rangle ={\frac {e^{i\alpha }|\uparrow \rangle +e^{i\beta }|\downarrow \rangle }{\sqrt {2}}}$.

Here $|\uparrow \rangle$ and $|\downarrow \rangle$ are the eigenstates of the Pauli spin matrix ${\hat {\sigma }}_{z}$:

${\hat {\sigma }}_{z}|\uparrow \rangle =+1|\uparrow \rangle ,\ {\hat {\sigma }}_{z}|\downarrow \rangle =-1|\downarrow \rangle$.

What is the spin-polarization of these two currents? Comment on the physical meaning of the difference between the spin state transported by two currents.

HINT: Compute the x, y, and z components of the spin polarization vector using both of these density matrices following the quantum-mechanical definition of an average value $P_{x,y,z}=\langle \sigma _{x,y,z}\rangle =\mathrm {Tr} \,[{\hat {\rho }}{\hat {\sigma }}_{x,y,z}]$.

Problem 2: Dynamics of Bloch vector

The Hamiltonian of a single spin of an electron in external magnetic field $\mathbf {B}$ is given by (assuming that gyromagnetic ration is unity):

${\hat {H}}=-{\frac {\hbar }{2}}\mathbf {B} \cdot {\boldsymbol {\sigma }}$

where ${\boldsymbol {\sigma }}=({\hat {\sigma }}_{x},{\hat {\sigma }}_{y},{\hat {\sigma }}_{z})$ is the vector of the Pauli matrices. Show that the equation of motion

$i\hbar {\frac {\partial {\hat {\rho }}}{\partial t}}=[{\hat {H}},{\hat {\rho }}]$

for the density matrix of spin-${\frac {1}{2}}$ discussed in the class

${\hat {\rho }}={\frac {1}{2}}\left(1+\mathbf {P} \cdot {\boldsymbol {\sigma }}\right)$

can be recast into the equation of motion for the spin-polarization (or Bloch) vector

${\frac {d\mathbf {P} }{dt}}=-\mathbf {B} \times \mathbf {P}$

since $\mathbf {\rho }$ and $\mathbf {P}$ are in one-to-one correspondence.

HINT: Use the following property of the Pauli matrices:

${\hat {\sigma }}_{\alpha }{\hat {\sigma }}_{\beta }-{\hat {\sigma }}_{\beta }{\hat {\sigma }}_{\alpha }=2i\epsilon _{\alpha \beta \gamma }{\hat {\sigma }}_{\gamma }$.

Problem 3: Does entropy increase in closed quantum systems?

In classical Hamiltonian systems the nonequilibrium entropy

$S=-k_{B}\int \rho \ln \rho$

is constant. In this problem we want to demonstrate that in microscopic evolution of an isolated quantum system, the entropy is also time independent, even for general time-dependent density matrix ${\hat {\rho }}$. That is, using the equation of motion:

$i\hbar {\frac {\partial {\hat {\rho }}}{\partial t}}=[{\hat {H}},{\hat {\rho }}]$

prove that von Neumann entropy

$S(t)=-k_{B}\mathrm {Tr} [{\hat {\rho }}(t)\ln {\hat {\rho }}(t)]$

is time independent for arbitrary density matrix ${\hat {\rho }}(t)$.

HINT: Use $\mathrm {Tr} ({\hat {A}}{\hat {B}}{\hat {C}})=\mathrm {Tr} ({\hat {C}}{\hat {A}}{\hat {B}})$ for any operators ${\hat {A}}$, ${\hat {B}}$, ${\hat {C}}$, as well as that an operator ${\hat {M}}$ commutes with any function $f({\hat {M}})$:

$[{\hat {M}},f({\hat {M}})]=0$.

Problem 4: Successive measurements on subsystems of composite bipartite quantum system

Consider a composite quantum system composed of two spins, labeled as subsystem A and B. The quantum state of the composite system is described by the following density matrix:

${\hat {\rho }}={\frac {1}{8}}{\hat {I}}+{\frac {1}{2}}|\Psi \rangle \langle \Psi |$

where ${\hat {I}}$ denotes the $4\times 4$ unit matrix and

$|\Psi \rangle ={\frac {1}{\sqrt {2}}}\left(|\uparrow \rangle |\downarrow \rangle -|\downarrow \rangle |\uparrow \rangle \right)$

is entangled (in the context of spins also called "singlet") pure state of two spins.

Suppose we measure the first spin (subsystem A) along the axis described by the unit vector $\mathbf {n}$, and the second spin (subsystem B) along the axis described by the unit vector $\mathbf {m}$, where $\mathbf {n} \cdot \mathbf {m} =\cos \theta$. What is the probability that both spins are "spin-up" along their respective axes?

HINT: In general, the probability to measure eigenvalue $\lambda$ of a physical quantity in the quantum state described by the density matrix ${\hat {\rho }}$ is given by $p=\mathrm {Tr} [{\hat {\rho }}{\hat {P}}_{\lambda }]$. Here ${\hat {P}}_{\lambda }$ is the projection operator on the eigensubspace corresponding to eigenvalue $\lambda$. To find the probability of measurement on the subsystem, one should use the density matrix of that subsystem, obtained by partial trace over the states of the second subsystem. This means that the probability asked in the problem is formally obtained from:

$p=\mathrm {Tr} _{B}\left[{\hat {P}}_{\mathbf {m} }^{B}{\hat {\rho }}^{B}\right]=\mathrm {Tr} _{B}\left[{\hat {P}}_{\mathbf {m} }^{B}\mathrm {Tr} _{A}\left[({\hat {P}}_{\mathbf {n} }^{A}\otimes {\hat {I}}^{B}){\hat {\rho }}\right]\right]$

The eigenprojector for the "spin-up" (i.e., +1) eigenvalue along the $\mathbf {n}$-axis is simply:

${\hat {P}}_{\mathbf {n} }^{A}=|\uparrow _{\mathbf {n} }\rangle \langle \uparrow _{\mathbf {n} }|={\frac {1}{2}}\left({\hat {I}}^{A}+\mathbf {n} \cdot {\hat {\boldsymbol {\sigma }}}^{A}\right)$.

We also use the fact that resulting state of the composite system after the selective measurement on subsystem A is described by ${\hat {P}}_{\mathbf {n} }^{A}{\hat {\rho }}{\hat {P}}_{\mathbf {n} }^{A}$, so that subsystem B after the measurement on system A is "collapsed" onto the state described by the density matrix ${\hat {\rho }}^{B}=\mathrm {Tr} _{A}[{\hat {P}}_{\mathbf {n} }^{A}{\hat {\rho }}{\hat {P}}_{\mathbf {n} }^{A}]=\mathrm {Tr} _{A}[{\hat {P}}_{\mathbf {n} }^{A}{\hat {\rho }}]$.