- #1
rbwang1225
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The Hamiltonian is ##H=\hbar \omega (a^\dagger a+b^\dagger b)+\hbar\kappa(a^\dagger b+ab^\dagger)## with commutation relations ##[a,a^\dagger]=1 \hspace{1 mm} and \hspace{1 mm}[b,b^\dagger]=1##.
I want to calculate the Heisenberg equations of motion for a and b.
Beginning with ##\dot a=\frac{i}{\hbar}[H,a]=-i\omega a-i\kappa b ## and
##\dot b=\frac{i}{\hbar}[H,b]=-i\omega b-i\kappa a##,
I got ##\ddot a=-(\omega^2+\kappa^2)a-2\omega\kappa b## and
##\ddot b=-(\omega^2+\kappa^2)b-2\omega\kappa a##.
The solution is ##a+b=[a(0)+b(0)]e^{-i(\omega+\kappa)t}## and from this I got
##b=[a(0)+b(0)]e^{-i(\omega+\kappa)t}-a## and then
##\dot a=-i(\omega+\kappa)a-i\kappa[a(0)+b(0)]e^{-i(\omega+\kappa)t}##.
The solution of ##a## is ##a=-i\kappa[a(0)+b(0)]te^{-i(\omega+\kappa)t}+a(0)e^{-i(\omega+\kappa)t}## and therefore
##b=i\kappa[a(0)+b(0)]te^{-i(\omega+\kappa)t}+b(0)e^{-i(\omega+\kappa)t}##.
However, my result did not preserve the commutator, i.e., ##[a,a^\dagger]=2\kappa^2t^2+1##.
I don't know which step is wrong in my derivation.
The solution in the book of Carmichael is ##a=e^{-i\omega t}[a(0)\cos\kappa t-ib(0)\sin\kappa t]## and
##b=e^{-i\omega t}[b(0)\cos\kappa t-ia(0)\sin\kappa t]##, which preserves the commutators.
I want to calculate the Heisenberg equations of motion for a and b.
Beginning with ##\dot a=\frac{i}{\hbar}[H,a]=-i\omega a-i\kappa b ## and
##\dot b=\frac{i}{\hbar}[H,b]=-i\omega b-i\kappa a##,
I got ##\ddot a=-(\omega^2+\kappa^2)a-2\omega\kappa b## and
##\ddot b=-(\omega^2+\kappa^2)b-2\omega\kappa a##.
The solution is ##a+b=[a(0)+b(0)]e^{-i(\omega+\kappa)t}## and from this I got
##b=[a(0)+b(0)]e^{-i(\omega+\kappa)t}-a## and then
##\dot a=-i(\omega+\kappa)a-i\kappa[a(0)+b(0)]e^{-i(\omega+\kappa)t}##.
The solution of ##a## is ##a=-i\kappa[a(0)+b(0)]te^{-i(\omega+\kappa)t}+a(0)e^{-i(\omega+\kappa)t}## and therefore
##b=i\kappa[a(0)+b(0)]te^{-i(\omega+\kappa)t}+b(0)e^{-i(\omega+\kappa)t}##.
However, my result did not preserve the commutator, i.e., ##[a,a^\dagger]=2\kappa^2t^2+1##.
I don't know which step is wrong in my derivation.
The solution in the book of Carmichael is ##a=e^{-i\omega t}[a(0)\cos\kappa t-ib(0)\sin\kappa t]## and
##b=e^{-i\omega t}[b(0)\cos\kappa t-ia(0)\sin\kappa t]##, which preserves the commutators.