Homodyne detection is an extension of the photocurrent method where the output is mixed with a strong external source allowing to get information about the phase of the system. With this method, the resulting detection rate depends is
With \(\gamma\), the strength of the external beam and \(C\) the collapse operator. When the beam is very strong \((\gamma >> C^\dag C)\), the rate becomes a constant term plus a term proportional to the quadrature of the system.
In closed systems, the resulting stochastic differential equation is
Here \(\delta \omega\) is a Wiener increment.
In QuTiP, this is available with the function
times = np.linspace(0.0, 10.0, 201) psi0 = tensor(fock(2, 0), fock(10, 5)) a = tensor(qeye(2), destroy(10)) sm = tensor(destroy(2), qeye(10)) H = 2*np.pi*a.dag()*a + 2*np.pi*sm.dag()*sm + 2*np.pi*0.25*(sm*a.dag() + sm.dag()*a) data = ssesolve(H, psi0, times, sc_ops=[np.sqrt(0.1) * a], e_ops=[a.dag()*a, sm.dag()*sm], method="homodyne") plt.figure() plt.plot(times, data.expect, times, data.expect) plt.title('Homodyne time evolution') plt.xlabel('Time') plt.ylabel('Expectation values') plt.legend(("cavity photon number", "atom excitation probability")) plt.show()
In open systems, 2 types of collapse operators are considered, \(S_i\) represent the dissipation in the environment, \(C_i\) are monitored operators. The deterministic part of the evolution is the liouvillian with both types of collapses
The stochastic part is given by
resulting in the stochastic differential equation
smesolve covert these cases in QuTiP.
With heterodyne detection, two measurements are made in order to obtain information about 2 orthogonal quadratures at once.