GFRM laser quantum phase noise

An implementation of Gillespies first reaction method for calculating quantum phase noise in laser rate equations used in https://arxiv.org/abs/2412.14347. See also https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.130.253801 for even more context.

Installation

Download the folder Stochastic and add the Stochastic module to the current environment with the command (press ] to get to the package manager):

pkg> dev Stochastic\\

Subsequently, the package is loaded by calling.

using Stochastic

Examples

In the following, we showcase different functionalities of the code, which are necessary to reproduce the results in REFERENCE.

Setup problem

We first define the parameters. Here, we consider only one particular pump rate, but by changing the index i, other pump rates can be investigated.

using Stochastic
using Statistics

#Define parameters (same as fig. 2 in the main text of paper)
g,kappa,gamma_A,gamma_D,n0,alpha = 0.1,0.04,0.0012,1,5,0

#Define the pump rates and choose one by index i
pump_rates = 10 .^(range(-4,stop=log(20)/log(10),length=50))

i = 1
P = pump_rates[i]

#Define the decay rate and collect the parameters
gamma_r = 4*g^2 ./ (kappa+gamma_A+gamma_D + P)
params = [P,gamma_r,kappa,gamma_A,n0,alpha]

In the code, one defines the different events and their effect on the populations by giving a matrix A. The matrix should have dimensions (rows $\times$ columns), where rows is the number of stochastic variables (here three: na, ne, and phi) and columns is the number of events (here 6).

#Define matrix A, which defines the population changes for each event.
A =[-1 -1  0  1  1  0; #Photon population change
     0  1 -1 -1 -1  1; #Emitter population change
     0  0  0  0  0  0]; #Phase population is unchanged

Then, a function that gives the rates at each timestep should be defined. Here, a is a vector of length columns that contains the rates, N is a vector of length rows containing the stochastic variables, and params are any parameters necessary for the simulation.

#Define how the six rates are updated (see eq. 1 and 2 in the main text of the paper)
function update_rates!(a,N,params)
    P,gamma_r,kappa,gamma_A,n0,alpha = params

    a[1] = kappa*N[1]                   #Cavity loss
    a[2] = gamma_r*N[1]*(n0-N[2])  #Stimulated absorption
    a[3] = gamma_A*N[2]                   #Non-radiative decay
    a[4] = gamma_r*N[2]*N[1]              #Stimulated emission
    a[5] = gamma_r*N[2]                   #Spontaneous emission
    a[6] = P*(n0 - N[2])     #Pump-event
    return
end

We then provide a function that updates the population (this function is run at each timestep). k here denotes which event (index of column) happened, and the population is changed according to that. Note that N[end], which is the phase phi, is changed only if k==5. All other variables (na and ne) are changed according to A (notice the loop over the rows of A at the column index k)

#Define how the population is updated. Here, we also add the phase noise if k==5 (spontaneous decay)
function update_population!(N,A,k,dt,params)
    if k==5
        if N[1] == 0
            N[end] += 2*pi*(rand()-0.5)
        else
            phase_adjustment = angle(sqrt(N[1]) * exp(im * N[end]) + exp(2 * pi * (rand()-0.5) * im))
            N[end] += phase_adjustment - angle(exp(im * N[end]))
        end
    end
    # param[6] is alpha and the following line adds the phase noise due to refractive index changes
    N[end] += params[6]*N[2]*params[2]*dt
    
    #Update the number of photons and emitters
    for i in 1:length(N)-1
        N[i] += A[i,k]
    end
    return
end

Combining all of this, we can define a stochastic rate equation problem:

#Define stochastic rate equation problem
prob = StochasticRateEquation(A,params,update_rates!,update_population!)    

Getting outcoupled field and calculating spectrum

When solving the problem, we can provide an output function that specifies which events should be recorded as outcoupled events (here, k=1 as it corresponds to a cavity decay event):

#Define the output function. Here, we output the phase of the cavity field if k==1 (cavity decay event)
function fout(N,k)
    if k==1
        N[end],true
    else
        nothing,false
    end
end

We then solve the problem using Gillespies First Reaction Method (GFRM) and specify that we want 1 million outcoupling events defined by fout:

#Solve the problem and output the result. Here, we require 1_000_000 outcoupling events.
result_out = gfrm_out(prob,1_000_000,fout)

From the solution, we can extract first and second-order statistical moments:

# First-order moments of the stochastic variables (na,ne, and phi) are stored in result_out["averages"][1:3]
pops = result_out["averages"]
na = pops[1]
ne = pops[2]
phi_avg = pops[3]

#Second order moments (<na^2>,<ne^2> and <phi^2>) are stored in pops[4:6]
#We calculate the second-order correlation function g2
g2 = (pops[4] - pops[1])/pops[1]^2

We can also see the outcoupled electric field as a function of time:

#The outcoupled series (phase at each outcoupling event) is stored in result_out["out_series"], and the corresponding times in result_out["out_times"]
using PyPlot
pygui(true)
fig,ax = subplots(1,1,figsize=(4.5,4.5))
ax.plot(result_out["out_times"][1:100],real.(exp.(im .* result_out["out_series"][1:100])))
ax.set_xlabel("Time")
ax.set_ylabel("Outcoupled Field")
plt.tight_layout()

The duration of each event is stored in result_out["out_decay"], and we can calculate the mean outcoupled duration as:

julia> mean(result_out["out_decay"])
4.407

For the actual spectrum and linewidth, we specify the appropriate frequency range and calculate the emission spectrum, assuming each pulse has a length of mean(result_out["out_decay"]). We could have also input just result_out["out_decay"] to get varying pulse durations (see out commented line).:

#We choose an appropriate frequency range. We here choose 10 times the Schawlow-Townes linewidth
st_lw = (pops[2]) .* gamma_r ./ (2*pops[1])/(2*pi)
freq = range(-10*st_lw,10*st_lw,length=201)

spec_out = calc_spec(result_out["out_times"],exp.(im .* result_out["out_series"]),mean(result_out["out_decay"]),freq;mode=:lor)
#Equivalently one can assume variable durations of outcoupling events and use a sinc function to calculate the spectrum
#spec_out = Stochastic.exponential_decay_fourier(result_out["out_times"],result_out["out_series"],result_out["out_decay"],freq;mode=:sinc)

This gives the spectrum:

#Extract the linewidth with a cumulative Lorenzian fit
fit_x_c,_, B,fit_y_c = fit_lorr_cum(freq,spec_out,st_lw)

#Plot the spectrum
fig,ax = subplots(1,1,figsize=(4.5,4.5))
ax.plot(freq,spec_out ./ max(spec_out...),"r-",label="Stochastic")
#Plot fit 
ax.plot(fit_x_c,fit_y_c ./ max(fit_y_c...),"bo",label="Fit")
ax.set_xlabel("Frequency")
ax.set_ylabel("Spectrum")
ax.legend()

Population sweep

If one wants only the averages. One can use just the gfrm solver. This is more efficient for sweeping over parameters since a time series is not saved. Here, we sweep over the pump rates and calculate the average number of photons in the cavity

na_list = zeros(length(pump_rates))
for (i,P) in enumerate(pump_rates)
    gamma_r_avg = 4*g^2 ./ (kappa+gamma_A+gamma_D + P)
    params_avg = [P,gamma_r_avg,kappa,gamma_A,n0,alpha]
    prob_avg = StochasticRateEquation(A,params_avg,update_rates!,update_population!)    
    result_avg = gfrm(prob_avg,1_000_000)
    na_list[i] = result_avg["averages"][1]
end
#Plot the number of photons
fig,ax = subplots(1,1,figsize=(4.5,4.5))
ax.loglog(pump_rates,na_list)
ax.set_xlabel("Pump rate")
ax.set_ylabel("Number of photons")
plt.tight_layout()

Calculate RIN

Here, we provide the code necessary to calculate the intra and outer cavity RIN, which follows https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.130.253801 closely.

If one wants the time series of the intra-cavity field, one can use the gfrm_intra solver

result_in = gfrm_intra(prob,1_000_000)

The interpolated intra-cavity field and RIN are then:

#Interpolate the intra-cavity field to a fixed time step
dt = 10.0
photons_in = result_in["time_series"][1,:]
photons_in_interpolated = interpolate_previous(result_in["times"],photons_in,dt)
rin_in,freq_rin_in = RIN(photons_in_interpolated,dt*1e-12,result_in["averages"][1];N = 10000)

Similarly, the outer cavity RIN is given as:

#The outcoupled RIN is similarly given (we have the outcoupled field from above and just need to interpolate it)
photons_out_interpolated = interpolate_outcoupled(result_out["out_times"],abs.(exp.(im .* result_out["out_series"])),dt)
rin_out,freq_rin_out = RIN(photons_out_interpolated,dt*1e-12,sum(photons_out_interpolated)/length(photons_out_interpolated);N = 10000)

Finally, we can plot them both together with the shot noise limit:

#Plot the RIN
fig,ax = subplots(1,1,figsize=(4.5,4.5))
ax.plot(freq_rin_in[2:end÷2] * 10^-9,10/log(10)*log.(rin_in[2:end÷2]),"b",label="Intra cavity RIN")
ax.plot(freq_rin_out[2:end÷2]* 10^-9,10/log(10)*log.(rin_out[2:end÷2]),"r",label="Outcoupled RIN")
ax.axhline(10/log(10)*log.(2/(kappa*result_out["averages"][1])*1e-12),color="black",ls="--",label="Shot noise limit")
ax.legend()
xlabel("Frequency [GHz]")
ylabel("RIN [dB/Hz]")
plt.tight_layout()