QContinuousMeasurement

Installation

Requires Julia v1.6.

From the Julia REPL, open the package manager with the key ] and run the commands

pkg> add https://github.com/matteoacrossi/BlockDiagonalMatrices.jl
pkg> add https://github.com/matteoacrossi/QContinuousMeasurement.jl

This will install all the dependencies, including a Python environment with QuTiP.

To make local changes to the package, use

pkg> dev QContinuousMeasurement

Usage

First, we initialize a ModelParameters object.

params = ModelParameters(Nj=3,
                         kind=1.0,
                         Gamma=1.0,
                         omega=1.0,
                         eta=1.0,
                         dt=0.001,
                         Tfinal=2.0,
                         outpoints=200)

Then, we can initialize a LocalDephasingModel object and a State object (in this case, a coherent spin state).

model = LocalDephasingModel(params)
initial_state = coherentspinstate(model)

To simulate one trajectory run

result = simulate_trajectory(model, initial_state);

To plot the FI for the trajectory, use

using Plots
plot(get_time(model), result.FI)

To run simulations with large numbers of trajectories, and for saving to .h5, use the script/simulate.jl script. The script can be modified as needed.

The old interface of ContinuousMeasurementFI still works for calculating averages.

Usage (old interface)

This will evaluate the effective QFI for 3 spins with 100 trajectories and compare it with the unconditional dynamics

using QContinuousMeasurement

result = Eff_QFI_HD_Dicke(3, 100, 2., 0.001)

result_uncond = Unconditional_QFI_Dicke(3, 2., 0.001)

This will produce the following plot

using Plots
plot(result.t, result.FI, label="FI")
plot!(result.t, result.FI + result.QFI, label="Eff. QFI")
plot!(result_uncond.t, result_uncond.QFI, label="Uncond. QFI")

result also contains the expectation values for the global operators jx, jy, jz and their variances Δjx, Δjy, Δjz, and a timer object showing detailed timing information.

Known issues

If you obtain the following error when importing QContinuousMeasurement:

/usr/bin/../lib64/libstdc++.so.6: version `CXXABI_1.3.9' not found

run the following before starting Julia:

export LD_PRELOAD=$HOME/.julia/conda/3/lib/libstdc++.so

Citation

This code has been developed for the following paper:

Matteo A. C. Rossi, Francesco Albarelli, Dario Tamascelli, Marco G. Genoni, "Noisy quantum metrology enhanced by continuous nondemolition measurement", Phys. Rev. Lett. 125, 200505 (2020)

Please cite it if you are using this code.