In depth: vqeeCalculator

vqeeCalculator is a command-line executable for accessing Qristal’s MPI-enabled Variational Quantum Eigensolver (VQE) functionality.

Compiling

cd $QRISTAL_INSTALL_DIR/examples/cpp/vqeeCalculator
mkdir build
cd build
cmake .. -DENABLE_MPI=ON
make

Limitations

• Noise model: none

JSON input and output

Instead of supplying command line options, vqeeCalculator reads a JSON file (and output results to another JSON file) when called with these options:

mpiexec -n 2 vqeeCalculator --fromJson=$json_input_file --jsonID=0 --outputJson=$json_output_file

Input

The fields that are recognized in $json_input_file are shown below:

Key	Example value	Details
nQubits	"nQubits": 5	Number of physical qubits
nShots	"nShots": 128	Number of circuit execution shots
pauli	"pauli": "5.907 - 2.1433 X0X1 - 2.1433 Y0Y1 + .21829 Z0 - 6.125 Z1"	Qubit Hamiltonian expressed as a weighted sum of Pauli terms
geometry	"geometry": "H 0.0 0.0 0.0; H 0.0 0.0 0.7408481486"	Alternative to specifying pauli: atoms in X-Y-Z co-ordinates. Unit is angstroms
ansatz	"ansatz": "ASWAP"	Predefined quantum circuit conditioned by parameter thetas
nElectrons	"nElectrons": 2	Number of particles in the ansatz
circuit	See this link for information	Alternative to specifying ansatz and nElectrons
thetas	"thetas": [-0.2,0.14,1.01]	Initial values for ansatz parameters. A value is needed for each parameter in the ansatz.
acceleratorName	"acceleratorName": "qpp"	Backend for executing quantum circuits
maxIters	"maxIters": 100	Maximum iteration count for the classical optimizer
algorithm	"algorithm": "nelder-mead"	Selects the algorithm used for optimization
extraOptions	"extraOptions": "{<key1>: <value1>, <key2>: <value2>, ...}"	Extra options specific to the optimization algorithm set in algorithm. More details on the key-value pairs relevant to each algorithm are at this link.
isDeterministic	"isDeterministic": true	When set to false, expectations are calculated from samples (nShots in size) When "acceleratorName": "qpp" and "isDeterministic": true, expectations are calculated directly using linear algebra.
enableVis	"enableVis": true	Enable convergence trace visualisation
showTheta	"showTheta": true	Enable the display of theta elements in the convergence trace visualisation
tolerance	"tolerance": 1.0e-5	Function tolerance (used as a termination criterion in the classical optimizer)
nThreads	"nThreads": 2	Number of OpenMP threads to use

Example JSON input file

[
    {
        "nQubits": 5,
        "acceleratorName": "qpp",
        "pauli": "0.00021123007138713814  +-0.0004797898199899509 X1 Z2 X3 +-0.0004797898199899509 Y1 Z2 Y3 +-0.0008504196943407897 Z1 +-0.0004797898199899509 X2 Z3 X4 +-0.0004797898199899509 Y2 Z3 Y4 +-0.0008504196943407897 Z2 +-0.0009971065473443474 Z3 +-0.0009971065473443474 Z4",
        "ansatz": "ASWAP",
        "nElectrons": 2,
        "maxIters": 1,
        "tolerance": 1e-05,
        "nShots": 1,
        "isDeterministic": true,
        "thetas": [
            0.19282382007161974,
            0.2635945440300202,
            -0.04929291223947813,
            0.04121265889451157,
            0.16609364599654375,
            0.2502175895079587,
            0.0488588580664832,
            1.6109279933369605,
            2.921249087681776
        ]
    }
]

Output

The fields that are output in $json_output_file are shown below:

Key	Details
energy	The optimum (min.) energy at the end of VQE optimization routine
iterations	Number of iterations reached before termination of the optimization routine
pauli	The Hamiltonian expressed as a weighted sum of Pauli terms
theta	The value of the ansatz parameters that produces the corresponding optimum (min.) energy
vis	Energy and ansatz parameter trace visualization
walltime_ms	The execution wall time, in ms

Example output JSON file

{
    "energy": -6.722747367844044e-05,
    "iterations": 1,
    "pauli": "0.00021123007138713814  +-0.0004797898199899509 X1 Z2 X3 +-0.0004797898199899509 Y1 Z2 Y3 +-0.0008504196943407897 Z1 +-0.0004797898199899509 X2 Z3 X4 +-0.0004797898199899509 Y2 Z3 Y4 +-0.0008504196943407897 Z2 +-0.0009971065473443474 Z3 +-0.0009971065473443474 Z4",
    "theta": [
        0.19282382007161974,
        0.2635945440300202,
        -0.04929291223947813,
        0.04121265889451157,
        0.16609364599654375,
        0.2502175895079587,
        0.0488588580664832,
        1.6109279933369605,
        2.921249087681776
    ],
    "walltime_ms": 30.81
}