VAns: Variable Ansatz for Variational Quantum Algorithms

A semi-agnostic ansatz with variable structure for variational quantum algorithms

This repository contains the official implementation of VAns (Variable Ansatz), a variable structure approach to build ansatzes for Variational Quantum Algorithms (VQAs). VAns applies a set of rules to both grow and remove quantum gates in an informed manner during optimization, making it ideally suited to mitigate trainability and noise-related issues by keeping the ansatz shallow.

📚 Citation

If you use VAns in your research, please cite:

@article{bilkis2023semi,
  title={A semi-agnostic ansatz with variable structure for variational quantum algorithms},
  author={Bilkis, M. and Cerezo, M. and Verdon, G. and Coles, P. J. and Cincio, L.},
  journal={Quantum Machine Intelligence},
  volume={5},
  number={43},
  year={2023},
  publisher={Springer},
  doi={10.1007/s42484-023-00132-1},
  arxiv={2103.06712}
}

🎯 Overview

VAns explores the hyperspace of architectures of parametrized quantum circuits to create short-depth ansatzes for VQA applications. The algorithm:

1. Starts with a (potentially non-trivial) initial circuit and optimizes its parameters until convergence
2. Inserts blocks of gates initialized to the identity, so that ansatzes at contiguous steps belong to an equivalence class
3. Simplifies the circuit by eliminating gates and finding the shortest circuit
4. Accepts or rejects modifications based on cost function improvements

This mechanism prevents circuits from over-growing and gets the most out of available quantum resources.

✨ Features

VAns has been successfully applied to:

1. Variational Quantum Eigensolver (VQE)

• Condensed Matter Systems: Transverse Field Ising Model (TFIM), XXZ model
• Quantum Chemistry: H₂, H₄, LiH molecules
• Finds lower energy states than fixed-structure ansatzes using the same resources

2. Quantum Autoencoder

• Data compression for quantum states
• Optimizes circuit structure for efficient state encoding/decoding

3. Unitary Compilation

• Compiles target unitaries into optimized quantum circuits
• Reduces circuit depth while maintaining fidelity

🚀 Quick Start

Prerequisites

• Python 3.7-3.9 (for TensorFlow Quantum compatibility)
• pip or conda

Installation

1. Clone the repository:

   git clone https://github.com/matibilkis/qvans.git
   cd qvans

2. Create a virtual environment (recommended):

   python3 -m venv venv
   source venv/bin/activate  # On Windows: venv\Scripts\activate

3. Install dependencies:

   pip install -r requirements.txt

   For different TensorFlow Quantum versions, see Version Management.

4. Run VAns:

   python3 main.py --problem TFIM --J 0.6 --g 1.0 --n_qubits 4

📖 Usage

Basic VQE Example

For a Transverse Field Ising Model (TFIM) with 4 qubits, J=0.6, g=1.0:

python3 main.py --problem TFIM --J 0.6 --g 1.0 --n_qubits 4 \
    --reps 150 --qepochs 10000 --qlr 0.01

Quantum Chemistry Example

For H₂ molecule at bond length 1.5 Å:

python3 main.py --n_qubits 4 --problem_config '{"problem": "H2", "geometry": [("H", (0., 0., 0.)), ("H", (0., 0., 1.5))], "multiplicity": 1, "charge": 0, "basis": "sto-3g"}' \
    --reps 200 --qepochs 10000

Parameter Sweeps

Use meta_main.py to sweep over parameters:

# Edit meta_main.py to customize sweeps
js = np.arange(0.5, 1.6, 0.1)
for J in js:
    problem_config = dict_to_json({"problem": "TFIM", "g": 1.0, "J": J})
    instruction = "python3 main.py --n_qubits 4 --problem_config {}".format(problem_config)
    insts.append(instruction)

Then run:

python3 meta_main.py

Command-Line Arguments

General Configuration

• --n_qubits: Number of qubits (default: 8)
• --reps: Number of VAns iterations (default: 150)
• --path_results: Path to save results (default: "../data-vans/")
• --problem_config: JSON string with problem configuration

VQE Options

• --qepochs: Maximum training epochs per VQE (default: 10000)
• --qlr: Learning rate (default: 0.01)
• --training_patience: Early stopping patience (default: 1000)
• --optimizer: Optimizer choice: "adam", "sgd", "adagrad" (default: "adam")
• --max_vqe_time: Maximum time per VQE in seconds (default: 300)

VAns Hyperparameters

• --initialization: Initial ansatz: "hea", "separable", "xz" (default: "hea")
• --init_layers_hea: Number of initial HEA layers (default: 1)
• --acceptance_percentage: Energy improvement threshold for acceptance (default: 0.01)
• --rate_iids_per_step: Rate of identity insertions per step (default: 1.5)
• --selector_temperature: Temperature for gate selection (default: 10.0)
• --wait_to_get_back: Iterations before reverting to best circuit (default: 25)

📚 Tutorials

Interactive Jupyter notebooks are available in the tutorials/ directory:

• 0_introduction.ipynb: Basic circuit construction and VQE
• 1_VANs_methods.ipynb: VAns algorithm components
• 2_VANs_loop.ipynb: Complete VAns workflow

To run tutorials:

jupyter notebook tutorials/

🔬 Reproducibility

Environment Setup

For reproducibility, we recommend using the exact versions specified in requirements.txt:

pip install -r requirements.txt

Paper Results

The results from the paper can be reproduced using the default parameters. Key configurations:

• TFIM: --problem TFIM --J 0.5 --g 1.0 --n_qubits 4
• XXZ: --problem XXZ --J 1.0 --g 1.0 --n_qubits 4
• H₂: See quantum chemistry example above
• H₄: --problem H4 --geometry [...] --n_qubits 8

Version Compatibility

Component	Version	Notes
Python	3.7-3.9	Required for TFQ compatibility
TensorFlow	2.3.1	Compatible with TFQ 0.4.0
TensorFlow Quantum	0.4.0	Primary version
Cirq	0.9.1	Quantum circuit library
OpenFermion	1.0.0	Quantum chemistry support

🏗️ Project Structure

qvans/
├── main.py                 # Main VAns execution script
├── meta_main.py            # Parameter sweep script
├── requirements.txt        # Python dependencies
├── utilities/              # Core VAns modules
│   ├── variational.py     # VQE and Autoencoder classes
│   ├── evaluator.py       # Circuit evaluation and acceptance
│   ├── idinserter.py      # Identity insertion (growth)
│   ├── simplifier.py      # Circuit simplification
│   ├── unitary_killer.py  # Gate removal
│   ├── circuit_basics.py  # Basic circuit operations
│   ├── chemical.py        # Quantum chemistry Hamiltonians
│   └── qmodels.py         # Quantum neural network models
├── tutorials/              # Jupyter notebook tutorials
├── examples_repository/    # Example results
└── multivans/              # Experimental: Quantum combs (see below)

🔬 Experimental: MultiVAns (Quantum Combs)

⚠️ Status: Experimental/Unfinished

The multivans/ directory contains an experimental extension of VAns for optimizing quantum combs with communication channels. This work extends VAns to multi-party quantum protocols.

Note: This code was previously maintained in a separate repository (github.com/matibilkis/multivans) but has been merged into this repository for convenience. The separate repository is no longer actively maintained.

Important Notes

• State: Unfinished research code (4+ years old)
• Framework: Written in PennyLane (older version)
• Compatibility: May not work with current dependencies
• Purpose: Research exploration of quantum combs optimization

MultiVAns Features

• Quantum channel discrimination
• Multi-party quantum protocols
• Circuit database representation
• Template-based circuit construction

Usage Warning

MultiVAns is provided for research purposes only. The codebase:

• Uses older PennyLane APIs
• May require manual dependency resolution
• Has incomplete documentation
• Is not actively maintained

To explore MultiVAns:

cd multivans
# Review README.md for specific setup instructions
# Note: May require older dependency versions

🧪 Testing

Run basic functionality tests:

python3 -c "from utilities.variational import VQE; print('VQE import successful')"
python3 -c "from utilities.evaluator import Evaluator; print('Evaluator import successful')"

📊 Results

Example results are stored in examples_repository/. Each run generates:

• Circuit structures (.pkl files)
• Energy evolution (.npy files)
• Training metadata

🤝 Contributing

Contributions are welcome! Please feel free to submit a Pull Request. For major changes, please open an issue first to discuss what you would like to change.

📝 License

This software is provided for research and educational purposes. See the LICENSE file for details and disclaimers regarding government funding and use restrictions.

🙏 Acknowledgments

• Los Alamos National Laboratory (LANL)
• Sandbox@Alphabet for TPU-based Floq simulator access
• Port d'Informació Científica, Barcelona for computational resources

📧 Contact

For questions or issues, please open a GitHub issue or contact the authors.

📖 References

1. Bilkis, M., et al. "A semi-agnostic ansatz with variable structure for variational quantum algorithms." Quantum Machine Intelligence 5, 43 (2023).
2. Preskill, J. "NISQ-era and beyond." Quantum 2, 79 (2018).
3. Cerezo, M., et al. "Variational quantum algorithms." Nature Reviews Physics 3, 625-644 (2021).

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Note: This implementation corresponds to the published paper. For the latest experimental features, see the multivans/ directory (experimental status).