Qubit Tapering for Quantum Chemistry

Qubit Tapering for Quantum Chemistry

This PennyLane tutorial explains qubit tapering, a powerful technique for reducing the number of qubits needed to simulate a molecule by exploiting the symmetries of its Hamiltonian, often shrinking the problem by several qubits at no loss of accuracy. Molecular Hamiltonians possess symmetries, such as the conservation of the number of electrons and of spin, that correspond to Pauli operators commuting with the whole Hamiltonian (a Z2 symmetry group). Tapering uses these symmetries to transform the Hamiltonian so that certain qubits are acted on only trivially and can be removed, replaced by classical plus-or-minus-one values fixed by the relevant symmetry sector. The tutorial shows how to find a Hamiltonian's symmetries in PennyLane, construct the Clifford transformation that exposes the removable qubits, taper them away to obtain a smaller effective Hamiltonian, and correspondingly transform the state-preparation circuit so a VQE calculation runs on fewer qubits. It demonstrates the qubit savings on a sample molecule and verifies that the tapered calculation reproduces the original energy. By cutting qubit cost through symmetry, the tutorial gives a practical, widely-used tool for making larger chemistry problems tractable on near-term hardware in PennyLane.

Run it

pip install -r requirements.txt
python demo.py

Source and license

Imported from demonstrations_v2/tutorial_qubit_tapering/demo.py in PennyLaneAI/demos at c52c0abeb5122218aa96b38eea848864cce7323f, under the Apache License 2.0. Original authors: Xanadu and the PennyLane community. The upstream LICENSE is included alongside this example.