Implements optimizations A.1 and A.2 for Quantum Shannon Decomposition

[codrut3]

Fixes #6777

I checked that with the current PR, the circuit depth after transpiler matches that obtained by Qiskit.

The runtime is slow due merge_single_qubit_gates_to_phased_x_and_z. I wrote a different implementation for the special case used by the Shannon decomposition. With this, the runtime is halved. I believe merge_single_qubit_gates_to_phased_x_and_z can be optimized too.

[codrut3]

@NoureldinYosri Please have a look!

[codrut3]

For future reference, this is the script I used to benchmark the decomposition:

# pylint: disable=wrong-or-nonexistent-copyright-notice
"""Tests performance of quantum Shannon decomposition.
"""

import sys
import time

from scipy.stats import unitary_group

import cirq
import qiskit.qasm2


if not sys.warnoptions:
    import warnings
    warnings.simplefilter("ignore")


def main():
    for n_qubits in range(3, 9):
        U = unitary_group.rvs(2**n_qubits)
        qubits = [cirq.NamedQubit(f'q{i}') for i in range(n_qubits)]
        start_time = time.time()
        circuit = cirq.Circuit(cirq.quantum_shannon_decomposition(qubits, U))
        elapsed_time = time.time() - start_time
        print(f'Depth for {n_qubits} qubits (no transpiler): {len(circuit)} ({elapsed_time:.2f} sec)')
        print(f'Number of CZ gates (no transpiler): {len(list(circuit.findall_operations_with_gate_type(cirq.CZPowGate)))}')
        print(f'Number of CNOT gates (no transpiler): {len(list(circuit.findall_operations_with_gate_type(cirq.CXPowGate)))}')


        # Export to qasm
        qasm_str = cirq.qasm(circuit, args=cirq.QasmArgs(version="2.0"))
        qiskit_circuit = qiskit.qasm2.loads(qasm_str, include_path=('.',), custom_instructions=qiskit.qasm2.LEGACY_CUSTOM_INSTRUCTIONS, custom_classical=(), strict=False)

        # Run qiskit transpiler
        start_time = time.time()
        opt_circuit = qiskit.compiler.transpile(qiskit_circuit, basis_gates=["cz", "u3"], optimization_level=3)
        elapsed_time = time.time() - start_time
        print(f'Qiskit depth for {n_qubits} qubits (with transpiler): {opt_circuit.depth()} ({elapsed_time:.2f} sec)')
        print(f'Qiskit gate counts for {n_qubits} qubits (with transpiler): {dict(opt_circuit.count_ops())}')

        # Run cirq transpiler
        start_time = time.time()
        circuit = cirq.optimize_for_target_gateset(circuit, gateset=cirq.CZTargetGateset(preserve_moment_structure=False), max_num_passes=None)
        elapsed_time = time.time() - start_time
        print(f'Cirq depth for {n_qubits} qubits (with transpiler): {len(circuit)} ({elapsed_time:.2f} sec)')
        # Count the operations
        operation_counts = {}
        for op in circuit.all_operations():
            gate_type = op.gate.__class__
            if isinstance(op.gate, cirq.CZPowGate):
                gate_type = 'cz'
            elif isinstance(op.gate, cirq.PhasedXZGate):
                gate_type = 'u3'
            if gate_type not in operation_counts:
                operation_counts[gate_type] = 0
            operation_counts[gate_type] += 1
        print(f'Cirq gate counts for {n_qubits} qubits (with transpiler): {operation_counts}')


if __name__ == '__main__':
    main()

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.68%. Comparing base (12eab31) to head (b31d996).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7390   +/-   ##
=======================================
  Coverage   98.68%   98.68%           
=======================================
  Files        1112     1112           
  Lines       97642    97703   +61     
=======================================
+ Hits        96359    96421   +62     
+ Misses       1283     1282    -1     

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