qibotn/benchmark_mps.py

"""Benchmark: qibojit (reference) vs qibotn/quimb MPS, with error comparison."""
import time
import argparse
import os
import numpy as np
import qibo
import quimb.tensor as qtn
from qibo import Circuit, gates

DATA_DIR = os.path.join(os.path.dirname(__file__), "data")


def make_circuit(circuit_type, nqubits, nlayers=1, add_measurements=False):
    c = Circuit(nqubits)
    if circuit_type == "qft":
        from qibo.models import QFT
        c = QFT(nqubits)
    elif circuit_type == "variational":
        for layer in range(nlayers):
            for q in range(nqubits):
                c.add(gates.RY(q, theta=np.random.uniform(0, 2 * np.pi)))
            offset = layer % 2
            for q in range(offset, nqubits - 1, 2):
                c.add(gates.CZ(q, q + 1))
    elif circuit_type == "ghz":
        c.add(gates.H(0))
        for q in range(nqubits - 1):
            c.add(gates.CNOT(q, q + 1))
    else:
        raise ValueError(f"Unknown circuit: {circuit_type}")
    if add_measurements:
        c.add(gates.M(*range(nqubits)))
    return c


def run_qibojit(circuit):
    qibo.set_backend("qibojit", platform="numba")
    t0 = time.time()
    result = circuit()
    elapsed = time.time() - t0
    sv = result.state()
    return sv, elapsed


def run_quimb_mps(circuit, max_bond, svd_cutoff, optimizer, nshots=None):
    qibo.set_backend("qibotn", platform="quimb")
    b = qibo.get_backend()
    b.configure_tn_simulation(ansatz="mps", max_bond_dimension=max_bond, svd_cutoff=svd_cutoff)
    b.contractions_optimizer = optimizer

    t0 = time.time()
    if nshots:
        result = b.execute_circuit(circuit, nshots=nshots)
        elapsed = time.time() - t0
        return result.frequencies(), elapsed, 0.0
    else:
        # MPS simulation
        circ_quimb = qtn.CircuitMPS.from_openqasm2_str(
            circuit.to_qasm(),
            gate_opts={"max_bond": max_bond, "cutoff": svd_cutoff},
        )
        t_mps = time.time() - t0
        # to_dense separately
        t1 = time.time()
        #sv = circ_quimb.psi.to_dense().reshape(-1)
        sv = None
        t_dense = time.time() - t1
        return sv, t_mps, t_dense


def run_quimb_permmps(circuit, max_bond, svd_cutoff, nshots=None):
    gates_list = [
        qtn.Gate(g.name, params=list(g.parameters), qubits=list(g.qubits))
        for g in circuit.queue
        if g.name.lower() != "measure"
    ]
    t0 = time.time()
    circ = qtn.CircuitPermMPS.from_gates(
        gates_list,
        N=circuit.nqubits,
        max_bond=max_bond,
        cutoff=svd_cutoff,
    )
    if nshots:
        from collections import Counter
        result = Counter(circ.sample(nshots))
        elapsed = time.time() - t0
        return dict(result), elapsed
    else:
        mps = circ.get_psi_unordered()
        sv = mps.to_dense().reshape(-1)
        elapsed = time.time() - t0
        return sv, elapsed


def compare_statevector(sv_ref, sv_mps):
    sv_ref = np.array(sv_ref, dtype=complex).flatten()
    sv_mps = np.array(sv_mps, dtype=complex).flatten()
    fidelity = abs(np.dot(sv_ref.conj(), sv_mps)) ** 2
    l2_err = np.linalg.norm(sv_ref - sv_mps)
    return fidelity, l2_err


def compare_frequencies(freq_ref, freq_mps, nshots):
    all_keys = set(freq_ref) | set(freq_mps)
    tvd = 0.5 * sum(abs(freq_ref.get(k, 0) - freq_mps.get(k, 0)) for k in all_keys) / nshots
    return tvd


def jit_cache_path(circuit_type, nqubits, nlayers):
    os.makedirs(DATA_DIR, exist_ok=True)
    return os.path.join(DATA_DIR, f"jit_{circuit_type}_n{nqubits}_l{nlayers}.npy")


def main():
    parser = argparse.ArgumentParser()
    parser.add_argument("--nqubits", type=int, default=10)
    parser.add_argument("--circuit", type=str, default="ghz",
                        choices=["qft", "variational", "ghz"])
    parser.add_argument("--nlayers", type=int, default=3)
    parser.add_argument("--max-bond", type=int, default=None,
                        help="Max bond dimension for MPS (None = unlimited)")
    parser.add_argument("--svd-cutoff", type=float, default=1e-6)
    parser.add_argument("--optimizer", type=str, default="eager")
    parser.add_argument("--nshots", type=int, default=None,
                        help="Use sampling mode with given number of shots instead of statevector")
    parser.add_argument("--permmps", action="store_true",
                        help="Use CircuitPermMPS directly instead of qibotn backend")
    parser.add_argument("--skip-jit", action="store_true",
                        help="Skip qibojit run, load cached statevector if available")
    parser.add_argument("--no-compare", action="store_true",
                        help="Skip correctness comparison entirely")
    args = parser.parse_args()

    print(f"Circuit: {args.circuit}, nqubits={args.nqubits}, nlayers={args.nlayers}")
    print(f"MPS config: max_bond={args.max_bond}, svd_cutoff={args.svd_cutoff}, optimizer={args.optimizer}")

    ref = None
    t_ref = None

    if not args.no_compare:
        cache_path = jit_cache_path(args.circuit, args.nqubits, args.nlayers)
        if args.nshots:
            # frequency mode: run qibojit with same nshots
            np.random.seed(42)
            circuit_ref = make_circuit(args.circuit, args.nqubits, args.nlayers, add_measurements=True)
            qibo.set_backend("qibojit", platform="numba")
            t0 = time.time()
            result_ref = circuit_ref(nshots=args.nshots)
            t_ref = time.time() - t0
            ref = dict(result_ref.frequencies())
            print(f"\n[qibojit]  time={t_ref:.4f}s")
        elif args.skip_jit and os.path.exists(cache_path):
            ref = np.load(cache_path)
            print(f"\n[qibojit]  loaded from cache: {cache_path}")
        else:
            np.random.seed(42)
            circuit_ref = make_circuit(args.circuit, args.nqubits, args.nlayers)
            ref, t_ref = run_qibojit(circuit_ref)
            np.save(cache_path, ref)
            print(f"\n[qibojit]  time={t_ref:.4f}s  (saved to {cache_path})")

    np.random.seed(42)
    circuit_mps = make_circuit(args.circuit, args.nqubits, args.nlayers)
    label = "quimb PermMPS" if args.permmps else "quimb MPS"
    try:
        if args.permmps:
            out, t_mps = run_quimb_permmps(circuit_mps, args.max_bond, args.svd_cutoff, args.nshots)
            t_dense = 0.0
        else:
            out, t_mps, t_dense = run_quimb_mps(circuit_mps, args.max_bond, args.svd_cutoff, args.optimizer, args.nshots)
        print(f"[{label}] MPS sim={t_mps:.4f}s  to_dense={t_dense:.4f}s  total={t_mps+t_dense:.4f}s")
        if not args.no_compare:
            if args.nshots:
                tvd = compare_frequencies(ref, out, args.nshots)
                print(f"\nTVD      : {tvd:.6f}  (0=perfect)")
            else:
                fidelity, l2_err = compare_statevector(ref, out)
                print(f"\nFidelity : {fidelity:.8f}  (1=perfect)")
                print(f"L2 error : {l2_err:.2e}")
            if t_ref is not None and t_mps > 0:
                print(f"Speedup  : {t_ref/t_mps:.2f}x")
    except Exception as e:
        print(f"[quimb MPS] FAILED: {e}")
        raise


if __name__ == "__main__":
    main()