Qudit_interface

examples/vqe_qudit_example.py

@@ -0,0 +1,249 @@
+r"""
+You must set the backend explicitly via --backend {jax, tensorflow}.
+AD-based optimization (gradient descent) is enabled for these backends.
+A fallback random-search optimizer is also provided.
+
+Example runs:
+  python vqe_qudit_example.py --backend jax --optimizer gd --dim 3 --layers 2 --steps 200 --lr 0.1 --jit
+  python vqe_qudit_example.py --backend tensorflow --optimizer gd --dim 3 --layers 2 --steps 200 --lr 0.1
+  python vqe_qudit_example.py --backend jax --optimizer random --dim 3 --layers 2 --iters 300
+
+What this script does:
+  - Builds a 2-qudit (d>=3) ansatz with native RY/RZ single-qudit rotations on adjacent levels
+    and an RXX entangler on (0,1) level pairs.
+  - Minimizes the expectation of a simple 2-site Hermitian Hamiltonian:
+        H = N(0) + N(1) + J * [ X_sym(0)\otimes X_sym(1) + Z_sym(0)\otimes Z_sym(1) ]
+    where N = diag(0,1,...,d-1), X_sym = (X + X^\dagger)/2, Z_sym = (Z + Z^\dagger)/2.
+"""
+
+import argparse
+import math
+import sys
+from typing import Sequence, Tuple
+
+import numpy as np
+import tensorcircuit as tc
+from tensorcircuit.quditcircuit import QuditCircuit
+
+
+# ---------- Hamiltonian helpers ----------
+def symmetrize_hermitian(U: np.ndarray) -> np.ndarray:
+    return 0.5 * (U + U.conj().T)
+
+
+def build_2site_hamiltonian(
+    d: int, J: float
+) -> Tuple[np.ndarray, np.ndarray, np.ndarray, np.ndarray, float]:
+    N = np.diag(np.arange(d))
+    Xsym = symmetrize_hermitian(tc.backend.numpy(tc.quditgates._x_matrix_func(d)))
+    Zsym = symmetrize_hermitian(tc.backend.numpy(tc.quditgates._z_matrix_func(d)))
+    H0 = N.copy()
+    H1 = N.copy()
+    return H0, H1, Xsym, Zsym, J
+
+
+# ---------- Ansatz ----------
+
+
+def apply_single_qudit_layer(c: QuditCircuit, qudit: int, thetas: Sequence) -> None:
+    """
+    Apply RY(j,j+1) then RZ(j) for each adjacent level pair.
+    Number of params per site = 2*(d-1).
+    """
+    d = c._d
+    idx = 0
+    for j, k in [(p, p + 1) for p in range(d - 1)]:
+        c.ry(qudit, theta=thetas[idx], j=j, k=k)
+        idx += 1
+        c.rz(qudit, theta=thetas[idx], j=j)
+        idx += 1
+
+
+def apply_entangler(c: QuditCircuit, theta) -> None:
+    # generalized RXX on (0,1) level pair for both qudits
+    c.rxx(0, 1, theta=theta, j1=0, k1=1, j2=0, k2=1)
+
+
+def build_ansatz(nlayers: int, d: int, params: Sequence) -> QuditCircuit:
+    c = QuditCircuit(2, dim=d)
+    per_site = 2 * (d - 1)
+    per_layer = 2 * per_site + 1  # two sites + entangler
+    assert (
+        len(params) == nlayers * per_layer
+    ), f"params length {len(params)} != {nlayers * per_layer}"
+    off = 0
+    for _ in range(nlayers):
+        th0 = params[off : off + per_site]
+        off += per_site
+        th1 = params[off : off + per_site]
+        off += per_site
+        thE = params[off]
+        off += 1
+        apply_single_qudit_layer(c, 0, th0)
+        apply_single_qudit_layer(c, 1, th1)
+        apply_entangler(c, thE)
+    return c
+
+
+# ---------- Energy ----------
+
+
+def energy_expectation_backend(
+    params_b,
+    d: int,
+    nlayers: int,
+    ham: Tuple[np.ndarray, np.ndarray, np.ndarray, np.ndarray, float],
+):
+    """
+    params_b: 1D backend tensor (jax/tf) of shape [nparams].
+    Returns backend scalar.
+    """
+    bk = tc.backend
+    # Keep differentiability by passing backend scalars into gates
+    plist = [params_b[i] for i in range(params_b.shape[0])]
+    c = build_ansatz(nlayers, d, plist)
+    E = 0.0 + 0.0j
+    H0, H1, Xsym, Zsym, J = ham
+    # Local number operators
+    E = E + c.expectation((tc.gates.Gate(H0), [0]))
+    E = E + c.expectation((tc.gates.Gate(H1), [1]))
+    # Coupling terms as products on separate sites (avoids 9x9 reshaping issues)
+    E = E + J * c.expectation((tc.gates.Gate(Xsym), [0]), (tc.gates.Gate(Xsym), [1]))
+    E = E + J * c.expectation((tc.gates.Gate(Zsym), [0]), (tc.gates.Gate(Zsym), [1]))
+    return bk.real(E)
+
+
+def energy_expectation_numpy(
+    params_np: np.ndarray,
+    d: int,
+    nlayers: int,
+    ham: Tuple[np.ndarray, np.ndarray, np.ndarray, np.ndarray, float],
+) -> float:
+    c = build_ansatz(nlayers, d, params_np.tolist())
+    E = 0.0 + 0.0j
+    H0, H1, Xsym, Zsym, J = ham
+    E += c.expectation((tc.gates.Gate(H0), [0]))
+    E += c.expectation((tc.gates.Gate(H1), [1]))
+    # Coupling terms as products on separate sites (avoids 9x9 reshaping issues)
+    E += J * c.expectation((tc.gates.Gate(Xsym), [0]), (tc.gates.Gate(Xsym), [1]))
+    E += J * c.expectation((tc.gates.Gate(Zsym), [0]), (tc.gates.Gate(Zsym), [1]))
+    return float(np.real(E))
+
+
+# ---------- Optimizers ----------
+
+
+def random_search(fun_numpy, x0_shape, iters=300, seed=42):
+    rng = np.random.default_rng(seed)
+    best_x, best_y = None, float("inf")
+    for _ in range(iters):
+        x = rng.uniform(-math.pi, math.pi, size=x0_shape)
+        y = fun_numpy(x)
+        if y < best_y:
+            best_x, best_y = x, y
+    return best_x, float(best_y)
+
+
+def gradient_descent_ad(energy_bk, x0_np: np.ndarray, steps=200, lr=0.1, jit=False):
+    """
+    energy_bk: (backend_tensor[nparams]) -> backend_scalar
+    Simple gradient descent in numpy space with backend-gradients.
+    """
+    bk = tc.backend
+    if jit:
+        energy_bk = bk.jit(energy_bk)
+    grad_f = bk.grad(energy_bk)
+
+    x_np = x0_np.copy()
+    best_x, best_y = x_np.copy(), float("inf")
+
+    def to_np(x):
+        return x if isinstance(x, np.ndarray) else bk.numpy(x)
+
+    for _ in range(steps):
+        x_b = bk.convert_to_tensor(x_np)  # numpy -> backend tensor
+        g_b = grad_f(x_b)  # backend gradient
+        g = to_np(g_b)  # backend -> numpy
+        x_np = x_np - lr * g  # SGD step in numpy
+        y = float(to_np(energy_bk(bk.convert_to_tensor(x_np))))
+        if y < best_y:
+            best_x, best_y = x_np.copy(), y
+    return best_x, float(best_y)
+
+
+# ---------- CLI ----------
+
+
+def main():
+    ap = argparse.ArgumentParser(description="Qudit VQE (explicit backend)")
+    ap.add_argument(
+        "--backend",
+        required=True,
+        choices=["jax", "tensorflow"],
+        help="tensorcircuit backend",
+    )
+    ap.add_argument("--dim", type=int, default=3, help="local qudit dimension d (>=3)")
+    ap.add_argument("--layers", type=int, default=2, help="# ansatz layers")
+    ap.add_argument("--J", type=float, default=0.5, help="coupling strength")
+    ap.add_argument(
+        "--optimizer",
+        type=str,
+        default="gd",
+        choices=["gd", "random"],
+        help="gradient descent (AD) or random search",
+    )
+    ap.add_argument("--steps", type=int, default=200, help="GD steps")
+    ap.add_argument("--lr", type=float, default=0.1, help="GD learning rate")
+    ap.add_argument("--iters", type=int, default=300, help="random search steps")
+    ap.add_argument("--seed", type=int, default=42, help="RNG seed")
+    ap.add_argument(
+        "--jit",
+        action="store_true",
+        help="enable backend JIT (all backends implement .jit; numpy backend no-ops)",
+    )
+    args = ap.parse_args()
+
+    tc.set_backend(args.backend)
+
+    if args.dim < 3:
+        print("Please use dim >= 3 for qudits.", file=sys.stderr)
+        sys.exit(1)
+
+    d, L = args.dim, args.layers
+    per_layer = 4 * (d - 1) + 1
+    nparams = L * per_layer
+
+    ham = build_2site_hamiltonian(d, args.J)
+
+    print(
+        f"[info] backend={args.backend}, d={d}, layers={L}, params={nparams}, J={args.J}"
+    )
+
+    if args.optimizer == "random":
+
+        def obj_np(theta_np):
+            return energy_expectation_numpy(theta_np, d, L, ham)
+
+        x, y = random_search(
+            obj_np, x0_shape=(nparams,), iters=args.iters, seed=args.seed
+        )
+    else:
+
+        def obj_bk(theta_b):
+            return energy_expectation_backend(theta_b, d, L, ham)
+
+        rng = np.random.default_rng(args.seed)
+        x0 = rng.uniform(-math.pi, math.pi, size=(nparams,))
+        x, y = gradient_descent_ad(
+            obj_bk, x0_np=x0, steps=args.steps, lr=args.lr, jit=args.jit
+        )
+
+    print("\n=== Result ===")
+    print(f"Energy      : {y:.6f}")
+    print(f"Params shape: {x.shape}")
+    np.set_printoptions(precision=4, suppress=True)
+    print(x[: min(10, x.size)])
+
+
+if __name__ == "__main__":
+    main()

tensorcircuit/__init__.py

@@ -23,8 +23,10 @@
     runtime_contractor,
 )  # prerun of set hooks
 from . import gates
+from . import quditgates
 from . import basecircuit
 from .gates import Gate
+from .quditcircuit import QuditCircuit
 from .circuit import Circuit, expectation
 from .mpscircuit import MPSCircuit
 from .densitymatrix import DMCircuit as DMCircuit_reference

tensorcircuit/basecircuit.py

@@ -480,28 +480,12 @@ def measure_jit(
 
     def amplitude_before(self, l: Union[str, Tensor]) -> List[Gate]:
         r"""
-        Returns the tensornetwor nodes for the amplitude of the circuit given a computational-basis label ``l``.
-        For a state simulator, it computes :math:`\langle l \vert \psi\rangle`;
-        for a density-matrix simulator, it computes :math:`\mathrm{Tr}(\rho \vert l\rangle\langle l\vert)`.
+        Returns the tensornetwor nodes for the amplitude of the circuit given the bitstring l.
+        For state simulator, it computes :math:`\langle l\vert \psi\rangle`,
+        for density matrix simulator, it computes :math:`Tr(\rho \vert l\rangle \langle 1\vert)`
         Note how these two are different up to a square operation.
 
-        :Example:
-
-        >>> c = tc.Circuit(2)
-        >>> c.X(0)
-        >>> c.amplitude("10")  # d=2, per-qubit digits
-        array(1.+0.j, dtype=complex64)
-        >>> c.CNOT(0, 1)
-        >>> c.amplitude("11")
-        array(1.+0.j, dtype=complex64)
-
-        For qudits (d>2, d<=36):
-        >>> c = tc.Circuit(3, dim=12)
-        >>> c.amplitude("0A2")  # base-12 string, A stands for 10
-
-        :param l: Basis label.
-            - If a string: it must be a base-d string of length ``nqubits``, using 0–9A–Z (A=10,…,Z=35) when ``d<=36``.
-            - If a tensor/array/list: it should contain per-site integers in ``[0, d-1]`` with length ``nqubits``.
+        :param l: The bitstring of 0 and 1s.
         :type l: Union[str, Tensor]
         :return: The tensornetwork nodes for the amplitude of the circuit.
         :rtype: List[Gate]