def test_optimize_grad(self):
"""Test that the gradient of VQECost is accessible and correct when using observable
optimization and the autograd interface."""
if not qml.tape_mode_active():
pytest.skip("This test is only intended for tape mode")
dev = qml.device("default.qubit", wires=4)
hamiltonian = big_hamiltonian
cost = qml.VQECost(qml.templates.StronglyEntanglingLayers,
hamiltonian,
dev,
optimize=True)
cost2 = qml.VQECost(qml.templates.StronglyEntanglingLayers,
hamiltonian,
dev,
optimize=False)
w = qml.init.strong_ent_layers_uniform(2, 4, seed=1967)
dc = qml.grad(cost)(w)
exec_opt = dev.num_executions
dev._num_executions = 0
dc2 = qml.grad(cost2)(w)
exec_no_opt = dev.num_executions
assert exec_no_opt > exec_opt
assert np.allclose(dc, big_hamiltonian_grad)
assert np.allclose(dc2, big_hamiltonian_grad)
def test_integration_hamiltonian_to_vqe_cost(monkeypatch, mol_name, terms_ref,
expected_cost, tol):
r"""Test if `convert_hamiltonian()` in qchem integrates with `VQECost()` in pennylane"""
qOp = QubitOperator()
if terms_ref is not None:
monkeypatch.setattr(qOp, "terms", terms_ref)
vqe_hamiltonian = qchem.convert_hamiltonian(qOp)
# maybe make num_qubits a @property of the Hamiltonian class?
num_qubits = max(
1, len(set([w for op in vqe_hamiltonian.ops for w in op.wires])))
dev = qml.device("default.qubit", wires=num_qubits)
print(vqe_hamiltonian.terms)
# can replace the ansatz with more suitable ones later.
def dummy_ansatz(phis, wires):
for phi, w in zip(phis, wires):
qml.RX(phi, wires=w)
dummy_cost = qml.VQECost(dummy_ansatz, vqe_hamiltonian, dev)
params = [0.1 * i for i in range(num_qubits)]
res = dummy_cost(params)
assert np.allclose(res, expected_cost, **tol)
def test_module_example(self, tol):
"""Test the example in the QAOA module docstring"""
# Defines the wires and the graph on which MaxCut is being performed
wires = range(3)
graph = Graph([(0, 1), (1, 2), (2, 0)])
# Defines the QAOA cost and mixer Hamiltonians
cost_h, mixer_h = qaoa.maxcut(graph)
# Defines a layer of the QAOA ansatz from the cost and mixer Hamiltonians
def qaoa_layer(gamma, alpha):
qaoa.cost_layer(gamma, cost_h)
qaoa.mixer_layer(alpha, mixer_h)
# Repeatedly applies layers of the QAOA ansatz
def circuit(params, **kwargs):
for w in wires:
qml.Hadamard(wires=w)
qml.layer(qaoa_layer, 2, params[0], params[1])
# Defines the device and the QAOA cost function
dev = qml.device('default.qubit', wires=len(wires))
cost_function = qml.VQECost(circuit, cost_h, dev)
res = cost_function([[1, 1], [1, 1]])
expected = -1.8260274380964299
assert np.allclose(res, expected, atol=tol, rtol=0)
def test_integration_observable_to_vqe_cost(monkeypatch, mol_name, terms_ref,
expected_cost, custom_wires, tol):
r"""Test if `convert_observable()` in qchem integrates with `VQECost()` in pennylane"""
qOp = QubitOperator()
if terms_ref is not None:
monkeypatch.setattr(qOp, "terms", terms_ref)
vqe_observable = qchem.convert_observable(qOp, custom_wires)
num_qubits = len(vqe_observable.wires)
assert vqe_observable.terms.__repr__() # just to satisfy codecov
if custom_wires is None:
wires = num_qubits
elif isinstance(custom_wires, dict):
wires = qchem.structure._process_wires(custom_wires)
else:
wires = custom_wires[:num_qubits]
dev = qml.device("default.qubit", wires=wires)
# can replace the ansatz with more suitable ones later.
def dummy_ansatz(phis, wires):
for phi, w in zip(phis, wires):
qml.RX(phi, wires=w)
dummy_cost = qml.VQECost(dummy_ansatz, vqe_observable, dev)
params = [0.1 * i for i in range(num_qubits)]
res = dummy_cost(params)
assert np.allclose(res, expected_cost, **tol)
def test_optimize_grad_tf(self, tf_support):
"""Test that the gradient of VQECost is accessible and correct when using observable
optimization and the TensorFlow interface."""
if not qml.tape_mode_active():
pytest.skip("This test is only intended for tape mode")
if not tf_support:
pytest.skip("This test requires TensorFlow")
dev = qml.device("default.qubit", wires=4)
hamiltonian = big_hamiltonian
cost = qml.VQECost(qml.templates.StronglyEntanglingLayers,
hamiltonian,
dev,
optimize=True,
interface="tf")
w = tf.Variable(qml.init.strong_ent_layers_uniform(2, 4, seed=1967))
with tf.GradientTape() as tape:
res = cost(w)
dc = tape.gradient(res, w).numpy()
assert np.allclose(dc, big_hamiltonian_grad)
def test_optimize_grad_torch(self, torch_support):
"""Test that the gradient of VQECost is accessible and correct when using observable
optimization and the Torch interface."""
if not qml.tape_mode_active():
pytest.skip("This test is only intended for tape mode")
if not torch_support:
pytest.skip("This test requires Torch")
dev = qml.device("default.qubit", wires=4)
hamiltonian = big_hamiltonian
cost = qml.VQECost(
qml.templates.StronglyEntanglingLayers,
hamiltonian,
dev,
optimize=True,
interface="torch",
)
w = torch.tensor(qml.init.strong_ent_layers_uniform(2, 4, seed=1967),
requires_grad=True)
res = cost(w)
res.backward()
dc = w.grad.detach().numpy()
assert np.allclose(dc, big_hamiltonian_grad)
def test_gradient(self, tol):
"""Test differentiation works"""
dev = qml.device("default.qubit", wires=1)
def ansatz(params, **kwargs):
qml.RX(params[0], wires=0)
qml.RY(params[1], wires=0)
coeffs = [0.2, 0.5]
observables = [qml.PauliX(0), qml.PauliY(0)]
H = qml.vqe.Hamiltonian(coeffs, observables)
a, b = 0.54, 0.123
params = Variable([a, b], dtype=tf.float64)
cost = qml.VQECost(ansatz, H, dev, interface="tf")
with tf.GradientTape() as tape:
loss = cost(params)
res = np.array(tape.gradient(loss, params))
expected = [
-coeffs[0] * np.sin(a) * np.sin(b) - coeffs[1] * np.cos(a),
coeffs[0] * np.cos(a) * np.cos(b)
]
assert np.allclose(res, expected, atol=tol, rtol=0)
def test_cost_evaluate(self, params, ansatz, coeffs, observables):
"""Tests that the cost function evaluates properly"""
hamiltonian = qml.vqe.Hamiltonian(coeffs, observables)
dev = qml.device("default.qubit", wires=3)
expval = qml.VQECost(ansatz, hamiltonian, dev)
assert type(expval(params)) == np.float64
assert np.shape(expval(params)) == () # expval should be scalar
def test_gradient(self, tol):
"""Test differentiation works"""
dev = qml.device("default.qubit", wires=1)
def ansatz(params, **kwargs):
qml.RX(params[0], wires=0)
qml.RY(params[1], wires=0)
coeffs = [0.2, 0.5]
observables = [qml.PauliX(0), qml.PauliY(0)]
H = qml.vqe.Hamiltonian(coeffs, observables)
a, b = 0.54, 0.123
params = torch.autograd.Variable(torch.tensor([a, b]),
requires_grad=True)
cost = qml.VQECost(ansatz, H, dev, interface="torch")
loss = cost(params)
loss.backward()
res = params.grad.numpy()
expected = [
-coeffs[0] * np.sin(a) * np.sin(b) - coeffs[1] * np.cos(a),
coeffs[0] * np.cos(a) * np.cos(b)
]
assert np.allclose(res, expected, atol=tol, rtol=0)
def test_gradient(self, tol, interface):
"""Test differentiation works"""
dev = qml.device("default.qubit", wires=1)
def ansatz(params, **kwargs):
qml.RX(params[0], wires=0)
qml.RY(params[1], wires=0)
coeffs = [0.2, 0.5]
observables = [qml.PauliX(0), qml.PauliY(0)]
H = qml.vqe.Hamiltonian(coeffs, observables)
a, b = 0.54, 0.123
params = np.array([a, b])
cost = qml.VQECost(ansatz, H, dev, interface=interface)
dcost = qml.grad(cost, argnum=[0])
res = dcost(params)
expected = [
-coeffs[0] * np.sin(a) * np.sin(b) - coeffs[1] * np.cos(a),
coeffs[0] * np.cos(a) * np.cos(b)
]
assert np.allclose(res, expected, atol=tol, rtol=0)
def test_all_interfaces_gradient_agree(self, tol):
"""Test the gradient agrees across all interfaces"""
dev = qml.device("default.qubit", wires=2)
coeffs = [0.2, 0.5]
observables = [qml.PauliX(0) @ qml.PauliZ(1), qml.PauliY(0)]
H = qml.vqe.Hamiltonian(coeffs, observables)
# TensorFlow interface
params = Variable(
qml.init.strong_ent_layers_normal(n_layers=3, n_wires=2, seed=1))
ansatz = qml.templates.layers.StronglyEntanglingLayers
cost = qml.VQECost(ansatz, H, dev, interface="tf")
with tf.GradientTape() as tape:
loss = cost(params)
res_tf = np.array(tape.gradient(loss, params))
# Torch interface
params = torch.tensor(
qml.init.strong_ent_layers_normal(n_layers=3, n_wires=2, seed=1))
params = torch.autograd.Variable(params, requires_grad=True)
ansatz = qml.templates.layers.StronglyEntanglingLayers
cost = qml.VQECost(ansatz, H, dev, interface="torch")
loss = cost(params)
loss.backward()
res_torch = params.grad.numpy()
# NumPy interface
params = qml.init.strong_ent_layers_normal(n_layers=3,
n_wires=2,
seed=1)
ansatz = qml.templates.layers.StronglyEntanglingLayers
cost = qml.VQECost(ansatz, H, dev, interface="numpy")
dcost = qml.grad(cost, argnum=[0])
res = dcost(params)
assert np.allclose(res, res_tf, atol=tol, rtol=0)
assert np.allclose(res, res_torch, atol=tol, rtol=0)
def test_vqe_cost():
"""Tests that VQECost raises a UserWarning but otherwise behaves as ExpvalCost"""
h = qml.Hamiltonian([1], [qml.PauliZ(0)])
dev = qml.device("default.qubit", wires=1)
ansatz = qml.templates.StronglyEntanglingLayers
with pytest.warns(UserWarning, match="Use of VQECost is deprecated"):
cost = qml.VQECost(ansatz, h, dev)
assert isinstance(cost, qml.ExpvalCost)
def test_optimize(self, interface, tf_support, torch_support):
"""Test that a VQECost with observable optimization gives the same result as another
VQECost without observable optimization."""
if not qml.tape_mode_active():
pytest.skip("This test is only intended for tape mode")
if interface == "tf" and not tf_support:
pytest.skip("This test requires TensorFlow")
if interface == "torch" and not torch_support:
pytest.skip("This test requires Torch")
dev = qml.device("default.qubit", wires=4)
hamiltonian = big_hamiltonian
cost = qml.VQECost(
qml.templates.StronglyEntanglingLayers,
hamiltonian,
dev,
optimize=True,
interface=interface,
)
cost2 = qml.VQECost(
qml.templates.StronglyEntanglingLayers,
hamiltonian,
dev,
optimize=False,
interface=interface,
)
w = qml.init.strong_ent_layers_uniform(2, 4, seed=1967)
c1 = cost(w)
exec_opt = dev.num_executions
dev._num_executions = 0
c2 = cost2(w)
exec_no_opt = dev.num_executions
assert exec_opt == 5 # Number of groups in the Hamiltonian
assert exec_no_opt == 15
assert np.allclose(c1, c2)
def test_single_qubit_vqe_using_vqecost(self, tol):
"""Test single-qubit VQE using VQECost
has the correct QNG value every step, the correct parameter updates,
and correct cost after 200 steps"""
dev = qml.device("default.qubit", wires=1)
def circuit(params, wires=0):
qml.RX(params[0], wires=wires)
qml.RY(params[1], wires=wires)
coeffs = [1, 1]
obs_list = [
qml.PauliX(0),
qml.PauliZ(0)
]
h = qml.Hamiltonian(coeffs=coeffs, observables=obs_list)
cost_fn = qml.VQECost(ansatz=circuit, hamiltonian=h, device=dev)
def gradient(params):
"""Returns the gradient"""
da = -np.sin(params[0]) * (np.cos(params[1]) + np.sin(params[1]))
db = np.cos(params[0]) * (np.cos(params[1]) - np.sin(params[1]))
return np.array([da, db])
eta = 0.01
init_params = np.array([0.011, 0.012])
num_steps = 200
opt = qml.QNGOptimizer(eta)
theta = init_params
# optimization for 200 steps total
for t in range(num_steps):
theta_new = opt.step(cost_fn, theta)
# check metric tensor
res = opt.metric_tensor
exp = np.diag([0.25, (np.cos(theta[0]) ** 2)/4])
assert np.allclose(res, exp, atol=tol, rtol=0)
# check parameter update
dtheta = eta * sp.linalg.pinvh(exp) @ gradient(theta)
assert np.allclose(dtheta, theta - theta_new, atol=tol, rtol=0)
theta = theta_new
# check final cost
assert np.allclose(cost_fn(theta), -1.41421356, atol=tol, rtol=0)
def test_optimize_outside_tape_mode(self):
"""Test that an error is raised if observable optimization is requested outside of tape
mode."""
if qml.tape_mode_active():
pytest.skip("This test is only intended for non-tape mode")
dev = qml.device("default.qubit", wires=2)
hamiltonian = qml.vqe.Hamiltonian([1], [qml.PauliZ(0)])
with pytest.raises(
ValueError,
match="Observable optimization is only supported in tape"):
qml.VQECost(lambda params, **kwargs: None,
hamiltonian,
dev,
optimize=True)
def test_passing_kwargs(self, coeffs, observables, expected):
"""Test that the step size and order used for the finite differences
differentiation method were passed to the QNode instances using the
keyword arguments."""
dev = qml.device("default.qubit", wires=2)
hamiltonian = qml.vqe.Hamiltonian(coeffs, observables)
cost = qml.VQECost(lambda params, **kwargs: None,
hamiltonian,
dev,
h=123,
order=2)
# Checking that the qnodes contain the step size and order
for qnode in cost.qnodes:
assert qnode.h == 123
assert qnode.order == 2
def test_metric_tensor(self):
"""Test that an error is raised if the metric tensor is requested in optimize=True mode."""
if not qml.tape_mode_active():
pytest.skip("This test is only intended for tape mode")
dev = qml.device("default.qubit", wires=4)
hamiltonian = big_hamiltonian
cost = qml.VQECost(qml.templates.StronglyEntanglingLayers,
hamiltonian,
dev,
optimize=True)
with pytest.raises(
ValueError,
match="Evaluation of the metric tensor is not supported"):
cost.metric_tensor(None)
def test_integration_mol_file_to_vqe_cost(name, core, active, mapping,
expected_cost, custom_wires, tol):
r"""Test if the output of `decompose()` works with `convert_observable()`
to generate `VQECost()`"""
ref_dir = os.path.join(os.path.dirname(os.path.realpath(__file__)),
"test_ref_files")
hf_file = os.path.join(ref_dir, name)
qubit_hamiltonian = qchem.decompose(
hf_file,
mapping=mapping,
core=core,
active=active,
)
vqe_hamiltonian = qchem.convert_observable(qubit_hamiltonian, custom_wires)
assert len(vqe_hamiltonian.ops) > 1 # just to check if this runs
num_qubits = len(vqe_hamiltonian.wires)
assert num_qubits == 2 * len(active)
if custom_wires is None:
wires = num_qubits
elif isinstance(custom_wires, dict):
wires = qchem.structure._process_wires(custom_wires)
else:
wires = custom_wires[:num_qubits]
dev = qml.device("default.qubit", wires=wires)
# can replace the ansatz with more suitable ones later.
def dummy_ansatz(phis, wires):
for phi, w in zip(phis, wires):
qml.RX(phi, wires=w)
phis = np.load(os.path.join(ref_dir, "dummy_ansatz_parameters.npy"))
dummy_cost = qml.VQECost(dummy_ansatz, vqe_hamiltonian, dev)
res = dummy_cost(phis)
assert np.abs(res - expected_cost) < tol["atol"]
def test_integration_mol_file_to_vqe_cost(hf_filename, docc_mo, act_mo,
type_of_transformation,
expected_cost, tol):
r"""Test if the output of `decompose_hamiltonian()` works with `convert_hamiltonian()`
to generate `VQECost()`"""
ref_dir = os.path.join(os.path.dirname(os.path.realpath(__file__)),
"test_ref_files")
transformed_hamiltonian = qchem.decompose_hamiltonian(
hf_filename,
ref_dir,
mapping=type_of_transformation,
docc_mo_indices=docc_mo,
active_mo_indices=act_mo,
)
vqe_hamiltonian = qchem.convert_hamiltonian(transformed_hamiltonian)
assert len(vqe_hamiltonian.ops) > 1 # just to check if this runs
num_qubits = max(
1,
len(
set([
w for op in vqe_hamiltonian.ops for ws in op.wires for w in ws
])))
assert num_qubits == 2 * len(act_mo)
dev = qml.device("default.qubit", wires=num_qubits)
# can replace the ansatz with more suitable ones later.
def dummy_ansatz(phis, wires):
for phi, w in zip(phis, wires):
qml.RX(phi, wires=w)
phis = np.load(os.path.join(ref_dir, "dummy_ansatz_parameters.npy"))
dummy_cost = qml.VQECost(dummy_ansatz, vqe_hamiltonian, dev)
res = dummy_cost(phis)
assert np.abs(res - expected_cost) < tol["atol"]
##############################################################################
# The ground state for each inter-atomic distance is characterized by a different Y-rotation angle.
# The values of these Y-rotations can be found by minimizing the ground state energy as outlined in
# :doc:`tutorial_vqe`. In this tutorial, we load pre-optimized rotations and focus on
# comparing the speed of evaluating the potential energy surface with sequential and parallel
# evaluation. These parameters can be downloaded by clicking :download:`here
# <../demonstrations/vqe_parallel/RY_params.npy>`.
params = np.load("vqe_parallel/RY_params.npy")
##############################################################################
# Finally, the energies as functions of rotation angle can be given using
# :class:`~.pennylane.VQECost`.
energies = [qml.VQECost(circuit, h, devs) for h in hamiltonians]
##############################################################################
# Calculating the potential energy surface
# ----------------------------------------
#
# :class:`~.pennylane.VQECost` returns a :class:`~.pennylane.QNodeCollection` which can be
# evaluated using the input parameters to the ansatz circuit. The
# :class:`~.pennylane.QNodeCollection` can be evaluated asynchronously by passing the keyword
# argument ``parallel=True``. When ``parallel=False`` (the default behaviour), the QNodes are
# instead evaluated sequentially.
#
# We can use this feature to compare the sequential and parallel times required to calculate the
# potential energy surface. The following function calculates the surface:
def circuit(params, wires=0):
qml.RX(params[0], wires=wires)
qml.RY(params[1], wires=wires)
##############################################################################
# We then define our cost function using the ``VQECost`` class, which supports the computation of
# block-diagonal or diagonal approximations to the Fubini-Study metric tensor. This tensor is a
# crucial component for optimizing with quantum natural gradients.
coeffs = [1, 1]
obs = [qml.PauliX(0), qml.PauliZ(0)]
H = qml.Hamiltonian(coeffs, obs)
cost_fn = qml.VQECost(circuit, H, dev)
##############################################################################
# To analyze performance of the quantum natural gradient on VQE calculations,
# we will set up and execute optimizations using the ``GradientDescentOptimizer``
# and the ``QNGOptimizer`` using the block-diagonal approximation to the metric tensor.
#
# To perform a fair comparison, we fix the initial parameters for the two optimizers.
init_params = np.array([3.97507603, 3.00854038])
##############################################################################
# We will carry out each optimization over a maximum of 500 steps. As was done in the VQE
# tutorial, we aim to reach a convergence tolerance of around :math:`10^{-6}`. We use a step size of 0.01.
max_iterations = 500
######################################################################
#
# Note that :func:`~.pennylane.layer` allows us to pass variational parameters
# ``params[0]`` and ``params[1]`` into each layer of the circuit. That's it! The last
# step is PennyLane's specialty: optimizing the circuit parameters.
#
# The cost function is the expectation value of :math:`H_C`, which we want to minimize. The
# function :func:`~.pennylane.VQECost` is designed for this purpose: it returns the
# expectation value of an input Hamiltonian with respect to the circuit's output state.
# We also define the device on which the simulation is
# performed. We use the PennyLane-Qulacs plugin to
# run the circuit on the Qulacs simulator:
#
dev = qml.device("qulacs.simulator", wires=wires)
cost_function = qml.VQECost(circuit, cost_h, dev)
######################################################################
#
# Finally, we optimize the cost function using the built-in
# :func:`~.pennylane.GradientDescentOptimizer`. We perform optimization for seventy steps and initialize the
# parameters:
optimizer = qml.GradientDescentOptimizer()
steps = 70
params = [[0.5, 0.5], [0.5, 0.5]]
######################################################################
#
# Notice that we set each of the initial parameters to :math:`0.5`. For demonstration purposes,
# we chose initial parameters that we know work fairly well, and don't get stuck in any local minima.
hf_state = qchem.hf_state(electrons, qubits)
print(hf_state)
##############################################################################
# Finally, we can use the :func:`~.pennylane.templates.subroutines.UCCSD` function to define
# our VQE ansatz,
ansatz = partial(UCCSD, init_state=hf_state, s_wires=s_wires, d_wires=d_wires)
##############################################################################
# Next, we use the PennyLane class :class:`~.pennylane.VQECost` to define the cost function.
# This requires specifying the circuit, target Hamiltonian, and the device. It returns
# a cost function that can be evaluated with the circuit parameters:
cost_fn = qml.VQECost(ansatz, H, dev)
##############################################################################
# As a reminder, we also built the total spin operator :math:`\hat{S}^2` for which
# we can now define a function to compute its expectation value:
S2_exp_value = qml.VQECost(ansatz, S2, dev)
##############################################################################
# The total spin :math:`S` of the trial state can be obtained from the
# expectation value :math:`\langle \hat{S}^2 \rangle` as,
#
# .. math::
#
# S = -\frac{1}{2} + \sqrt{\frac{1}{4} + \langle \hat{S}^2 \rangle}.
#
qml.CNOT(wires=[3, 1])
##############################################################################
# .. note::
#
# The qubit register has been initialized to :math:`|1100\rangle` which encodes the
# Hartree-Fock state of the hydrogen molecule described with a `minimal basis
# <https://en.wikipedia.org/wiki/Basis_set_(chemistry)#Minimal_basis_sets>`__.
#
# The cost function for optimizing the circuit can be created using the :class:`~.VQECost`
# class, which is tailored for VQE optimization. It requires specifying the
# circuit, target Hamiltonian, and the device, and returns a cost function that can
# be evaluated with the circuit parameters:
cost_fn = qml.VQECost(circuit, h, dev)
##############################################################################
# Wrapping up, we fix an optimizer and randomly initialize circuit parameters. For reliable
# results, we fix the seed of the random number generator, since in practice it may be necessary
# to re-initialize the circuit several times before convergence occurs.
opt = qml.GradientDescentOptimizer(stepsize=0.4)
np.random.seed(0)
params = np.random.normal(0, np.pi, (qubits, 3))
print(params)
##############################################################################
# We carry out the optimization over a maximum of 200 steps, aiming to reach a convergence
# tolerance (difference in cost function for subsequent optimization steps) of :math:`\sim 10^{
def test_cost_expvals(self, coeffs, observables, expected):
"""Tests that the cost function returns correct expectation values"""
dev = qml.device("default.qubit", wires=2)
hamiltonian = qml.vqe.Hamiltonian(coeffs, observables)
cost = qml.VQECost(lambda params, **kwargs: None, hamiltonian, dev)
assert cost([]) == sum(expected)
def test_cost_invalid_ansatz(self, ansatz, mock_device):
"""Tests that the cost function raises an exception if the ansatz is not valid"""
hamiltonian = qml.vqe.Hamiltonian((1.0, ), [qml.PauliZ(0)])
with pytest.raises(ValueError, match="not a callable function."):
cost = qml.VQECost(4, hamiltonian, mock_device)
