tfq.differentiators.ParameterShift

Calculate the general version of parameter-shift rule based gradients.

Inherits From: Differentiator

Used in the notebooks

Used in the tutorials

This ParameterShift is the gradient estimator of the following paper:

arXiv:1905.13311, Gavin E. Crooks.

This ParameterShift is used for any programs with parameterized gates. It internally decomposes any programs into array of gates with at most two distinct eigenvalues.

non_diff_op = tfq.get_expectation_op()
linear_differentiator = tfq.differentiators.ParameterShift()
# Get an expectation op, with this differentiator attached.
op = linear_differentiator.generate_differentiable_op(
    analytic_op=non_diff_op
)
qubit = cirq.GridQubit(0, 0)
circuit = tfq.convert_to_tensor([
    cirq.Circuit(cirq.X(qubit) ** sympy.Symbol('alpha'))
])
psums = tfq.convert_to_tensor([[cirq.Z(qubit)]])
symbol_values_array = np.array([[0.123]], dtype=np.float32)
# Calculate tfq gradient.
symbol_values_tensor = tf.convert_to_tensor(symbol_values_array)
with tf.GradientTape() as g:
    g.watch(symbol_values_tensor)
    expectations = op(circuit, ['alpha'], symbol_values_tensor, psums)
# This value is now computed via the ParameterShift rule.
# https://arxiv.org/abs/1905.13311
grads = g.gradient(expectations, symbol_values_tensor)
grads
tf.Tensor([[-1.1839752]], shape=(1, 1), dtype=float32)

Methods

differentiate_analytic

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Calculate the gradient.

The gradient calculations follows the following steps:

  1. Compute the decomposition of the incoming circuits so that we have their generator information (done using cirq in a tf.py_function)
  2. Use formula (31) from paper inside of TensorFlow to calculate gradients from all the decomposed circuits.
  3. Sum up terms and reshape for the total gradient that is compatible with TensorFlow.

Args
programs tf.Tensor of strings with shape [batch_size] containing the string representations of the circuits to be executed.
symbol_names tf.Tensor of strings with shape [n_params], which is used to specify the order in which the values in symbol_values should be placed inside of the circuits in programs.
symbol_values tf.Tensor of real numbers with shape [batch_size, n_params] specifying parameter values to resolve into the circuits specified by programs, following the ordering dictated by symbol_names.
pauli_sums tf.Tensor of strings with shape [batch_size, n_ops] containing the string representation of the operators that will be used on all of the circuits in the expectation calculations.
forward_pass_vals tf.Tensor of real numbers with shape [batch_size, n_ops] containing the output of the forward pass through the op you are differentiating.
grad tf.Tensor of real numbers with shape [batch_size, n_ops] representing the gradient backpropagated to the output of the op you are differentiating through.

Returns
Backward gradient values for each program & each pauli sum. It has the shape of [batch_size, n_symbols].

differentiate_sampled

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Calculate the gradient.

The gradient calculations follows the following steps:

  1. Compute the decomposition of the incoming circuits so that we have their generator information (done using cirq in a tf.py_function)
  2. Use formula (31) from paper inside of TensorFlow to calculate gradients from all the decomposed circuits.
  3. Sum up terms and reshape for the total gradient that is compatible with TensorFlow.

Args
programs tf.Tensor of strings with shape [batch_size] containing the string representations of the circuits to be executed.
symbol_names tf.Tensor of strings with shape [n_params], which is used to specify the order in which the values in symbol_values should be placed inside of the circuits in programs.
symbol_values tf.Tensor of real numbers with shape [batch_size, n_params] specifying parameter values to resolve into the circuits specified by programs, following the ordering dictated by symbol_names.
pauli_sums tf.Tensor of strings with shape [batch_size, n_ops] containing the string representation of the operators that will be used on all of the circuits in the expectation calculations.
num_samples tf.Tensor of positiver integers indicating the number of samples used per term to calculate the expectation value in the forward pass.
forward_pass_vals tf.Tensor of real numbers with shape [batch_size, n_ops] containing the output of the forward pass through the op you are differentiating.
grad tf.Tensor of real numbers with shape [batch_size, n_ops] representing the gradient backpropagated to the output of the op you are differentiating through.

Returns
Backward gradient values for each program & each pauli sum. It has the shape of [batch_size, n_symbols].

generate_differentiable_op

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Generate a differentiable op by attaching self to an op.

This function returns a tf.function that passes values through to forward_op during the forward pass and this differentiator (self) to backpropagate through the op during the backward pass. If sampled_op is provided the differentiators differentiate_sampled method will be invoked (which requires sampled_op to be a sample based expectation op with num_samples input tensor). If analytic_op is provided the differentiators differentiate_analytic method will be invoked (which requires analytic_op to be an analytic based expectation op that does NOT have num_samples as an input). If both sampled_op and analytic_op are provided an exception will be raised.

This generate_differentiable_op() can be called only ONCE because of the one differentiator per op policy. You need to call refresh() to reuse this differentiator with another op.

Args
sampled_op A callable op that you want to make differentiable using this differentiator's differentiate_sampled method.
analytic_op A callable op that you want to make differentiable using this differentiators differentiate_analytic method.

Returns
A callable op that who's gradients are now registered to be a call to this differentiators differentiate_* function.

refresh

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Refresh this differentiator in order to use it with other ops.