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tfq.get_sampled_expectation_op

Get a TensorFlow op that will calculate sampled expectation values.

Used in the notebooks

Used in the tutorials

This function produces a non-differentiable TF op that will calculate batches of expectation values given tensor batches of cirq.Circuits, parameter values, and cirq.PauliSum operators to measure. Expectation is estimated by taking num_samples shots per term in the corresponding PauliSum.

# Simulate circuits with C++.
my_op = tfq.get_sampled_expectation_op()
# Prepare some inputs.
qubit = cirq.GridQubit(0, 0)
my_symbol = sympy.Symbol('alpha')
my_circuit_tensor = tfq.convert_to_tensor([
    cirq.Circuit(cirq.H(qubit) ** my_symbol)
])
my_values = np.array([[0.123]])
my_paulis = tfq.convert_to_tensor([[
    3.5 * cirq.X(qubit) - 2.2 * cirq.Y(qubit)
]])
my_num_samples = np.array([[100]])
# This op can now be run with:
output = my_op(
    my_circuit_tensor, ['alpha'], my_values, my_paulis, my_num_samples)
output
tf.Tensor([[0.71530885]], shape=(1, 1), dtype=float32)

In order to make the op differentiable, a tfq.differentiator object is needed. see tfq.differentiators for more details. Below is a simple example of how to make my_op from the above code block differentiable:

diff = tfq.differentiators.ForwardDifference()
my_differentiable_op = diff.generate_differentiable_op(
    analytic_op=my_op
)

backend Optional Python object that specifies what backend this op should use when evaluating circuits. Can be any cirq.Sampler. If not provided the default C++ sampled expectation op is returned.
quantum_concurrent Optional Python bool. True indicates that the returned op should not block graph level parallelism on itself when executing. False indicates that graph level parallelism on itself should be blocked. Defaults to value specified in tfq.get_quantum_concurrent_op_mode which defaults to True (no blocking). This flag is only needed for advanced users when using TFQ for very large simulations, or when running on a real chip.

A callable with the following signature:

op(programs, symbol_names, symbol_values, pauli_sums, num_samples)

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 with num_samples[i][j] is equal to the number of samples to draw in each term of pauli_sums[i][j] when estimating the expectation. Therefore, num_samples must have the same shape as pauli_sums.
Returns tf.Tensor with shape [batch_size, n_ops] that holds the expectation value for each circuit with each op applied to it (after resolving the corresponding parameters in).