tfq.get_expectation_op

Get a TensorFlow op that will calculate batches of expectation values.

tfq.get_expectation_op(
    backend=None,
    *,
    quantum_concurrent=quantum_context.get_quantum_concurrent_op_mode()
)

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.

# Simulate circuits with C++.
my_op = tfq.get_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)
]])
# This op can now be run with:
output = my_op(
    my_circuit_tensor, ['alpha'], my_values, my_paulis)
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
)

Args
backend	Optional Python object that specifies what backend this op should use when evaluating circuits. Can be cirq.DensityMatrixSimulator or any cirq.sim.simulator.SimulatesExpectationValues. If not provided the default C++ analytical expectation calculation 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.

Returns
A callable with the following signature: op(programs, symbol_names, symbol_values, pauli_sums)
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.
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).