cirq.protocols.apply_mixture

High performance evolution under a mixture of unitaries evolution.

Follows the steps below to attempt to apply a mixture:

A. Try to use val._apply_mixture_(args).

1. If `_apply_mixture_` is not present or returns NotImplemented
    go to step B.
2. If '_apply_mixture_' is present and returns None conclude that
    `val` has no effect and return.
3. If '_apply_mixture_' is present and returns a numpy array conclude
    that the mixture was applied successfully and forward result to
    caller.

B. Construct an ApplyUnitaryArgs object uargs from args and then try to use cirq.apply_unitary(val, uargs, None).

1. If `None` is returned then go to step C.
2. If a numpy array is returned forward this result back to the caller
    and return.

C. Try to use val._mixture_().

1. If '_mixture_' is not present or returns NotImplemented
    go to step D.
2. If '_mixture_' is present and returns None conclude that `val` has
    no effect and return.
3. If '_mixture_' returns a list of tuples, loop over the list and
    examine each tuple. If the tuple is of the form
    `(probability, np.ndarray)` use matrix multiplication to apply it.
    If the tuple is of the form `(probability, op)` where op is any op,
    attempt to use <a href="../../cirq/protocols/apply_unitary"><code>cirq.apply_unitary(op, uargs, None)</code></a>. If this
    operation returns None go to step D. Otherwise return the resulting
    state after all of the tuples have been applied.

D. Raise TypeError or return default.

val The value with a mixture to apply to the target.
args A mutable cirq.ApplyMixtureArgs object describing the target tensor, available workspace, and left and right axes to operate on. The attributes of this object will be mutated as part of computing the result.
default What should be returned if val doesn't have a mixture. If not specified, a TypeError is raised instead of returning a default value.

If the receiving object is not able to apply a mixture, the specified default value is returned (or a TypeError is raised). If this occurs, then target_tensor should not have been mutated.

If the receiving object was able to work inline, directly mutating target_tensor it will return target_tensor. The caller is responsible for checking if the result is target_tensor.

If the receiving object wrote its output over out_buffer, the result will be out_buffer. The caller is responsible for checking if the result is out_buffer (and e.g. swapping the buffer for the target tensor before the next call).

Note that it is an error for the return object to be either of the auxiliary buffers, and the method will raise an AssertionError if this contract is violated.

The receiving object may also write its output over a new buffer that it created, in which case that new array is returned.

TypeError val doesn't have a mixture and default wasn't specified.
ValueError Different left and right shapes of args.target_tensor selected by left_axes and right_axes or qid_shape(val) doesn't equal the left and right shapes.
AssertionError _apply_mixture_ returned an auxiliary buffer.