CostKey

qualtran.resource_counting.CostKey

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Abstract base class for different types of costs.

One important aspect of a bloq is the resources required to execute it on an error corrected quantum computer. Since we’re usually trying to minimize these resource requirements we will generally use the catch-all term “costs”.

There are a variety of different types or flavors of costs. Each is represented by an instance of a sublcass of CostKey. For example, gate counts (including T-gate counts), qubit requirements, and circuit depth are all cost metrics that may be of interest.

Each CostKey primarily encodes the behavior required to compute a cost value from a bloq. Often, these costs are defined recursively: a bloq’s costs is some combination of the costs of the bloqs in its decomposition (i.e. the bloq ‘callees’). Implementors must override the compute method to define the cost computation.

Each cost key has an associated CostValT. For example, the CostValT of a “t count” CostKey could be an integer. For a more complicated gateset, the value could be a mapping from gate to count. This abstract base class is generic w.r.t. CostValT. Subclasses should have a concrete value type. The validate_val method can optionally be overridden to raise an exception if a bad value type is encountered. The zero method must return the zero (additive identity) cost value of the correct type.

Methods

compute

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@abc.abstractmethod
compute(
    bloq: 'Bloq', get_callee_cost: Callable[['Bloq'], CostValT]
) -> qualtran.resource_counting.CostValT

Compute this type of cost.

When implementing a new CostKey, this method must be overridden. Users should not call this method directly. Instead: use the qualtran.resource_counting functions like get_cost_value, get_cost_cache, or query_costs. These provide caching, logging, generalizers, and support for static costs.

For recursive computations, use the provided callable to recurse.

Args

bloq

The bloq to compute the cost of.

get_callee_cost

A qualtran-provided function for computing costs for “callees” of the bloq; i.e. bloqs in the decomposition. Use this function to accurately cache intermediate cost values and respect bloqs’ static costs.

Returns

zero

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@abc.abstractmethod
zero() -> qualtran.resource_counting.CostValT

The value corresponding to zero cost.

validate_val

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validate_val(
    val: qualtran.resource_counting.CostValT
)

Assert that val is a valid CostValT.

This method can be optionally overridden to raise an error if an invalid value is encountered. By default, no validation is performed.