State
Number with an error bar
A scalar estimate with an additive-error guarantee, the shot or query budget it consumed, and the deepest circuit it actually ran. Every claim about an application eventually has to arrive here.
A state is an object you can be holding, named once so that two routes reaching the same thing are drawn as reaching the same thing. It says nothing about how you got here or where you can go next — that is entirely in the processes below.
This is a kind of
Anything that asks for one of these will accept this, because it is narrower. The reverse does not hold.
- Answer about the solution
Whatever a route hands back about the state of the system at the end — a state you can measure, a history over the whole interval, or a single number. Which of the three you get is a real difference between routes, so each is its own kind below this one.
Narrower kinds of this
- Bias-reduced expectation value
The same number after post-processing, with less bias and more variance. The variance is a sampling overhead and it grows with circuit volume, which is what bounds how far mitigation alone can be pushed.
Records that are this object
Nothing in the catalogue has been joined to this state. That is a gap in the join rather than a claim that no such object exists; the shelf on /repository lists what is joined and what is not, with the reason.
Work that arrives here
- Estimate an observable
Given the ability to prepare and a description of an observable , return a classical scalar within of at confidence . The state is never returned; only the number is.
- Estimate a Hamiltonian's ground-state energy
Given a Hamiltonian you can query and some way of preparing trial states, return an estimate of its lowest eigenvalue to a stated additive error. The state achieving that energy may or may not come back with the number; almost every application wants the number.
- Estimate an excited-state energy
Given a Hamiltonian you can query and a statement of which state above the lowest one is wanted, return an estimate of that state's energy — or of its distance from the ground state, which is the quantity an experiment usually measures. Whether the answer comes back as an absolute energy or as an excitation energy is a property of the method, and the two are not interchangeable.
- Estimate the eigenphase of a unitary
Given a circuit whose controlled powers you can apply, and a routine preparing a state with non-negligible overlap on one of its eigenvectors, return that eigenvector's phase as a number with an error bar. The phase is read out of an ancilla, never out of the system register — the system is only ever the thing the controlled powers act on.
Work that starts here
No recorded process takes this as its input. Nothing in this graph leaves from here.