High-level interface: quantum programs
In the definition of qoqo/roqoqo, a quantum program accepts input from a user/calling function and runs operations on a quantum computer. It then returns an output in the form of expectation values, instead of raw output from the quantum computer.
To represent quantum programs qoqo/roqoqo provides QuantumProgram.
It is intended as a high-level interface between a complex program and the quantum computer, which can be called (almost) as easily as a normal function and handles variable replacement and the post-processing of the raw output of the quantum computer/simulator.
Measurements
Post-processing the raw output is handled by measurements.
For many applications, the measurement results of several circuits need to be combined to extract the required information.
Measurement objects group several quantum circuits and a measurement input that determines how the raw output is post-processed.
A qoqo measurement combines one optional constant_circuit that is always executed first and a list of circuits that are each executed after the constant circuit.
The type of measurement and measurement input depends on the type of readout used in the circuits.
The following measurements are available in qoqo/roqoqo:
The PauliZProduct measurement is based on measuring the product of PauliZ operators for given qubits. Combined with a basis rotation of the measured qubits, it can be used to measure arbitrary expectation values. It uses projective qubit readouts like MeasureQubit or PragmaRepeatedMeasurement. It can be run on real quantum computer hardware and simulators. For further details please refer to section PauliZProduct.
The ClassicalRegister measurement returns the raw output of the classical registers written during circuit execution.
The CheatedPauliZProduct measurement directly calculates operator expectation values from the expectation values of products of Pauli matrices. The Pauli product expectation values are read out by the PragmaGetPauliProduct operation. It can only be used on simulator backends.
The Cheated measurement calculates expectation values by directly extracting the state vector PragmaGetStateVector or the density matrix PragmaGetDensityMatrix from a simulator and applying matrix multiplication.
For examples how to construct measurement input and measurements see the pages for the separate measurement types.
Variable replacement
qoqo/roqoqo supports symbolic parameters in Operations, for example the rotation angles \( \theta \) in a RotateX gate operation. A Circuit with symbolic parameters cannot be executed on real hardware or simulated. The symbolic parameters need to be replaced with real floating point numbers first. A QuantumProgram contains a list of the free parameters (input_parameter_names).
When calling its run method, it replaces the free parameters with the given input, executes the measurement on the given backend and returns the result.
For an example how to run a QuantumProgram see here.