Transpileur quantique

Classical transpilation describes the translation of code written in one programming language into code written in another language. It is a process analogous to compilation.

In the context of quantum computing, transpilation aims to ensure that a quantum circuit uses only the native gates of the quantum machine on which it will be executed. Transpilation also ensures that multi-qubit operations are assigned to physically connected qubits on the quantum chip.

Transpilation Steps

Measurement Decomposition

Measurements are performed in a given basis, such as the X, Y, or Z bases, among others. Most quantum computers measure in the Z basis (computational basis). If another basis is required, rotations must be added at the end of the circuit to adjust the measurement basis.

Intermediate Decomposition

An initial decomposition of operations is necessary to execute the circuit on a quantum machine in order to limit the number of different operations used by the circuit. For example, operations with more than two qubits must be decomposed into two-qubit or one-qubit operations.

Placement

The idea is to establish an association between the wires of the created quantum circuit and the physical qubits of the machine. This step can be reduced to a subgraph isomorphism problem.

Routing

Despite the placement step, some two-qubit operations may not be correctly assigned to available physical couplers on the machine. In this case, swap operations are used to virtually bring the affected qubits closer together and allow their connection. However, these swap operations are very costly, making optimal initial placement essential to minimize their use.

Optimization

Qubits accumulate errors and lose coherence over time. To limit these effects, the optimization process reduces the number of operations applied to each qubit using various classical algorithms. For example, it removes trivial and inverse operations; combines rotations on the same axis; and more generally, replaces sections of circuits with equivalent circuits that generate fewer errors.

Native Gate Decomposition

Each quantum computer has a finite set of basic operations (native gates), from which all other operations can be composed. For example, MonarQ has a set of 13 native gates. Transpilation thus decomposes all non-native operations of a circuit into native operations.

Using the Calcul Québec Transpiler with MonarQ

Calcul Québec has developed a transpiler that allows circuits to be sent to MonarQ transparently, using the transpilation steps mentioned above. This transpiler is integrated into a PennyLane device and is therefore designed to be specifically used with PennyLane. For details, consult the documentation.