Microsoft Introduces Integrated Hybrid Processing Feature for the Quantinuum Processor

Diagram of Hybrid Quantum Architectures. Credit: Microsoft

It is well understood in the quantum industry that successful quantum programs will require the integration of classical and quantum processors. The issue is that initially, the interfaces between the two were simple and had long latencies that could dramatically increase the runtimes of a program. In particular, a new class of algorithms like Variable Quantum Eigensolver (VQE) and Quantum Approximate Optimization Algorithm (QAOA) were introduced that had an interactive architecture that required shuttling data back and forth hundreds or even thousands of times between the classical and quantum processor to arrive at a solution.

In recent years, a number of companies including Rigetti and IBM have created more integrated solutions by redoing the system architectures and streamlining the interfaces to make the two work in a more integrated fashion. Microsoft does this too and it is shown as Interactive architecture in the above diagram.

However, the Quantinuum family of processors have a feature called mid-circuit measurement that can potentially make an integration of classical and hybrid computing even more powerful. This feature allows the Quantinuum QPU to measure and individual qubit while still maintaining the remaining qubits in a quantum state. We call this a "Quantum IF Statement". (See our September 2020 article about this here.) This capability can provide a number of benefits. First, it allows for qubit reuse. A qubit can be put to one use in the early part of an algorithm, measured, and then set to zero allowing it to be reused for a later part of the algorithm. This can reduce the number of qubits overall needed to process certain algorithms. The other use can be used for conditional branching which all classical programmers know and love. This capability can open up new types of algorithms can be more efficient and compact. It is a new capability and only a handful of recent papers are coming out that are using this and could also be handy for implementing various error correction algorithms. The IBM processors also have this capability which they call dynamic circuits, but we are not aware of anyone else who supports this feature yet.

To fully support all the capabilities of mid-circuit measurement, though, it requires a very fast interface to the classical processor. The quantum processor must measure a particular qubit, send the results back to the classical processor, have the classical processor make a decision on what to do next, and send the new instructions back to the quantum processor. And all this must be done before the remaining qubits decohere! So this is what Microsoft has just implemented with the Quantinuum processors in their Azure Quantum cloud system. In the diagram above, it is shown as the Integrated architecture step. Although there are several other processors connected to Microsoft's Azure Quantum system, we do not believe this Integrated step is available with them because they do not currently support this mid-circuit measurement.

For more about Microsoft's introduction of this new capability, you can read a blog entry from Fabrice Frachon describing it here, a document and video that provides more detail about the function of each architecture here, and a more general blog about the importance of hybrid classical/quantum processing from Krysta Svore here.

March 9, 2023