A specification for C-like interfaces to quantum computers.
CQ is a specification for a C-like interface to quantum computers. It is not a library, or a DSL, or a framework, but you could use it to implement such things. In fact you are encouraged to do so. CQ specifically assumes a host-device model, where the host is a stricly classical computer, and the device is... well, also a classical computer, but one which directly controls a quantum computer.
Although the interface is C-like, we don't insist that implementations are written in C. You could certainly write a compatible implementation in C++, or even in python or Fortran (if you must).
There are really two questions to answer here.
We believe standard APIs are fundamentally useful. They allow application developers to reason about their applications, without needing to worry about the specific underlying technology. If they use the same interface, the same source code can be recompiled and rerun elsewhere, enabling code portability. This is a Good Thing ™️.
Okay, so why not specify a RESTful API? That's a very good question, with a simple answer -- we're interested in the long-term view of quantum computers as tightly-coupled accelerators. For reasons, C and Fortran remain the ubiquitous programming languages across HPC. C++ is increasingly prevalent, especially in accelerator programming, but it's easier to access a C interface from C++ than the other way round so a C-like interface seemed to us to be the obvious starting place.
This does not mean we expect everyone to program quantum computers in C. We expect CQ implementations to act as a performant intermediate layer between higher level abstractions and programming models, and lower-level QHDLs like OpenQASM and QIR.
For simplicity we do not include any namespace in the specification, but in line with best practice, we encourage implementers to use approriate namespacing, i.e. cq_ or CQ::.
- cq: The core CQ specification, for interfacing with quantum hardware.
- datatypes: Basic and complex datatypes which should be provided by a CQ implementation.
- host_operations: Operations which are valid on the host, but not the device. Mainly focused on resource management and offloading.
- device_operations: Functions which are valid on the quantum device. This is where all the quantum stuff lives.
We plan to add at least one reference implementation to this repository, possibly in the form of C-based QASM writer.
We currently foresee the following CQ extensions:
- cq-emulation: An extension for routines that are only applicable when emulating a quantum computer, rather than using a real one. For instance allowing direct access to amplitudes.
- cq-analogue: An extension for dealing with analogue rather than digital quantum computers.
- cq-algorithms: An extension for higher-level functionality.
The following resources were used in the construction of CQ: