Quantum Computation Language


Quantum Computation Language is one of the first implemented quantum programming languages. The most important feature of QCL is the support for user-defined operators and functions. Its syntax resembles the syntax of the C programming language and its classical data types are similar to primitive data types in C. One can combine classical code and quantum code in the same program.
The QCL standard library provides standard quantum operators used in quantum algorithms such as:
The basic built-in quantum data type in QCL is the qureg. It can be interpreted as an array of qubits.

qureg x1; // 2-qubit quantum register x1
qureg x2; // 2-qubit quantum register x2
H; // Hadamard operation on x1
H; // Hadamard operation on the first qubit of the register x2

Since the qcl interpreter uses qlib simulation library, it is possible to observe the internal state of the quantum machine during execution of the quantum program.

qcl> dump
0.35355 |0> + 0.35355 |1> + 0.35355 |2> + 0.35355 |3>
+ 0.35355 |8> + 0.35355 |9> + 0.35355 |10> + 0.35355 |11>

Note that the dump operation is different from measurement, since it does not influence the state of the quantum machine and can be realized only using a simulator.
Like in modern programming languages, it is possible to define new operations which can be used to manipulate quantum data. For example:

operator diffuse

defines inverse about the mean operator used in Grover's algorithm. This allows one to define algorithms on a higher level of abstraction and extend the library of functions available for programmers.