Structure

cQASM 1.x files consist of a version statement, optionally a qubits statement (mandatory for 1.0, optional for 1.1+), optionally an error_model statement, and zero or more of any combination and ordering of the following statement types:

• mappings,
• variables (version 1.1+),
• subcircuit headers,
• bundles, and
• structured control-flow statements (version 1.2+).

A simple algorithm might look like this.

version 1.0
qubits 2

map input = q[0]
map output = q[1]

.initialize
prep_z input | prep_z output
x output
h input | h output

.oracle
cnot input, output

.measure
h input
measure input

File extension

The file extension for cQASM files is preferably .cq. Historically everyone seems to use .qasm, but this conflicts with the OpenQASM format. libqasm doesn’t care, but DQCsim goes as far as to require that the file extension is .cq, as it uses file extensions to disambiguate the input file format (the problem, here, is that it also has an OpenQASM frontend).

Whitespace and comments

cQASM 1.x is newline-sensitive. In most contexts, newlines are used to terminate statements. A semicolon (;) may be used instead of the newline if the code is more readable when two statements occupy the same line. Newlines can also be “escaped” using a backslash (\) immediately in front; this disables the functional behavior of the newline and thus allows statements to be split over a single line.

statement 1
statement 2; statement 3
a very long statement 4,    \
    but a single statement  \
    nontheless.

Besides newlines, cQASM 1.x is not whitespace-sensitive; tabs and spaces can go between any two tokens.

Single-line comments use the hash (#) prefix as in Python and most other scripting languages. Block comments use /* */ as in C and various C-like languages. Like spaces and tabs, block comments can go between any two tokens.

statement 1 # I'm a comment
statement /* I'm a much
longer comment */ 2

cQASM 1.x files are not case-sensitive.

h q[1]
H Q[1] # this means the same thing

Keywords

cQASM 1.x recognizes the following keywords. These words cannot be used to name mappings or variables.

break  cond  continue  else  for  foreach  if
map  repeat  set  qubits  until  var  while

Note

All gates, axes (x, y, z), the qubit register (q), and the bit register (b) used to be keywords. This is no longer the case, so you can freely rename a qubit to for instance b now. Be aware, however, that this shadows the bit register, meaning that you won’t be able to use it anymore later. The type system is such that the operands of a gate are used to disambiguate the gate; the gate can thus not be used to disambiguate between b as in a mapping to a qubit and b as in the bit register. The same applies for q, x, y, and z (the latter three only if you use a gate that takes an axis operand). You can work around this by first remapping the entire register or the axis to a longer word, for instance using map qubits = q.