Indices

An index is a reference to one or more positions within a register. The first position of a register is 0. Positions are expressed as a list of comma-separated entries. Each entry can be a single integer or a range. A range is written as two integers separated by a colon, pointing to a first and to a last element within a register. The last position is also part of the range, i.e., it does not point to the first element out of the range.

Grammar for indices

index:
  identifier [ index-list ]

index-list:
  index-entry
  index-list , index-entry

index-entry:
  integer-literal
  index-range

index-range:
  index-entry : index-entry

Indices, index entries, and index ranges

// Qubit register containing 7 qubits, numbered from 0 to 6
qubit[7] q 

// q[1, 3:5] is a qubit index with two entries: 
// the first entry points to qubit 1 in q, and the second to qubits 3, 4, and 5
H q[1, 3:5]