zilkf92 / qasm-parser

qasm-parser

The project includes a stand-alone parser for QASM files. The parser is a part of the compiler pipeline of the PhotonQ compiler project.

Overview

This repository contains the source files of a parser built with Flex and Bison that translates standard QASM 2.0 files consisting of ${R_z(\theta), R_x(\theta), H, CNOT, CZ}$ gates into QASM 2.0 files consisting of ${HR_z(\theta), CZ}$ gates using gate identities.

The first test versions of the parser were based on examples taken from Levine's book and then adapted accordingly for use in translating QASM files.

The lex file parser.l contains Flex code that defines the regular expressions and character tokens for which the input file is scanned.

The yacc file parser.y contains Bison code that defines the relationship between the input tokens as recursive grammar rules.

Cloning the parser to your computer

To try the parser yourself you can clone the repo with

git clone https://github.com/zilkf92/qasm-parser.git

The repository contains 2 branches:

pi-constant branch

Here, the string "pi" is interpreted as a mathematical constant from the <math.h> header and all other numbers are interpreted as integers or floats. E.g.:

rz(pi/4) q[8];
cx q[8],q[7];
rx(2*pi) q[7];

is converted to

rz(0.785398) q[8];
h q[8];
rz(0) q[8];
h q[8];
rz(0) q[7];
h q[7];
cz q[8], q[7];
rz(0) q[7];
h q[7];
rz(0) q[7];
h q[7];
rz(6.283185) q[7];
h q[7];

main branch

Here, each argument of a QASM gate (parameter and qubit assignment) is interpreted and parsed as a string. E.g.:

rz(pi/4) q[8];
cx q[8],q[7];

is converted to

rz(pi/4) q[8];
h q[8];
rz(0) q[8];
h q[8];
rz(0) q[7];
h q[7];
cz q[8],q[7];
rz(0) q[7];
h q[7];

Using the parser

After copying the repository, you can use, e.g., Qiskit to transpile an arbitrary quantum circuit into the gate set ${R_z(\theta), R_x(\theta), H, CNOT, CZ}$ and export the QASM file. You can use this QASM file as an input for the parser. Replace the input.qasm file with your generated QASM file and run the ./parser file in the "parser" folder. This will create an output.qasm file according to the following translation rules:

$R_z(\theta) = [H R_z(0)][H R_z(\theta)]$

$R_x(\theta) = [H R_z(\theta)][H R_z(0)]$

$H = [H R_z(0)]$

$CNOT =[\mathbb{I}\otimes H] CZ [\mathbb{I}\otimes H]$

$CZ = CZ$