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OpenQASM 3

qiskit.qasm3

Qiskit provides some tools for converting between OpenQASM 3(opens in a new tab) representations of quantum programs, and the QuantumCircuit class. These will continue to evolve as Qiskit’s support for the dynamic-circuit capabilities expressed by OpenQASM 3 increases.


Exporting to OpenQASM 3

The high-level functions are simply dump() and dumps(), which respectively export to a file (given as a filename) and to a Python string.

dump

qiskit.qasm3.dump(circuit, stream, **kwargs)GitHub(opens in a new tab)

Serialize a QuantumCircuit object as an OpenQASM 3 stream to file-like object.

Parameters

  • circuit (QuantumCircuit) – Circuit to serialize.
  • stream (TextIOBase) – stream-like object to dump the OpenQASM 3 serialization
  • **kwargs – Arguments for the Exporter constructor.

dumps

qiskit.qasm3.dumps(circuit, **kwargs)GitHub(opens in a new tab)

Serialize a QuantumCircuit object in an OpenQASM 3 string.

Parameters

Returns

The OpenQASM 3 serialization

Return type

str(opens in a new tab)

Both of these exporter functions are single-use wrappers around the main Exporter class. For more complex exporting needs, including dumping multiple circuits in a single session, it may be more convenient or faster to use the complete interface.

qiskit.qasm3.Exporter(includes=('stdgates.inc', ), basis_gates=('U', ), disable_constants=False, alias_classical_registers=None, allow_aliasing=None, indent='  ', experimental=ExperimentalFeatures.None)GitHub(opens in a new tab)

QASM3 exporter main class.

Parameters

  • includes (Sequence(opens in a new tab)[str(opens in a new tab)]) – the filenames that should be emitted as includes. These files will be parsed for gates, and any objects dumped from this exporter will use those definitions where possible.

  • basis_gates (Sequence(opens in a new tab)[str(opens in a new tab)]) – the basic defined gate set of the backend.

  • disable_constants (bool(opens in a new tab)) – if True, always emit floating-point constants for numeric parameter values. If False (the default), then values close to multiples of OpenQASM 3 constants (pi, euler, and tau) will be emitted in terms of those constants instead, potentially improving accuracy in the output.

  • alias_classical_registers (bool(opens in a new tab)) –

    If True, then bits may be contained in more than one register. If so, the registers will be emitted using “alias” definitions, which might not be well supported by consumers of OpenQASM 3.

    See also

    Parameter allow_aliasing

    A value for allow_aliasing overrides any value given here, and supersedes this parameter.

  • allow_aliasing (bool(opens in a new tab)) –

    If True, then bits may be contained in more than one register. If so, the registers will be emitted using “alias” definitions, which might not be well supported by consumers of OpenQASM 3. Defaults to False or the value of alias_classical_registers.

    New in version 0.25.0.

  • indent (str(opens in a new tab)) – the indentation string to use for each level within an indented block. Can be set to the empty string to disable indentation.

  • experimental (ExperimentalFeatures) – any experimental features to enable during the export. See ExperimentalFeatures for more details.

dump

dump(circuit, stream)

Convert the circuit to OpenQASM 3, dumping the result to a file or text stream.

dumps

dumps(circuit)

Convert the circuit to OpenQASM 3, returning the result as a string.

All of these interfaces will raise QASM3ExporterError on failure.

QASM3ExporterError

qiskit.qasm3.QASM3ExporterError(*message)GitHub(opens in a new tab)

An error raised during running the OpenQASM 3 exporter.

Set the error message.

Experimental features

The OpenQASM 3 language is still evolving as hardware capabilities improve, so there is no final syntax that Qiskit can reliably target. In order to represent the evolving language, we will sometimes release features before formal standardization, which may need to change as the review process in the OpenQASM 3 design committees progresses. By default, the exporters will only support standardised features of the language. To enable these early-release features, use the experimental keyword argument of dump() and dumps(). The available feature flags are:

qiskit.qasm3.ExperimentalFeatures(value)GitHub(opens in a new tab)

Flags for experimental features that the OpenQASM 3 exporter supports.

These are experimental and are more liable to change, because the OpenQASM 3 specification has not formally accepted them yet, so the syntax may not be finalized.

SWITCH_CASE_V1

= 1

Support exporting switch-case statements as proposed by https://github.com/openqasm/openqasm/pull/463(opens in a new tab) at commit bfa787aa3078(opens in a new tab).

These have the output format:

switch (i) {
    case 0:
    case 1:
        x $0;
    break;
 
    case 2: {
        z $0;
    }
    break;
 
    default: {
        cx $0, $1;
    }
    break;
}

This differs from the syntax of the switch statement as stabilized. If this flag is not passed, then the parser will instead output using the stabilized syntax, which would render the same example above as:

switch (i) {
    case 0, 1 {
        x $0;
    }
    case 2 {
        z $0;
    }
    default {
        cx $0, $1;
    }
}

If you want to enable multiple experimental features, you should combine the flags using the | operator, such as flag1 | flag2.

For example, to perform an export using the early semantics of switch support:

from qiskit import qasm3, QuantumCircuit, QuantumRegister, ClassicalRegister
 
# Build the circuit
qreg = QuantumRegister(3)
creg = ClassicalRegister(3)
qc = QuantumCircuit(qreg, creg)
with qc.switch(creg) as case:
    with case(0):
        qc.x(0)
    with case(1, 2):
        qc.x(1)
    with case(case.DEFAULT):
        qc.x(2)
 
# Export to an OpenQASM 3 string.
qasm_string = qasm3.dumps(qc, experimental=qasm3.ExperimentalFeatures.SWITCH_CASE_V1)
Note

All features enabled by the experimental flags are naturally transient. If it becomes necessary to remove flags, they will be subject to the standard Qiskit deprecation policy(opens in a new tab). We will leave these experimental flags in place for as long as is reasonable.

However, we cannot guarantee any support windows for consumers of OpenQASM 3 code generated using these experimental flags, if the OpenQASM 3 language specification changes the proposal that the flag is based on. It is possible that any tool you are using to consume OpenQASM 3 code created using these flags may update or remove their support while Qiskit continues to offer the flag. You should not rely on the resultant experimental OpenQASM 3 code for long-term storage of programs.


Importing from OpenQASM 3

Currently only two high-level functions are offered, as Qiskit support for importing from OpenQASM 3 is in its infancy, and the implementation is expected to change significantly. The two functions are load() and loads(), which are direct counterparts of dump() and dumps(), respectively loading a program indirectly from a named file and directly from a given string.

Note

While we are still in the exploratory release period, to use either function, the package qiskit_qasm3_import must be installed. This can be done by installing Qiskit Terra with the qasm3-import extra, such as by:

pip install qiskit-terra[qasm3-import]

We expect that this functionality will eventually be merged into core Terra, and no longer require an optional import, but we do not yet have a timeline for this.

load

qiskit.qasm3.load(filename)GitHub(opens in a new tab)

Load an OpenQASM 3 program from the file filename.

Parameters

filename (str(opens in a new tab)) – the filename to load the program from.

Returns

a circuit representation of the OpenQASM 3 program.

Return type

QuantumCircuit

Raises

QASM3ImporterError – if the OpenQASM 3 file is invalid, or cannot be represented by a QuantumCircuit.

loads

qiskit.qasm3.loads(program)GitHub(opens in a new tab)

Load an OpenQASM 3 program from the given string.

Parameters

program (str(opens in a new tab)) – the OpenQASM 3 program.

Returns

a circuit representation of the OpenQASM 3 program.

Return type

QuantumCircuit

Raises

QASM3ImporterError – if the OpenQASM 3 file is invalid, or cannot be represented by a QuantumCircuit.

Both of these two functions raise QASM3ImporterError on failure.

QASM3ImporterError

qiskit.qasm3.QASM3ImporterError(*message)GitHub(opens in a new tab)

An error raised during the OpenQASM 3 importer.

Set the error message.

For example, we can define a quantum program using OpenQASM 3, and use loads() to directly convert it into a QuantumCircuit:

import qiskit.qasm3
 
program = """
    OPENQASM 3.0;
    include "stdgates.inc";
 
    input float[64] a;
    qubit[3] q;
    bit[2] mid;
    bit[3] out;
 
    let aliased = q[0:1];
 
    gate my_gate(a) c, t {
      gphase(a / 2);
      ry(a) c;
      cx c, t;
    }
    gate my_phase(a) c {
      ctrl @ inv @ gphase(a) c;
    }
 
    my_gate(a * 2) aliased[0], q[{1, 2}][0];
    measure q[0] -> mid[0];
    measure q[1] -> mid[1];
 
    while (mid == "00") {
      reset q[0];
      reset q[1];
      my_gate(a) q[0], q[1];
      my_phase(a - pi/2) q[1];
      mid[0] = measure q[0];
      mid[1] = measure q[1];
    }
 
    if (mid[0]) {
      let inner_alias = q[{0, 1}];
      reset inner_alias;
    }
 
    out = measure q;
"""
circuit = qiskit.qasm3.loads(program)
circuit.draw("mpl")
../_images/qasm3-1.png

Experimental import interface

The import functions given above rely on the ANTLR-based reference parser from the OpenQASM project itself, which is more intended as a language reference than a performant parser. You need to have the extension qiskit-qasm3-import installed to use it.

Qiskit is developing a native parser, written in Rust, which is available as part of the core Qiskit package. This parser is still in its early experimental stages, so is missing features and its interface is changing and expanding, but it is typically orders of magnitude more performant for the subset of OpenQASM 3 it currently supports, and its internals produce better error diagnostics on parsing failures.

You can use the experimental interface immediately, with similar functions to the main interface above:

load_experimental

qiskit.qasm3.load_experimental(pathlike_or_filelike, /, *, custom_gates=None, include_path=None)

Load an OpenQASM 3 program from a source file into a QuantumCircuit.

Warning

This native version of the OpenQASM 3 importer is currently experimental. It is typically much faster than load(), but has a reduced supported feature set, which will expand over time.

Parameters

  • pathlike_or_filelike (str(opens in a new tab) |os.PathLike(opens in a new tab) |io.TextIOBase(opens in a new tab)) – the program source. This can either be given as a filepath, or an open text stream object. If the stream is already opened it is consumed in Python space, whereas filenames are opened and consumed in Rust space; there might be slightly different performance characteristics, depending on your system and how the streams are buffered by default.
  • custom_gates (Iterable[CustomGate]) – Python constructors to use for particular named gates. If not supplied, Qiskit will use its own standard-library constructors for gates defined in the OpenQASM 3.0 standard-library file stdgates.inc.
  • include_path (Iterable[str(opens in a new tab)]) – the path to search when resolving include statements. If not given, Qiskit will arrange for this to point to a location containing stdgates.inc only. Paths are tried in the sequence order.

Returns

the constructed circuit object.

Return type

QuantumCircuit

Raises

.QASM3ImporterError – if an error occurred during parsing or semantic analysis. In the case of a parsing error, most of the error messages are printed to the terminal and formatted, for better legibility.

loads_experimental

qiskit.qasm3.loads_experimental(source, /, *, custom_gates=None, include_path=None)

Load an OpenQASM 3 program from a string into a QuantumCircuit.

Warning

This native version of the OpenQASM 3 importer is currently experimental. It is typically much faster than loads(), but has a reduced supported feature set, which will expand over time.

Parameters

  • source (str(opens in a new tab)) – the program source in a Python string.
  • custom_gates (Iterable[CustomGate]) – Python constructors to use for particular named gates. If not supplied, Qiskit will use its own standard-library constructors for gates defined in the OpenQASM 3.0 standard-library file stdgates.inc.
  • include_path (Iterable[str(opens in a new tab)]) – the path to search when resolving include statements. If not given, Qiskit will arrange for this to point to a location containing stdgates.inc only. Paths are tried in the sequence order.

Returns

the constructed circuit object.

Return type

QuantumCircuit

Raises

.QASM3ImporterError – if an error occurred during parsing or semantic analysis. In the case of a parsing error, most of the error messages are printed to the terminal and formatted, for better legibility.

These two functions are both experimental, meaning they issue an ExperimentalWarning on usage, and their interfaces may be subject to change within the Qiskit 1.x release series. In particular, the native parser may be promoted to be the default version of load() and loads(). If you are happy to accept the risk of using the experimental interface, you can disable the warning by doing:

import warnings
from qiskit.exceptions import ExperimentalWarning
 
warnings.filterwarnings("ignore", category=ExperimentalWarning, module="qiskit.qasm3")

These two functions allow for specifying include paths as an iterable of paths, and for specifying custom Python constructors to use for particular gates. These custom constructors are specified by using the CustomGate object:

qiskit.qasm3.CustomGate

Information received from Python space about how to construct a Python-space object to represent a given gate that might be declared.

In custom_gates is not given, Qiskit will attempt to use its standard-library gate objects for the gates defined in OpenQASM 3 standard library file stdgates.inc. This sequence of gates is available on this module, if you wish to build on top of it:

qiskit.qasm3.STDGATES_INC_GATES

A tuple of CustomGate objects specifying the Qiskit constructors to use for the stdgates.inc include file.

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