How to modify the default circuit transpilation.

A Qibolab platform can execute pulse sequences. As shown in Circuit execution, Qibo circuits can be executed by invoking the qibolab.backends.QibolabBackend, which is the object integrating Qibolab to Qibo. When a Qibo circuit is executed, the backend will automatically transpile and compile it to a pulse sequence, which will be sent to the platform for execution. The default compiler outlined in the Transpiler and Compiler section will be used in this process. In this tutorial we will demonstrate how the user can modify this process for custom applications.

The compiler object used when executing a circuit are attributes of qibolab.backends.QibolabBackend. Creating an instance of the backend provides access to these objects:

from qibolab.backends import QibolabBackend

backend = QibolabBackend(platform="dummy")

print(type(backend.compiler))

The transpiler is responsible for transforming any circuit to one that respects the chip connectivity and native gates. The compiler then transforms this circuit to the equivalent pulse sequence. Note that there is no default transpiler, therefore the backend can only execute circuits that contain native gates by default. The user can modify the transpilation and compilation process by changing the transpiler and compiler attributes of the QibolabBackend.

In this example, we executed circuits using the backend backend.execute_circuit method, unlike the previous example (Circuit execution) where circuits were executed directly using circuit(nshots=1000). It is possible to perform transpiler and compiler manipulation in both approaches. When using circuit(nshots=1000), Qibo is automatically initializing a GlobalBackend() singleton that is used to execute the circuit. Therefore the previous manipulations can be done as follows:

import qibo
from qibo import gates
from qibo.models import Circuit
from qibo.backends import GlobalBackend

# define circuit
circuit = Circuit(1)
circuit.add(gates.U3(0, 0.1, 0.2, 0.3))
circuit.add(gates.M(0))

# set backend to qibolab
qibo.set_backend("qibolab", platform="dummy")
# disable the transpiler
GlobalBackend().transpiler = None

# execute circuit
result = circuit(nshots=1000)

Defining custom compiler rules

The compiler can be modified by adding new compilation rules or changing existing ones. As explained in Transpiler and Compiler section, a rule is a function that accepts a Qibo gate and a Qibolab platform and returns the pulse sequence implementing this gate.

The following example shows how to modify the transpiler and compiler in order to execute a circuit containing a Pauli X gate using a single pi-pulse:

from qibo import gates
from qibo.models import Circuit
from qibolab.backends import QibolabBackend
from qibolab.pulses import PulseSequence

# define the circuit
circuit = Circuit(1)
circuit.add(gates.X(0))
circuit.add(gates.M(0))


# define a compiler rule that translates X to the pi-pulse
def x_rule(gate, platform):
    """X gate applied with a single pi-pulse."""
    qubit = gate.target_qubits[0]
    sequence = PulseSequence()
    sequence.add(platform.create_RX_pulse(qubit, start=0))
    return sequence, {}


# the empty dictionary is needed because the X gate does not require any virtual Z-phases

backend = QibolabBackend(platform="dummy")
# disable the transpiler
backend.transpiler = None
# register the new X rule in the compiler
backend.compiler[gates.X] = x_rule

# execute the circuit
result = backend.execute_circuit(circuit, nshots=1000)

Here we completely disabled the transpiler to avoid transforming the X gate to a different gate and we added a rule that instructs the compiler how to transform the X gate.

The default set of compiler rules is defined in qibolab.compilers.default.

Note

If the compiler receives a circuit that contains a gate for which it has no rule, an error will be raised. This means that the native gate set that the transpiler uses, should be compatible with the available compiler rules. If the transpiler is disabled, a rule should be available for all gates in the original circuit.