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Introduction to Blqs_cirq

Blqs_cirq is a python library that uses blqs to make writing (Cirq)[https://github.com/quantumlib/cirq] programs more imperative and, hopefully, more straightforward.

A Quick Review of Cirq Circuits

In Cirq a cirq.Circuit is a container for cirq.Moments. Each cirq.Moment roughly corresponds to a slide of abstract time. A moment itself is made up of cirq.Operations, each of which operates on disjoint cirq.Qids.

For example, here is a simple circuit created in Cirq:

import cirq

q0, q1 = cirq.LineQubit.range(2)

circuit = cirq.Circuit()
circuit.append(cirq.H(q0))
circuit.append(cirq.CNOT(q0, q1))
circuit.append(cirq.measure(q0, q1, key="m"))
print(circuit)
> prints
> 0: ───H───@───M('m')───
>           │   │
> 1: ───────X───M────────

This circuit represents a Hadamard gate acting on the first qubit, a controlled NOT from the first qubit to the second, and then a measurement on the last two qubits storing the result in the m key.

Cirq comes with a large number of standard quantum gates and is easily extensible to add your own gates. Circuit objects can then be used in other ways by Cirq. For example, they can be optimized (compiled) into other circuits. Or they can be simulated using built in Cirq simulator or externally available simulators. They can also be sent to a quantum computer to run the actual program.

Cirq has a few ways to simplify writing circuits. One of these is that one can take use generators to stream gates into a cirq.Circuit object

def my_circuit():
    for q in cirq.LineQubit.range(3):
        yield cirq.H(q)

circuit = cirq.Circuit()
circuit.append(my_circuit())
print(circuit)
> prints
> 0: ───H───
>
> 1: ───H───
>
> 2: ───H───

This is nice as it allows for mixing of native python with the actual circuit construction. However, it is still a bit awkward.

Another important construction in Cirq are the basic of control flow (this is currently, as of Cirq v0.12 a work in progress). For example one can add a subcircuit to Cirq which repeats a circuit multiple times

q0, q1 = cirq.LineQubit.range(2)
circuit = cirq.Circuit()

subcircuit = cirq.Circuit()
subcircuit.append([cirq.H(q0), cirq.CNOT(q0, q1)])

circuit.append([cirq.H(q0), cirq.H(q1)])
circuit.append(cirq.CircuitOperation(subcircuit.freeze(), repetitions=10))
print(circuit)
> prints
>           Circuit_0xb2d97d2cac98317c:
> 0: ───H───[ 0: ───H───@───          ]─────────────
>           [           │             ]
>           [ 1: ───────X───          ](loops=10)
>           │
> 1: ───H───#2──────────────────────────────────────

This is rather cumbersome, and as more control flow is added to Cirq, this is likely to make writing Cirq more painful.

Blqs_cirq

In blqs_cirq we can do the above, in a more familiar imperative style.

import blqs_cirq as bc
from blqs_cirq import H, CX

@bc.build
def my_circuit():
    H(0)
    CX(0, 1)

circuit = my_circuit()
print(circuit)
> prints
> 0: ───H───@───
>           │
> 1: ───────X───

My using the blqs_cirq.build annotation on a function, we have turned it into a function that will return the cirq.Circuit corresponding to the imperatively written circuit inside the function. Further we have used the blqs_cirq gates, H and CX, directly corresponding to the Cirq gates, to construct the circuit. Finally, note that instead of using line qubits, we have used simple integers. If we example the circuit that has been constructed we can see that it has converted these ints to cirq.LineQubits.

Similarly, we can use blqs_cirq to write cirq.CircuitOperations in a cleaner form

@bc.build
def my_circuit():
    H(0)
    CX(0, 1)
    with bc.Repeat(repetitions=10):
        for i in range(2):
            H(i)

circuit = my_circuit()
print(circuit)
> prints
>               Circuit_0x91d1c5891174e662:
> 0: ───H───@───[ 0: ───H───              ]─────────────
>           │   [                         ]
>           │   [ 1: ───H───              ](loops=10)
>           │   │
> 1: ───────X───#2──────────────────────────────────────

Behind the scenes, blqs_cirq is using blqs which is turn capturing the

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