How to reset qubit in project Q?

[saifawan2703]

Hi everyone,

I'm new to ProjectQ and I'm looking for guidance on resetting qubits. I understand that the reset gate available in Qiskit isn't directly available in ProjectQ. Could anyone suggest an alternative method to reset a qubit to the ∣0⟩ state in ProjectQ?

[damiansteiger]

To reset a qubit, you can Measure the qubit and depending on the outcome flip it and then continue to use it. See example below. While the simulator allows a measured qubits to be used further, hardware backends might not allow operations on a qubit after it has been measured. In these cases it would require to implement the missing reset gate in Project and adopt it in the hardware backend which support this functionality.

from projectq import MainEngine
from projectq.ops import X, H, Measure

eng = MainEngine()

q1 = eng.allocate_qubit()

H | q1
Measure | q1
eng.flush() # To ensure that circuit got executed 
measurement_result = int(q1)
print("Measurement: {}".format(measurement_result))

# Flip qubit back to state 0
if measurement_result:
    X | q1

# Do sth else with q1 which is now in state 0
H | q1 #...
Measure | q1
eng.flush()

[saifawan2703]

@damiansteiger Hi everyone,

I would like to know if there is a way to reset a qubit to the |0⟩ state in ProjectQ without performing a measurement. The reason is that after measuring, I am unable to use the compute-uncompute pattern. like that

with Compute(eng):
    All(H) | k0
    All(H) | k1
    All(X) | k0
    All(X) | k1

with Control(eng, k0[0:4]):
        with Control(eng, k1[0:-1]):
            Z | k1[-1]  
Uncompute(eng)  #

Any guidance on this would be appreciated.

[damiansteiger]

Does https://github.com/ProjectQ-Framework/ProjectQ/issues/477 solve the issue?