Writing a Phase Estimation function in Cirq

Question

I am trying to write a phase estimation algorithm using Cirq. The algorithm works for different inputs but I receive a few errors in the estimate_phi(mystery) function.

from cirq import CZPowGate

def create_operator(phi):
    CU = CZPowGate(exponent = phi*2)
    return CU

def iqft(n,qubits,circuit):
    
    #Swap the qubits
    for i in range(n//2):
        circuit.append(SWAP(qubits[i],qubits[n-i-1]), strategy = InsertStrategy.NEW)
     
    #For each qubit
    for i in range(n-1,-1,-1):
        #Apply CR_k gates where j is the control and i is the target
        k=n-i #We start with k=n-i
        for j in range(n-1,i,-1):
            #Define and apply CR_k gate
            crk = CZPowGate(exponent = -2/2**(k))
            circuit.append(crk(qubits[j],qubits[i]),strategy = InsertStrategy.NEW)
            k=k-1 #Decrement at each step
            
        #Apply Hadamard to the qubit
        circuit.append(H(qubits[i]),strategy = InsertStrategy.NEW)

def qpe(t,control, target, circuit, CU):
    
    #Apply Hadamard to control qubits
    circuit.append(cirq.H.on_each(control))
    
    #Apply CU gates
    for i in range(t):
        #Obtain the power of CU gate 
        CUi = CU**(2**i)
        #Apply CUi gate where t-i-1 is the control
        circuit.append(CUi(control[t-i-1],*target))
        
    #Apply inverse QFT
    iqft(t,control,circuit)

This script works for different inputs:

import cirq
from cirq.circuits import InsertStrategy
from cirq          import H, X, SWAP, CZPowGate
# def estimate_phi(mystery):
mystery = 0.30
#     Let the size of the first register equal 10.
n = 1

for t in range (1,10):
    #Create cirucit
    circuit = cirq.Circuit()
    
   
    #Create t control qubits
    control = [cirq.LineQubit(i) for i in range(t) ]

    #Create n target qubits
    target = [cirq.LineQubit(i) for i in range(t, t + n) ]

    #Set target qubit to state |1> 
    circuit.append(X.on_each(target))
    
    #Apply QPE
    CU = create_operator(mystery)
    qpe(t,control, target, circuit, CU)

    circuit.append(cirq.measure(*control, key = 'result'))

    s       = cirq.Simulator()
    samples = s.run(circuit, repetitions = 1000)
    
    #Most frequent observation
    freq = list(samples.histogram(key = 'result').keys())[0]
    print("t:", t,"estimation:", freq/2**t)

but when I put it in a function, I receive two errors for the print(test_qpe(0.23)) command

unsupported operand type(s) for ** or pow(): 'complex' and 'LinearCombinationOfGates'

During handling of the above exception, another exception occurred: Simulator doesn't support (cirq.CZcirq.LinearCombinationOfGates({(cirq.CZ0.46): 2})).on(cirq.LineQubit(0), cirq.LineQubit(1))

import cirq
from cirq.circuits import InsertStrategy
from cirq          import H, X, SWAP, CZPowGate

def estimate_phi(mystery):

    for t in range (1,10):
    #Create cirucit
        circuit = cirq.Circuit()
        #Number of qubits in the register storing eigenvector
        n = 1 
    
    
    #Create t control qubits
        control = [cirq.LineQubit(i) for i in range(t) ]

    #Create n target qubits
        target = [cirq.LineQubit(i) for i in range(t, t + n) ]

    #Set target qubit to state |1> 
        circuit.append(X.on_each(target))
    
    #Apply QPE
        CU = create_operator(mystery)
        qpe(t,control, target, circuit, CU)

        circuit.append(cirq.measure(*control, key = 'result'))

        s       = cirq.Simulator()
        samples = s.run(circuit, repetitions = 1000)
    
    #Most frequent observation
        freq = list(samples.histogram(key = 'result').keys())[0]
        
        
    return freq/2**t

import math
def test_qpe(phi):
    operator = CZPowGate(exponent=2*phi)
    return estimate_phi(operator)

Answer 1

You are passing estimate_phi the an operator: CZPowGate(exponent=2*phi). Then in estimate_phi this operator becomes the mystery parameter. You then take this mystery variable and call create_operator on it. In create_operator this means that CU = CZPowGate(exponent = phi*2) is really CU = CZPowGate(CZPowGate(exponent=2*phi)). So you are creating a CZ raised to a CZ!

I think what you want to do is to pass in 2*phi in the test_qpe function.

Also, for creating the line qubits, you might be interested in the class function LineQubit.range which is just like python's range but returns LineQubits instead.