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Sorry that I keep asking questions for the same subject. It is driving me crazy.

I have followed the [5,1,3] stabilizer code circuit designed by Gottesman in page 35 in his paper (also illustrated in my another question). This code has four stabilizer generators $XZZXI$, $IXZZX$, $XIXZZ$, $ZXIXZ$.

The following is a minimal working example in which I only draw the stabilizer generator $IXZZX$,

from qiskit import *

encoded = QuantumRegister(5,'encoded')
stab = QuantumRegister(4,'stab')
syndr = ClassicalRegister(4, 'syndr')
five = QuantumCircuit(encoded)
fiveS = QuantumCircuit(encoded, stab, syndr)


#----------------------------------encode----------------------------------
five.h(encoded[0])
five.h(encoded[1])
five.h(encoded[2])
five.h(encoded[3])


five.z(encoded[0])
five.z(encoded[3])


five.cz(encoded[0],encoded[1])
five.cz(encoded[0],encoded[3])
five.cy(encoded[0],encoded[4])


five.cz(encoded[1],encoded[2])
five.cz(encoded[1],encoded[3])
five.cx(encoded[1],encoded[4])


five.cz(encoded[2],encoded[0])
five.cz(encoded[2],encoded[1])
five.cx(encoded[2],encoded[4])


five.cz(encoded[3],encoded[0])
five.cz(encoded[3],encoded[2])
five.cy(encoded[3],encoded[4])


five.barrier()


#-------------------stabilizer generator IXZZX------------------------------
fiveS.h(stab[1])
fiveS.cx(stab[1],encoded[1])
fiveS.cz(stab[1],encoded[2])
fiveS.cz(stab[1],encoded[3])
fiveS.cx(stab[1],encoded[4])
fiveS.h(stab[1])


fiveS.measure(stab,syndr)


#----------------circuit visualization--------------------
#%matplotlib inline
#(five+fiveS).draw(output='mpl')


counts = execute( five+fiveS, Aer.get_backend('qasm_simulator'), shots=10000).result().get_counts()
print(counts)

Measuring stabilizer generators should return $0000$ but instead I get half $0000$ and half $0010$ (Same results returns when measuring $XIXZZ$, $ZXIXZ$). Where am I wrong?

Question updated

I tried to add all qubits in one quantum register and produce one quantum circuit.

from qiskit import *

stab = QuantumRegister(9,'stab')
syndr = ClassicalRegister(4, 'syndr')
five = QuantumCircuit(stab, syndr)


#----------------------------------encode----------------------------------
five.h(stab[0])
five.h(stab[1])
five.h(stab[2])
five.h(stab[3])


five.z(stab[0])
five.z(stab[3])


five.cz(stab[0],stab[1])
five.cz(stab[0],stab[3])
five.cy(stab[0],stab[4])


five.cz(stab[1],stab[2])
five.cz(stab[1],stab[3])
five.cx(stab[1],stab[4])


five.cz(stab[2],stab[0])
five.cz(stab[2],stab[1])
five.cx(stab[2],stab[4])


five.cz(stab[3],stab[0])
five.cz(stab[3],stab[2])
five.cy(stab[3],stab[4])


five.barrier()


#-------------------stabilizer generator IXZZX------------------------------
five.h(stab[6])
five.cx(stab[6],stab[1])
five.cz(stab[6],stab[2])
five.cz(stab[6],stab[3])
five.cx(stab[6],stab[4])
five.h(stab[6])


five.measure(stab[5],syndr[0])
five.measure(stab[6],syndr[1])
five.measure(stab[7],syndr[2])
five.measure(stab[8],syndr[3])


#----------------circuit visualization--------------------
#%matplotlib inline
#five.draw(output='mpl')


counts = execute( five, Aer.get_backend('qasm_simulator'), shots=10000).result().get_counts()
print(counts)

However, the same happens. The following are my running results.enter image description here

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Jacey Li
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    Could you retry this while adding the encoding operation into the circuit/program fiveS instead of five? It might be the case that the 'encoded' register in fiveS does actually not have the encoded state - that means that the state of the data qubits is mostly shared even over the +1 and -1 eigenspace of the generator you are measuring – JSdJ Jan 07 '20 at 13:55
  • Thank you for your help! I tried. However, the same happens. I have updated new circuit version and running results in my original question. – Jacey Li Jan 07 '20 at 15:17
  • My Qiskit installation is broken, so I can't run any code of my own... It's weird, everything looks correct to me. Could you try a couple of things:
    • Try to run the stabilizer measurement without the encoding scheme,
    • Try a different stabilizer,

    And report back if you are still getting these -1 eigenvalues?

    I'm not familiar with the encoding scheme from Gottesman here (I don't know how he got it), so I don't really know if it is correct

     – JSdJ Jan 07 '20 at 15:45
  • -If I run without the encoding scheme, $IXZZX$ won't stabilize $|00000\rangle$, it returns half $+1$ half $-1$ as expected. -Only XZZXI returns all $+1$, the other three still get $-1$ eigenvalues. I could only guess that this encoding scheme does not match these stabilizer generators. – Jacey Li Jan 08 '20 at 02:16

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