# Verifying a Stabilizer Code¶

In this example we will see how VerifyStabilizers can be used to develop a simple, distance-three code. We begin by considering the generators in:

In this figure we see an example stabilizer code. Data qubits are indicated as white circles and are labelled by numbers. Polygons represent checks. The coloring of the polygons indicate how the associated check acts on qubits it touches. When the check is white on a qubit, the check acts as $$Z.$$ When black, $$X.$$ When brown, $$Y.$$

We now use VerifyStabilizers to represent the checks given in the previous figure:

>>> import pecos as pc
>>> qecc = pc.tools.VerifyStabilizers()
>>> qecc.check('X', (3, 4, 7, 8))
>>> qecc.check('X', (5, 6, 7, 9))
>>> qecc.check('Z', (2, 4, 5, 7))
>>> qecc.check('Z', (7, 8, 9))
>>> qecc.check(('Z', 'Z', 'Y'), (0, 1, 2))
>>> qecc.check(('X', 'X', 'Z', 'Y'), (0, 2, 3, 4))
>>> qecc.check(('X', 'X', 'Z', 'Y'), (1, 2, 6, 5))


Here we see that the check method can be used to specify a generator. If the first argument is a string, then this indicates the Pauli-type of the check. The second argument then indicates which qubits the check acts on. If the first argument is a tuple, then the tuple is a sequence of strings which indicate how the generator acts on the corresponding qubits indicated in the tuple of the second argument.

Once one has finished specifying the generators of the code, the compile method should be used:

>>> # Continuing the last listing.
>>> qecc.compile()   # doctest: +SKIP
Check:
check(('Z', 'Z', 'Y'), (0, 1, 2))
anticommutes with:
check(Z, (2, 4, 5, 7))


Once compile is called, VerifyStabilizers checks to see if all the generators anticommute. If any do VerifyStabilizers prints out a message indicating which checks anticommute.

Finding that our original stabilizer code design does have anticommuting generators, we can modify the QECC slightly to address the issue as seen in:

Re-specifying the generators according to the figure above, we run compile again and see that we have solved the commutation problem:

>>> qecc = pc.tools.VerifyStabilizers()
>>> qecc.check('X', (3, 4, 7, 8))
>>> qecc.check('X', (5, 6, 7, 9))
>>> qecc.check('Z', (2, 4, 5, 7))
>>> qecc.check('Z', (7, 8, 9))
>>> qecc.check(('Z', 'Z', 'Z'), (0, 1, 2))
>>> qecc.check(('X', 'X', 'Z', 'Y'), (0, 2, 3, 4))
>>> qecc.check(('X', 'X', 'Z', 'Y'), (1, 2, 6, 5))
>>> qecc.compile()


It is possible that we have specified a set of generators with redundant elements. That is, some of the generators can be written as products of the others. To check if this is the case, we use the method generators:

>>> # Following the last listing.
>>> qecc.generators()
Number of data qubits: 10
Number of checks: 7
Number of logical qubits: 3

Stabilizer generators:
XIXZYIIIII
IXXIIYZIII
IIZIZZIZII
ZZZIIIIIII
IIIIIIIZZZ
IIIXXIIXXI
IIIIIXXXIX

Destabilizer generators:
ZIIIIIIIII
IZIIIIIIII
ZIIIXIIIII
ZIXIXIIIII
ZIIIXIIXII
IIIIIIIIZI
IIIIIIIIIZ

Logical operators:
. Logical Z #1:
IIIZIIIIZI
. Logical X #1:
ZIIXIIIIII
. Logical Z #2:
IZIIIZIIIZ
. Logical X #2:
ZZIIXXIIII
. Logical Z #3:
IIIIIIZIIZ
. Logical X #3:
IZIIIIXIII


If we had redundant generators then generators would alert us. Luckily, we do not and generators has printed out some useful information including number of logical qubits, destabilizers, and a possible set of logical operators.

We can then use the distance method to determine the distance of the code. Note, to find the distance of a code, this method will try all combinations of possible Pauli errors. It starts with the smallest weight and evaluating larger and larger weights until a logical error is detected. Since this is a combinatorial search, the algorithm is not efficient and the runtime quickly grows with the size of the code. In practice, for smaller code of less than 20 or so qubits, the runtime is manageable.

We now run the distance method:

>>> # Following the last listing.
qecc.distance()
----
Checking errors of length 1...
Checking errors of length 2...
Logical error found: Xs - {0, 1} Zs - set()
This is a [[10, 3, 2]] code.


The last line of the code block indicates what type of QECC we have. The notation $$[[n, k, d]]$$ indicates that the code encodes $$k$$ qubits into $$n$$ physical qubits and has a distance of $$d$$. Since the number of errors a QECC can correct is $$t=\left \lfloor{(d-1)/2}\right\rfloor$$ and the distance of our code is two, this means our QECC can only detect but not correct errors. Because the distance method indicates the smallest logical error it found, we can use this information to mitigate the error by either introducing another check to detect the error or by including the logical error as a check. We do the later. Doing this, we find that we have not increased the distance of the code. If we repeat the process two more times we will end up with a code that has no logical qubits and, therefore, encodes a stabilizer state:

We seemly failed to create a higher distance code; however, we can persevere by removing a higher-weight stabilizer generator. If we remove the check that acts like Pauli $$Z$$ on qubits 7, 8, and 9, we will get the stabilizer code in:

Evaluating the distance of this new version of the code:

>>> qecc = pc.tools.VerifyStabilizers()
>>> qecc.check('Z', (2, 4, 5, 7))
>>> # qecc.check('Z', (7, 8, 9))
>>> qecc.check('X', (3, 4, 7, 8))
>>> qecc.check('X', (5, 6, 7, 9))
>>> qecc.check(('X', 'X', 'Z', 'Y'), (0, 2, 3, 4))
>>> qecc.check(('X', 'X', 'Z', 'Y'), (1, 2, 6, 5))
>>> qecc.check('Z', (0, 1, 2))
>>> qecc.check('X', (0, 1))
>>> qecc.check('Z', (3, 8))
>>> qecc.check('Z', (6, 9))
>>> qecc.compile()
>>> qecc.generators()
Number of data qubits: 10
Number of checks: 9
Number of logical qubits: 1
----

Stabilizer generators:
XIXZYIIIII
IXXIIYZIII
IIZIZZIZII
IIIZIIIIZI
XXIIIIIIII
ZZZIIIIIII
IIIIIIZIIZ
IIIXXIIXXI
IIIIIXXXIX

Destabilizer generators:
IIIIZIIIZI
ZZIIZIIIZI
ZZIIZXIIZI
IIIXZIIIZI
ZIIIZIIIZI
ZZXIZXIIZI
ZZIIZIXIZI
IIIIIIIIZI
IIIIIIIIIZ

Logical operators:
. Logical Z #1:
IIIIIIIZZZ
. Logical X #1:
ZZIIZXIXZI
>>> qecc.distance()

----
Checking errors of length 1...
Checking errors of length 2...
Checking errors of length 3...
Logical error found: Xs - {0, 2, 7} Zs - set()
This is a [[10, 1, 3]] code.


Thus, we have developed a simple distance three code.