Stabilizer Formalism and the Steane Code

Tags: algorithm error-correction

In Introduction to Quantum Error Correction (the first part), we built the 3-qubit bit-flip and phase-flip codes and Shor’s 9-qubit code, and at the end gave the name “stabilizer” to the parity operators.

This second part treats stabilizers formally. We then implement the CSS construction, which systematically builds quantum codes from classical codes, and its flagship example, the Steane code. The Steane code achieves the same $d=3$ as Shor’s code with seven qubits instead of nine.

What this article covers:

Stabilizer formalism — formalize stabilizers, generators, and the syndrome.
The CSS construction — build the Steane code’s stabilizers from a classical Hamming code.
Implement encoding, syndrome measurement, and correction for the Steane code.
Confirm that seven physical Hadamard gates act as a logical Hadamard.

Prerequisites: the content of the first part (syndrome measurement, Pauli errors, the term “stabilizer”).

# Install the latest Qamomile from pip.
# !pip install "qamomile[qiskit]"
# # or
# !uv add "qamomile[qiskit]"

1. Stabilizer Formalism¶

In the first part, we called parity operators such as $Z_0Z_1$ and $X_0X_1$ stabilizers. We first formalize this term.

1.1 What Is a Stabilizer¶

A stabilizer is a Pauli operator $S$ that leaves every state of the code space unchanged. For any codeword $\lvert\psi\rangle$ ,

S\lvert\psi\rangle = \lvert\psi\rangle

(1)

holds. Equivalently, the code space is the +1 eigenspace of the stabilizer.

For example, the codewords of the bit-flip code are $\lvert000\rangle$ and $\lvert111\rangle$ . Applying $Z_0Z_1$ gives $Z_0Z_1\lvert000\rangle=\lvert000\rangle$ and $Z_0Z_1\lvert111\rangle=(-1)(-1)\lvert111\rangle=\lvert111\rangle$ — neither changes. So $Z_0Z_1$ is a stabilizer of the bit-flip code.

1.2 Generators and the Syndrome¶

A code has many stabilizers, but we do not need to list them all. Choosing a few generators is enough; the rest are obtained as their products. For the bit-flip code, $Z_0Z_1$ and $Z_0Z_2$ are the two generators.

Measuring a stabilizer generator yields the eigenvalue +1 or -1.

With no error, the state is in the code space and every generator returns +1.
When an error $E$ occurs, the measured value of any generator that anticommutes with $E$ flips to -1.

This pattern of $\pm1$ (written as $0/1$ in bits) is the syndrome. The values we extracted into ancilla qubits in the first part were exactly the measurement results of stabilizer generators. Seeing which generators turned -1 tells us the location and type of the error.

1.3 The First Part’s Codes as Stabilizers¶

The three codes from the first part can all be described with stabilizer generators.

Code	Stabilizer generators
3-qubit bit-flip	$Z_0Z_1,\ Z_0Z_2$
3-qubit phase-flip	$X_0X_1,\ X_0X_2$
Shor 9-qubit	$Z_0Z_1,\ Z_0Z_2,\ Z_3Z_4,\ Z_3Z_5,\ Z_6Z_7,\ Z_6Z_8,\ X_0X_1X_2X_3X_4X_5,\ X_3X_4X_5X_6X_7X_8$

The bit-flip code has only $Z$ -type generators, and the phase-flip code only $X$ -type generators. Shor’s code has both, with the $Z$ -type detecting $X$ errors and the $X$ -type detecting $Z$ errors. This structure of “holding $X$ -type and $Z$ -type generators separately” leads to the CSS construction in the next section.

1.4 $[[n,k,d]]$ and the Code Distance¶

A stabilizer code is characterized by three numbers, $[[n,k,d]]$ .

$n$ : the number of physical qubits.
$k$ : the number of logical qubits protected. Each generator removes one degree of freedom, so $k = n - (\text{number of generators})$ .
$d$ : the code distance. It is the minimum weight (the number of qubits acted on) among the Pauli operators that commute with every stabilizer but are not themselves stabilizers — these are called logical operators.

A code of distance $d$ can correct up to $\lfloor(d-1)/2\rfloor$ errors. Correcting any single-qubit error requires $d\ge3$ . The 3-qubit codes from the first part had $d=1$ and Shor’s code had $d=3$ . The Steane code we are about to build is $[[7,1,3]]$ .

2. From Classical Hamming Codes to CSS Codes¶

A CSS code (Calderbank-Shor-Steane code) is a way to build quantum codes systematically from classical error-correcting codes. The Steane code is its flagship example, based on the classical Hamming code.

2.1 The Classical Hamming [7,4,3] Code¶

The classical Hamming [7,4,3] code protects 4 bits of information with 7 bits and can correct a single-bit error. It is defined by a parity-check matrix $H$ .

H = \begin{pmatrix} 0 & 0 & 0 & 1 & 1 & 1 & 1 \\ 0 & 1 & 1 & 0 & 0 & 1 & 1 \\ 1 & 0 & 1 & 0 & 1 & 0 & 1 \end{pmatrix}

(2)

A 7-bit word $c$ is a codeword if and only if $Hc=0$ (its parity with every row is even).

2.2 How the Columns Point to the Error Location¶

Look closely at the columns of $H$ : column $j$ is the binary representation of the number $j+1$ (column 0 is 001, column 1 is 010, …, column 6 is 111).

When a single-bit error occurs at position $j$ , $Hc$ equals exactly column $j$ . So reading the three bits of $Hc$ as a binary number gives the error location directly. This is the mechanism by which the classical Hamming code pinpoints the error location in one shot.

2.3 The CSS Construction: $X$ -Type and $Z$ -Type Stabilizers¶

The CSS construction builds two kinds of stabilizers from this parity-check matrix $H$ .

$Z$ -type stabilizers: read each row of $H$ as an operator placing $Z$ where the 1s are. Detects $X$ errors.
$X$ -type stabilizers: read the same rows as operators placing $X$ . Detects $Z$ errors.

An $X$ error anticommutes with the $Z$ -type stabilizers, and a $Z$ error anticommutes with the $X$ -type stabilizers. So the $Z$ -type locate the $X$ errors and the $X$ -type locate the $Z$ errors, each in exactly the same way as the classical Hamming code. The “correct $X$ and $Z$ independently” seen in Shor’s code in the first part comes out automatically from the classical code here.

2.4 The Six Generators of the Steane Code¶

From the three rows of the Hamming matrix $H$ we obtain three $Z$ -type and three $X$ -type generators — six stabilizer generators in total.

Type	Stabilizer	Detects
$X$ type	$X_3X_4X_5X_6$	$Z$
$X$ type	$X_1X_2X_5X_6$	$Z$
$X$ type	$X_0X_2X_4X_6$	$Z$
$Z$ type	$Z_3Z_4Z_5Z_6$	$X$
$Z$ type	$Z_1Z_2Z_5Z_6$	$X$
$Z$ type	$Z_0Z_2Z_4Z_6$	$X$

With 7 physical qubits and 6 generators, the number of logical qubits protected is $7-6=1$ . This is the Steane $[[7,1,3]]$ code.

Before getting into the implementation, we load Qamomile and the Qiskit integration and define helper functions. _bits7, _passes_hamming_checks, and _is_steane_zero_word are utilities that decide whether a measurement outcome is a Hamming codeword or a $\lvert0_L\rangle$ codeword. They are not central to QEC, so feel free to skip them.

import os

import qamomile.circuit as qmc
from qamomile.qiskit import QiskitTranspiler

transpiler = QiskitTranspiler()
docs_test_mode = os.environ.get("QAMOMILE_DOCS_TEST") == "1"
encoding_shots = 1 if docs_test_mode else 1024
correction_shots = 1 if docs_test_mode else 128

# Create a seeded backend for reproducible documentation output.
from qiskit_aer import AerSimulator

_seeded_backend = AerSimulator(seed_simulator=42, max_parallel_threads=1)
_seeded_executor = transpiler.executor(backend=_seeded_backend)


def _bits7(outcome) -> list[int]:
    """Return the outcome as a list of 7 bits in qubit-index order."""
    if isinstance(outcome, (list, tuple)):
        return list(outcome)
    return [(outcome >> i) & 1 for i in range(7)]


def _passes_hamming_checks(outcome) -> bool:
    """Return whether the outcome is a Hamming [7,4,3] codeword (passes all 3 parity checks)."""
    bits = _bits7(outcome)
    h_checks = [
        bits[3] ^ bits[4] ^ bits[5] ^ bits[6],
        bits[1] ^ bits[2] ^ bits[5] ^ bits[6],
        bits[0] ^ bits[2] ^ bits[4] ^ bits[6],
    ]
    return all(check == 0 for check in h_checks)


def _is_steane_zero_word(outcome) -> bool:
    """Return whether the outcome is a |0_L> codeword (an even-weight Hamming codeword)."""
    return _passes_hamming_checks(outcome) and sum(_bits7(outcome)) % 2 == 0

3. Encoding the Logical $\lvert0_L\rangle$ ¶

3.1 The Logical Zero Is a Superposition of Even-Weight Hamming Codewords¶

The logical $\lvert0_L\rangle$ of the Steane code is the superposition of all even-weight Hamming codewords.

\lvert0_L\rangle = \frac{1}{2\sqrt2}\sum_{c\in C,\ w(c)\,\text{even}} \lvert c\rangle

(3)

Here $C$ is the set of Hamming codewords and $w(c)$ is the weight. There are 8 even-weight Hamming codewords, so this is an 8-term superposition.

3.2 The Encoding Circuit¶

The following circuit builds $\lvert0_L\rangle$ from $\lvert0\rangle^{\otimes7}$ . It writes in the three $X$ -type stabilizer patterns ( $X_3X_4X_5X_6$ , $X_1X_2X_5X_6$ , $X_0X_2X_4X_6$ ) one by one with Hadamard and CNOT gates.

@qmc.qkernel
def encode_steane_zero(data: qmc.Vector[qmc.Qubit]) -> qmc.Vector[qmc.Qubit]:
    # Write in the X3 X4 X5 X6 pattern.
    data[3] = qmc.h(data[3])
    data[3], data[4] = qmc.cx(data[3], data[4])
    data[3], data[5] = qmc.cx(data[3], data[5])
    data[3], data[6] = qmc.cx(data[3], data[6])

    # Write in the X1 X2 X5 X6 pattern.
    data[1] = qmc.h(data[1])
    data[1], data[2] = qmc.cx(data[1], data[2])
    data[1], data[5] = qmc.cx(data[1], data[5])
    data[1], data[6] = qmc.cx(data[1], data[6])

    # Write in the X0 X2 X4 X6 pattern.
    data[0] = qmc.h(data[0])
    data[0], data[2] = qmc.cx(data[0], data[2])
    data[0], data[4] = qmc.cx(data[0], data[4])
    data[0], data[6] = qmc.cx(data[0], data[6])
    return data

@qmc.qkernel
def encode_zero_and_measure() -> qmc.Vector[qmc.Bit]:
    data = qmc.qubit_array(7, name="data")
    data = encode_steane_zero(data)
    return qmc.measure(data)

3.3 Checking the Encoding¶

We measure the encoder’s output and check that every observed bitstring is an even-weight Hamming codeword (a $\lvert0_L\rangle$ codeword).

print("Encode and measure |0_L>")
exe = transpiler.transpile(encode_zero_and_measure)
result = exe.sample(_seeded_executor, shots=encoding_shots).result()
total = sum(count for _, count in result.results)
valid = sum(count for outcome, count in result.results if _is_steane_zero_word(outcome))
print(f"  |0_L> codeword ratio: {valid / total:.3f}")
print(f"  distinct codewords observed: {len(result.results)}")
# The Steane |0_L> is an equal superposition of exactly 8 even-weight
# Hamming codewords, so every shot reads a valid |0_L> codeword and the
# distribution covers at most those 8 outcomes.
assert total == encoding_shots
assert valid == total
assert len(result.results) <= 8

Encode and measure |0_L>
  |0_L> codeword ratio: 1.000
  distinct codewords observed: 8

4. Syndrome Measurement and Correction¶

4.1 Decoding $X$ and $Z$ Independently¶

The advantage of a CSS code is that $X$ errors and $Z$ errors can be handled completely independently.

Measuring the $Z$ -type stabilizers gives the syndrome for $X$ errors.
Measuring the $X$ -type stabilizers gives the syndrome for $Z$ errors.

Each is a 3-bit syndrome, and the column mechanism of the Hamming matrix seen in 2.2 applies directly.

4.2 The Syndrome Table¶

The 3-bit syndrome $(s_2,s_1,s_0)$ represents the error location directly in binary.

Error location	Syndrome $(s_2,s_1,s_0)$
none	$(0,0,0)$
$q_0$	$(0,0,1)$
$q_1$	$(0,1,0)$
$q_2$	$(0,1,1)$
$q_3$	$(1,0,0)$
$q_4$	$(1,0,1)$
$q_5$	$(1,1,0)$
$q_6$	$(1,1,1)$

The $(s_2,s_1,s_0)$ obtained from the $Z$ -type stabilizers points to the $X$ error location, and the one from the $X$ -type points to the $Z$ error location.

4.3 Implementation¶

error_type is 1=X, 2=Y, 3=Z, and error_pos is the qubit index 0..6 to inject the error into.

@qmc.qkernel
def steane_run(
    error_type: qmc.UInt,
    error_pos: qmc.UInt,
) -> qmc.Vector[qmc.Bit]:
    # Allocate 7 data qubits and 6 ancilla qubits for syndrome measurement.
    data = qmc.qubit_array(7, name="data")
    anc = qmc.qubit_array(6, name="anc")

    # Encode into |0_L>.
    data = encode_steane_zero(data)

    # Inject the X / Y / Z error specified by error_type / error_pos.
    for i in qmc.range(7):
        if (error_type == 1) & (error_pos == i):  # 1: X error
            data[i] = qmc.x(data[i])
        if (error_type == 2) & (error_pos == i):  # 2: Y error
            data[i] = qmc.y(data[i])
        if (error_type == 3) & (error_pos == i):  # 3: Z error
            data[i] = qmc.z(data[i])

    # Z-type stabilizers: measure the X-error syndrome (sx_2, sx_1, sx_0).
    data[3], anc[0] = qmc.cx(data[3], anc[0])
    data[4], anc[0] = qmc.cx(data[4], anc[0])
    data[5], anc[0] = qmc.cx(data[5], anc[0])
    data[6], anc[0] = qmc.cx(data[6], anc[0])
    sx_2 = qmc.measure(anc[0])

    data[1], anc[1] = qmc.cx(data[1], anc[1])
    data[2], anc[1] = qmc.cx(data[2], anc[1])
    data[5], anc[1] = qmc.cx(data[5], anc[1])
    data[6], anc[1] = qmc.cx(data[6], anc[1])
    sx_1 = qmc.measure(anc[1])

    data[0], anc[2] = qmc.cx(data[0], anc[2])
    data[2], anc[2] = qmc.cx(data[2], anc[2])
    data[4], anc[2] = qmc.cx(data[4], anc[2])
    data[6], anc[2] = qmc.cx(data[6], anc[2])
    sx_0 = qmc.measure(anc[2])

    # X-type stabilizers: measure the Z-error syndrome (sz_2, sz_1, sz_0).
    anc[3] = qmc.h(anc[3])
    anc[3], data[3] = qmc.cx(anc[3], data[3])
    anc[3], data[4] = qmc.cx(anc[3], data[4])
    anc[3], data[5] = qmc.cx(anc[3], data[5])
    anc[3], data[6] = qmc.cx(anc[3], data[6])
    anc[3] = qmc.h(anc[3])
    sz_2 = qmc.measure(anc[3])

    anc[4] = qmc.h(anc[4])
    anc[4], data[1] = qmc.cx(anc[4], data[1])
    anc[4], data[2] = qmc.cx(anc[4], data[2])
    anc[4], data[5] = qmc.cx(anc[4], data[5])
    anc[4], data[6] = qmc.cx(anc[4], data[6])
    anc[4] = qmc.h(anc[4])
    sz_1 = qmc.measure(anc[4])

    anc[5] = qmc.h(anc[5])
    anc[5], data[0] = qmc.cx(anc[5], data[0])
    anc[5], data[2] = qmc.cx(anc[5], data[2])
    anc[5], data[4] = qmc.cx(anc[5], data[4])
    anc[5], data[6] = qmc.cx(anc[5], data[6])
    anc[5] = qmc.h(anc[5])
    sz_0 = qmc.measure(anc[5])

    # X-component correction: apply X at the location pointed to by (sx_2, sx_1, sx_0).
    if (~sx_2) & (~sx_1) & sx_0:
        data[0] = qmc.x(data[0])
    if (~sx_2) & sx_1 & (~sx_0):
        data[1] = qmc.x(data[1])
    if (~sx_2) & sx_1 & sx_0:
        data[2] = qmc.x(data[2])
    if sx_2 & (~sx_1) & (~sx_0):
        data[3] = qmc.x(data[3])
    if sx_2 & (~sx_1) & sx_0:
        data[4] = qmc.x(data[4])
    if sx_2 & sx_1 & (~sx_0):
        data[5] = qmc.x(data[5])
    if sx_2 & sx_1 & sx_0:
        data[6] = qmc.x(data[6])

    # Z-component correction: apply Z at the location pointed to by (sz_2, sz_1, sz_0).
    if (~sz_2) & (~sz_1) & sz_0:
        data[0] = qmc.z(data[0])
    if (~sz_2) & sz_1 & (~sz_0):
        data[1] = qmc.z(data[1])
    if (~sz_2) & sz_1 & sz_0:
        data[2] = qmc.z(data[2])
    if sz_2 & (~sz_1) & (~sz_0):
        data[3] = qmc.z(data[3])
    if sz_2 & (~sz_1) & sz_0:
        data[4] = qmc.z(data[4])
    if sz_2 & sz_1 & (~sz_0):
        data[5] = qmc.z(data[5])
    if sz_2 & sz_1 & sz_0:
        data[6] = qmc.z(data[6])

    return qmc.measure(data)

4.4 Verification: All 21 Single Errors¶

We try all 21 single errors — $X$ , $Y$ , $Z$ on each of the seven qubits. After correction, every measured bitstring should return to a $\lvert0_L\rangle$ codeword.

print("Steane code: correct X/Y/Z on all 7 locations")
print(f"  {'err':4s} | {'pos':5s} | |0_L> codeword")
print(f"  {'-' * 4}-+-{'-' * 5}-+-{'-' * 14}")
error_cases = [
    (name, error_type, pos)
    for name, error_type in [("X", 1), ("Y", 2), ("Z", 3)]
    for pos in range(7)
]
if docs_test_mode:
    error_cases = [("X", 1, 0), ("Y", 2, 3), ("Z", 3, 6)]

for name, error_type, pos in error_cases:
    exe = transpiler.transpile(
        steane_run,
        bindings={"error_type": error_type, "error_pos": pos},
    )
    result = exe.sample(_seeded_executor, shots=correction_shots).result()
    total = sum(count for _, count in result.results)
    valid = sum(
        count for outcome, count in result.results if _is_steane_zero_word(outcome)
    )
    print(f"  {name:4s} | q[{pos}]  | {valid / total:.3f}")
    # Steane corrects every single Pauli error on any of the 7 qubits,
    # so the post-correction state lies entirely in the |0_L> code
    # space — every shot reads a valid codeword.
    assert total == correction_shots
    assert valid == total

Steane code: correct X/Y/Z on all 7 locations
  err  | pos   | |0_L> codeword
  -----+-------+---------------
  X    | q[0]  | 1.000

  X    | q[1]  | 1.000

  X    | q[2]  | 1.000
  X    | q[3]  | 1.000

  X    | q[4]  | 1.000

  X    | q[5]  | 1.000
  X    | q[6]  | 1.000

  Y    | q[0]  | 1.000

  Y    | q[1]  | 1.000
  Y    | q[2]  | 1.000

  Y    | q[3]  | 1.000
  Y    | q[4]  | 1.000
  Y    | q[5]  | 1.000

  Y    | q[6]  | 1.000
  Z    | q[0]  | 1.000
  Z    | q[1]  | 1.000

  Z    | q[2]  | 1.000
  Z    | q[3]  | 1.000

  Z    | q[4]  | 1.000
  Z    | q[5]  | 1.000

  Z    | q[6]  | 1.000

A ratio of 1.000 means the state returned to the $\lvert0_L\rangle$ code space for that single Pauli error. A $Y=iXZ$ error triggers both the $X$ -component and the $Z$ -component correction, but in a CSS code these two are independent, so it is corrected as is.

5. The Transversal Hadamard Gate¶

5.1 What Is a Transversal Gate¶

When applying a logical gate to a logical qubit, if it suffices to apply a physical gate independently to each physical qubit, that gate is called transversal.

Transversal gates are important in fault-tolerant quantum computation. Because the physical gates are independent of one another, a failure in one physical gate does not spread the error across multiple qubits within a block.

A major feature of the Steane code is that the logical Hadamard $\bar H$ is transversal. Applying $H$ to each of the seven physical qubits is itself the logical Hadamard.

\bar H = H^{\otimes 7}

(4)

This follows from the $X$ -type and $Z$ -type stabilizers of the Steane code having the same Hamming pattern — from the CSS construction using the same matrix $H$ .

5.2 Verifying the Logical Hadamard¶

We verify $\bar H = H^{\otimes7}$ through two properties.

Property 1: $\bar H$ maps $\lvert0_L\rangle$ to $\lvert+_L\rangle$ . $\lvert+_L\rangle$ is a superposition of $\lvert0_L\rangle$ and $\lvert1_L\rangle$ . While $\lvert0_L\rangle$ consists of even-weight Hamming codewords, $\lvert1_L\rangle$ consists of odd-weight Hamming codewords. So measuring $\lvert+_L\rangle$ produces Hamming codewords of both even and odd weight (whereas $\lvert0_L\rangle$ produces only even-weight ones).

@qmc.qkernel
def transversal_h_to_plus() -> qmc.Vector[qmc.Bit]:
    data = qmc.qubit_array(7, name="data")
    data = encode_steane_zero(data)
    # Apply the transversal Hadamard once: |0_L> -> |+_L>.
    data = qmc.h(data)
    return qmc.measure(data)

print("Transversal H: |0_L> -> |+_L>")
exe = transpiler.transpile(transversal_h_to_plus)
result = exe.sample(_seeded_executor, shots=encoding_shots).result()
total = sum(count for _, count in result.results)
hamming = sum(
    count for outcome, count in result.results if _passes_hamming_checks(outcome)
)
odd = sum(count for outcome, count in result.results if sum(_bits7(outcome)) % 2 == 1)
print(f"  Hamming codeword ratio: {hamming / total:.3f}")
print(f"  odd-weight (|1_L>) fraction: {odd / total:.3f}")
# |+_L> is an equal superposition of all 16 Hamming codewords, so every
# shot reads some Hamming codeword and the odd-weight half (|1_L>) shows
# up roughly half the time — 5% window > 1024-shot standard error of < 0.02.
assert total == encoding_shots
assert hamming == total
assert len(result.results) <= 16
if not docs_test_mode:
    assert abs(odd / total - 0.5) < 0.05

Transversal H: |0_L> -> |+_L>

  Hamming codeword ratio: 1.000
  odd-weight (|1_L>) fraction: 0.518

The Hamming codeword ratio is 1.000, and odd-weight codewords appear about half the time. Unlike $\lvert0_L\rangle$ , which produces only even-weight codewords, this confirms the state has moved to $\lvert+_L\rangle$ .

Property 2: applying $\bar H$ twice returns to the identity ( $\bar H^2 = I$ ). Applying $H^{\otimes7}$ twice to $\lvert0_L\rangle$ should return to $\lvert0_L\rangle$ .

@qmc.qkernel
def transversal_h_round_trip() -> qmc.Vector[qmc.Bit]:
    data = qmc.qubit_array(7, name="data")
    data = encode_steane_zero(data)
    # Apply the transversal Hadamard twice: H^2 = I, so it returns to |0_L>.
    data = qmc.h(data)
    data = qmc.h(data)
    return qmc.measure(data)

print("Transversal H round trip: |0_L> -> H -> H -> |0_L>")
exe = transpiler.transpile(transversal_h_round_trip)
result = exe.sample(_seeded_executor, shots=encoding_shots).result()
total = sum(count for _, count in result.results)
valid = sum(count for outcome, count in result.results if _is_steane_zero_word(outcome))
print(f"  |0_L> codeword ratio: {valid / total:.3f}")
# H^2 = I, so the round trip restores |0_L> exactly — every shot reads a
# |0_L> codeword.
assert total == encoding_shots
assert valid == total

Transversal H round trip: |0_L> -> H -> H -> |0_L>
  |0_L> codeword ratio: 1.000

The ratio is 1.000 — two transversal Hadamards return to $\lvert0_L\rangle$ . The seven physical $H$ gates indeed act as the logical $\bar H$ .

6. Summary¶

In this article we covered the stabilizer formalism and the Steane $[[7,1,3]]$ code.

Stabilizer formalism — we viewed the code space as the +1 eigenspace of Pauli operators (stabilizers) and defined the syndrome as the measurement values of the generators.
The CSS construction — we built $X$ -type and $Z$ -type stabilizers from the parity-check matrix of the classical Hamming [7,4,3] code.
The Steane code — we implemented the encoding of $\lvert0_L\rangle$ , syndrome measurement with the six stabilizers, and correction of all 21 single Pauli errors.
The transversal Hadamard — we confirmed that seven physical $H$ gates act as the logical $\bar H$ .

The Steane code achieves the same $d=3$ as Shor’s code with seven qubits instead of nine, and is a clean example of the CSS construction and transversal Clifford gates.

Beyond¶

The surface code — a code whose stabilizers are local operators on a 2D lattice. It is the leading approach to error correction on today’s superconducting quantum computers, with repeated syndrome measurement as a new ingredient.
Fault-tolerant quantum computation — a framework for running logical gates while staying encoded and advancing the computation without amplifying errors. Transversal gates are its starting point.