System #0

You are a physics research assistant specializing in solving complex, research-level problems using precise, step-by-step reasoning.

Input Problems will be provided in Markdown format.

Output (Markdown format)

Step-by-Step Derivation - Show every non-trivial step in the solution. Justify steps using relevant physical laws, theorems, or mathematical identities.
Mathematical Typesetting - Use LaTeX for all mathematics: $...$ for inline expressions, $$...$$ for display equations.
Conventions and Units - Follow the unit system and conventions specified in the problem.
Final Answer - At the end of the solution, start a new line with “Final Answer:”, and present the final result.

For final answers involving values, follow the precision requirements specified in the problem. If no precision is specified: - If an exact value is possible, provide it (e.g., \$\sqrt(2)\$, \$\pi/4\$). - If exact form is not feasible, retain at least 12 significant digits in the result.
Formatting Compliance - If the user requests a specific output format (e.g., code, table), provide the final answer accordingly.

User #1

Problem setup:

In quantum error correction, you encode quantum states into logical states made of many qubits in order to improve their resilience to errors. In quantum error detection, you do the same but can only detect the presence of errors and not correct them. In this problem, we will consider a single [[4,2,2]] quantum error detection code, which encodes two logical qubits into four physical qubits, and investigate how robust logical quantum operations in this code are to quantum errors.

Our convention is that the four physical qubits in the [[4,2,2]] code are labelled 0,1,2,3. The two logical qubits are labelled A and B. The stabilizers are $XXXX$ and $ZZZZ$, where $X$ and $Z$ are Pauli matrices. The logical $X$ and $Z$ operators on the two qubits are $X_A = XIXI$, $X_B=XXII$, $Z_A = ZZII$, $Z_B = ZIZI$, up to multiplication by stabilizers.

We will consider different state preparation circuits consisting of controlled not $CNOT_{ij}$ gates, where $CNOT_{ij}$ has control qubit $i$ and target qubit $j$. As a simple model of quantum errors in hardware, we will suppose that each $CNOT_{ij}$ gate in the circuit has a two qubit depolarizing error channel following it that produces one of the 15 non-identity two-qubit Paulis with equal probability $p/15$. The probability $p$ indicates the probability of an error in a single two-qubit gate. We will assess the logical infidelity of certain state preparation protocols as a function of the physical infidelity $p$.

Checkpoints

Checkpoint 1

Suppose that we wish to prepare a logical two-qubit GHZ state $(|00\rangle_{AB}+|11\rangle_{AB})/\sqrt{2}$ in the [[4,2,2]] code. To do so, we use the following state preparation circuit:

\[ (CNOT_{03}) (H_0) (CNOT_{21}) (H_2). \]

Note that this equation is written in matrix multiplication order, while the quantum operations in the circuit occur in the reverse order (from right-to-left in the above equation). $H$ is a single-qubit Hadamard gate.

What is the physical state fidelity of the final physical 4-qubit state at the end of the circuit as a function of the two-qubit gate error rate $p$?

Assistant #2

Step-by-Step Derivation

Ideal state prepared by the circuit
Start in |0000⟩.
Apply H2: (|0⟩2 + |1⟩2)/√2 ⊗ |0⟩1 ⊗ |0⟩0 ⊗ |0⟩3.
Apply CNOT21 (control 2, target 1): produces a Bell pair on (2,1), |Φ+⟩21 = (|00⟩21 + |11⟩21)/√2.
Apply H0: puts qubit 0 into |+⟩0 = (|0⟩0 + |1⟩0)/√2.
Apply CNOT03 (control 0, target 3): produces a Bell pair on (0,3), |Φ+⟩03 = (|00⟩03 + |11⟩03)/√2.
Therefore the ideal 4-qubit state is a product of two Bell pairs:

\[ |\psi_{\text{ideal}}\rangle = |\Phi^+\rangle_{03} \otimes |\Phi^+\rangle_{21}. \]

Noise model and its placement
After each CNOT, a two-qubit depolarizing channel acts on that CNOT’s two qubits:

\[ \Lambda_p(\rho) = (1-p)\rho + \frac{p}{15}\sum_{P\in\mathcal{P}_2\setminus\{I\}} P\rho P, \]

where the sum runs over the 15 non-identity two-qubit Pauli operators on the gate’s two qubits.

The two CNOTs act on disjoint pairs (2,1) and (0,3), and so do their error channels. Moreover, the gates after the first error (H0 and CNOT03) act only on qubits (0,3), hence an error on (2,1) does not propagate to (0,3), and vice versa.
Structure of the final (noisy) state
Because the target state factorizes as |Φ+⟩03 ⊗ |Φ+⟩21 and the two error channels act on disjoint pairs, the final density matrix factorizes:

\[ \rho_{\text{final}} = \Lambda_p^{(03)}\big(|\Phi^+\rangle\langle\Phi^+|_{03}\big)\ \otimes\ \Lambda_p^{(21)}\big(|\Phi^+\rangle\langle\Phi^+|_{21}\big). \]

Thus the total fidelity with the ideal product state factorizes:

\[ \]

where each factor is the fidelity of a single Bell pair after a two-qubit depolarizing channel.

Fidelity of a single Bell pair under two-qubit depolarizing noise
Let ρ = |Φ+⟩⟨Φ+|. Then

\[ \]

Since |Φ+⟩ is a stabilizer state with stabilizer generated by X⊗X and Z⊗Z, the only non-identity Paulis that map |Φ+⟩ to itself (up to a global phase) are X⊗X, Y⊗Y, Z⊗Z. For these three, PρP = ρ and the contribution to fidelity is 1. For the remaining 12, P|Φ+⟩ is orthogonal to |Φ+⟩ and contributes 0.
Hence

\[ \]

Total physical state fidelity
Because the two pairs are independent and identically affected,

\[ = \left(1 - \frac{4p}{5}\right)^2 \]

Mathematical Typesetting

All mathematics has been typeset above using LaTeX.

Conventions and Units

No physical units are involved; probabilities are dimensionless.

Final Answer: F_phys(p) = (1 - 4p/5)^2 = 1 - (8/5)p + (16/25)p^2.

User #3

The correct answer is: $ F_{\rm{physical}}=(1-\frac{12}{15}p)^2 $

Now let’s solve the next Checkpoint.

Checkpoint 2

Suppose that we wish to prepare a logical two-qubit GHZ state $(|00\rangle_{AB}+|11\rangle_{AB})/\sqrt{2}$ in the [[4,2,2]] code. To do so, we use the following state preparation circuit:

\[ (CNOT_{03}) (H_0) (CNOT_{21}) (H_2). \]

What is the logical state fidelity of the final 2-qubit logical state at the end of the circuit as a function of the two-qubit gate error rate $p$, assuming the state is post-selected on all detectable errors in the code?

Assistant #4

Step-by-Step Derivation

Ideal state prepared by the circuit
Start from |0000⟩.
Apply H2, then CNOT21: creates a Bell pair on (2,1), |Φ+⟩21 = (|00⟩21 + |11⟩21)/√2.
Apply H0, then CNOT03: creates a Bell pair on (0,3), |Φ+⟩03.
Thus the ideal 4-qubit state is

\[ |\psi_{\text{ideal}}\rangle = |\Phi^+\rangle_{03}\otimes|\Phi^+\rangle_{21}. \]

This is the encoded logical Bell state (|00⟩AB + |11⟩AB)/√2 for the [[4,2,2]] code with stabilizers XXXX and ZZZZ and logicals X_A = X I X I, X_B = X X I I, Z_A = Z Z I I, Z_B = Z I Z I.

Noise model and placement
After each CNOT, apply a 2-qubit depolarizing channel on that CNOT’s two qubits:

\[ \Lambda_p(\rho)=(1-p)\rho+\frac{p}{15}\sum_{P\neq I}P\rho P, \]

where the sum runs over the 15 non-identity two-qubit Paulis acting on that pair.

The two CNOTs act on disjoint pairs (2,1) and (0,3), so the two error channels are independent and confined to their pairs.
Post-selection on detectable errors
We condition on measuring the stabilizers XXXX and ZZZZ with +1 outcomes. Equivalently, we accept if the total error E commutes with both stabilizers (i.e., is in the centralizer of the stabilizer group).
Classifying pair errors by syndrome parities
For a two-qubit Pauli on a given pair, define a parity vector

\[ v(E)=(v_X(E),v_Z(E))\in\{0,1\}^2, \]

where v_X(E)=1 iff the total number of factors in {Z,Y} on that pair is odd (so E anticommutes with XXXX), and v_Z(E)=1 iff the total number of factors in {X,Y} is odd (so E anticommutes with ZZZZ).

Counting the 15 non-identity Paulis on a pair:
- v=(0,0): exactly 3 elements {XX, YY, ZZ}.
- v=(1,0): 4 elements {(I,Z), (Z,I), (X,Y), (Y,X)}.
- v=(0,1): 4 elements {(I,X), (X,I), (Y,Z), (Z,Y)}.
- v=(1,1): 4 elements {(I,Y), (Y,I), (X,Z), (Z,X)}.
Hence, for a single pair,

\[ \]

Acceptance condition
The total error is E = E21 E03 with v(E)=v(E21)⊕v(E03). We accept iff v(E)=0, i.e., v(E21)=v(E03).
Therefore the acceptance probability is

\[ = \alpha^2+3\beta^2 = \left(1-\frac{4p}{5}\right)^2 + 3\left(\frac{4p}{15}\right)^2 \]

Which accepted errors are logically harmless?
The target logical Bell state |Φ+⟩AB is stabilized by X_A X_B and Z_A Z_B, so any logical operator in the set

\[ \mathcal{G}=\{I,\ X_A X_B,\ Z_A Z_B,\ Y_A Y_B\} \]

maps |Φ+⟩AB to itself up to a global phase (unit fidelity).

Single-pair undetectable errors are exactly {XX, YY, ZZ} on that pair. Using the given logicals and stabilizers:
- X2X1 = X_A X_B, Z2Z1 = Z_A Z_B, Y2Y1 = ±Y_A Y_B.
- X0X3 = X_A X_B·(XXXX), Z0Z3 = Z_A Z_B·(ZZZZ), Y0Y3 = ±Y_A Y_B·(XXXX·ZZZZ). So any event with v(E21)=v(E03)=(0,0) yields E ∈ 𝒢 (harmless).
If v(E21)=v(E03) is any nonzero vector (1,0), (0,1), or (1,1), then the product E is a centralizer element not in 𝒢, and implements a single-qubit logical Pauli on A or B (e.g., X_A, Z_A, Y_A, X_B, Z_B, Y_B). Such an operator maps |Φ+⟩AB to an orthogonal Bell state, yielding zero fidelity contribution.
- Example: E21 = X2I1 and E03 = X0I3 (both have v=(0,1)). Then E = X0X2 = X_A, which takes |Φ+⟩AB → |Ψ+⟩AB (orthogonal).
Good accepted probability and logical fidelity
Good accepted events are exactly those with v(E21)=v(E03)=(0,0), whose probability is α^2.
Therefore, the logical state fidelity conditioned on acceptance is

\[ = \frac{\alpha^2}{\alpha^2+3\beta^2} \]

Small-p expansion (no first-order term):

\[ F_{\text{logical}}(p) = 1 - \frac{16}{75}p^2 + O(p^3). \]

Mathematical Typesetting

All key expressions have been provided in LaTeX above.

Conventions and Units

Probabilities are dimensionless. Post-selection assumes perfect stabilizer measurements.

Final Answer: F_logical(p) = (1 - 4p/5)^2 / [ (1 - 4p/5)^2 + 3 (4p/15)^2 ] = (1 - 8p/5 + 16p^2/25) / (1 - 8p/5 + 64p^2/75).

User #5

Populate your final answer into the code template provided below. This step is purely for formatting/display purposes. No additional reasoning or derivation should be performed. Do not import any modules or packages beyond what is provided in the template.

import sympy as sp

p = sp.symbols('p')

def answer(p):
    r"""
    Return the expression of logical state fidelity of the final 2-qubit logical state
    at the end of the circuit as a function of the two-qubit gate error rate $p$ in Sympy format.

    Inputs
    ----------
    p: sympy.Symbol, the two-qubit gate error rate, $p$

    Outputs
    ----------
    F_logical: sympy.Expr, the logical state fidelity as a function of $p$
    """

    # ------------------ FILL IN YOUR RESULTS BELOW ------------------
    F_logical = ...  # a SymPy expression of inputs
    # ---------------------------------------------------------------

    return F_logical

Assistant #6

import sympy as sp

p = sp.symbols('p')

def answer(p):
    r"""
    Return the expression of logical state fidelity of the final 2-qubit logical state
    at the end of the circuit as a function of the two-qubit gate error rate $p$ in Sympy format.

    Inputs
    ----------
    p: sympy.Symbol, the two-qubit gate error rate, $p$

    Outputs
    ----------
    F_logical: sympy.Expr, the logical state fidelity as a function of $p$
    """

    # ------------------ FILL IN YOUR RESULTS BELOW ------------------
    F_logical = (1 - sp.Rational(4, 5)*p)**2 / ((1 - sp.Rational(4, 5)*p)**2 + 3*(sp.Rational(4, 15)*p)**2)
    # ---------------------------------------------------------------

    return F_logical