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 with |0000⟩ on qubits (0,1,2,3).
Apply H2 then CNOT21: this prepares a Bell pair on qubits (2,1),

\[ |\Phi^+\rangle_{21}=\frac{|00\rangle_{21}+|11\rangle_{21}}{\sqrt{2}}, \]

so the state is |Φ+⟩21 ⊗ |00⟩03.

Apply H0 then CNOT03: this prepares a Bell pair on qubits (0,3),

\[ |\Phi^+\rangle_{03}=\frac{|00\rangle_{03}+|11\rangle_{03}}{\sqrt{2}}, \]

so the final ideal state is

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

Stabilizers of the ideal state
Each Bell state |Φ+⟩ab is stabilized by X_a X_b and Z_a Z_b. Hence

\[ \mathcal{S}=\langle X_2 X_1,\; Z_2 Z_1,\; X_0 X_3,\; Z_0 Z_3\rangle. \]

Noise model and factorization
After CNOT21, a two-qubit depolarizing channel acts on qubits (2,1): with probability 1−p apply I, and with probability p/15 apply each of the 15 non-identity two-qubit Paulis.
After CNOT03, an independent depolarizing channel acts on qubits (0,3).
The subsequent gates after the first noise (H0 and CNOT03) act only on qubits (0,3), disjoint from (2,1). Therefore the error inserted after CNOT21 does not propagate or change; the total error at the end is simply

\[ E_{\mathrm{tot}}=E_{21}\otimes E_{03}, \]

where E21 acts on (2,1) and E03 acts on (0,3), independently.

Fidelity for one Bell pair under a two-qubit depolarizing channel
For a stabilizer state |s⟩ and a Pauli channel, the state fidelity is the total probability that the applied Pauli lies in the stabilizer (up to a global phase):

\[ F_{\text{pair}}(p)=\sum_E \Pr(E)\,|\langle \Phi^+|E|\Phi^+\rangle|^2. \]

The stabilizer of |Φ+⟩ contains (ignoring global phases) {II, XX, YY, ZZ}. Thus among the 15 non-identity two-qubit Paulis, exactly 3 (XX, YY, ZZ) keep |Φ+⟩ invariant up to a phase.
Therefore,

\[ F_{\text{pair}}(p)=(1-p)+\frac{3}{15}p=1-\frac{4}{5}p. \]

Total 4-qubit fidelity
Since the two error channels act independently on the two disjoint Bell pairs, the total fidelity factorizes:

\[ F_{\text{phys}}(p)=\big(F_{\text{pair}}(p)\big)^2=\left(1-\frac{4}{5}p\right)^2. \]

Mathematical Typesetting

All equations are given above in LaTeX.

Conventions and Units

We used the stated two-qubit depolarizing channel: identity with probability 1−p, each non-identity two-qubit Pauli with probability p/15.
Single-qubit gates are noiseless.

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

User #3

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 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 $p$.

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

    Outputs
    ----------
    F_physical: sympy.Expr, the physical state fidelity of the final physical 4-qubit state
    """

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

    return F_physical

Assistant #4

import sympy as sp

p = sp.symbols('p')

def answer(p):
    r"""
    Return the expression of 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 $p$.

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

    Outputs
    ----------
    F_physical: sympy.Expr, the physical state fidelity of the final physical 4-qubit state
    """

    # ------------------ FILL IN YOUR RESULTS BELOW ------------------
    F_physical = (1 - sp.Rational(4, 5)*p)**2  # a SymPy expression of inputs
    # ---------------------------------------------------------------

    return F_physical