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, mathematical identities or numerical codes.
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. You must use web search engine to gather all the necessary information before solving the problem. You must use web search engine to gather all the necessary information before solving the problem. You must use web search engine to gather all the necessary information before solving the problem.

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

1) Step-by-step derivation

Circuit, noise model, and target state
The circuit is applied right-to-left: 1) H2 2) CNOT21 3) H0 4) CNOT03
Only the two CNOTs are followed by two-qubit depolarizing channels acting on the gate’s two qubits. The model states that with probability p an error occurs and, conditioned on an error, one of the 15 non-identity two-qubit Paulis is applied uniformly at random, i.e.,

\[ \mathcal{E}^{(2)}(\rho)=(1-p)\rho+\frac{p}{15}\sum_{P\in\mathcal{P}_2\setminus\{II\}}P\rho P, \]

where $\mathcal{P}_2=\{I,X,Y,Z\}^{\otimes 2}$. This is the “uniform Pauli” form of the 2-qubit depolarizing/Pauli channel. ([qiskit.github.io](https://qiskit.github.io/qiskit-aer/stubs/qiskit_aer.noise.depolarizing_error.html?utm_source=openai), [docs.quantum.ibm.com](https://docs.quantum.ibm.com/api/qiskit/0.42/qiskit_aer.noise.depolarizing_error?utm_source=openai), [qml-tutorial.github.io](https://qml-tutorial.github.io/chapter2/2/?utm_source=openai))

H followed by CNOT on |00⟩ prepares a Bell state; hence each pair (2,1) and (0,3) is prepared into $|\Phi^+\rangle=(|00\rangle+|11\rangle)/\sqrt{2}$. Therefore, the ideal final 4-qubit state is a product of two Bell pairs,

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

([en.wikipedia.org](https://en.wikipedia.org/wiki/Bell_state))

Factorization of the noisy state
The first error channel acts on qubits (2,1) immediately after CNOT21. No later gate touches qubits 1 or 2. The second error channel acts on (0,3) after CNOT03. Therefore, the final state factors as a tensor product of two two-qubit states:

\[ \rho_{\text{final}}=\rho_{21}\otimes\rho_{03}, \]

with

\[ \]

Bell-pair survival under a 2-qubit depolarizing/Pauli channel
For a single pair (say, 21), the fidelity with the ideal Bell state is

\[ =(1-p)\underbrace{\langle\Phi^+|\Phi^+\rangle}_{=1} \]

Using the stabilizer properties of $|\Phi^+\rangle$,

\[ (Z\!\otimes\!Z)|\Phi^+\rangle=|\Phi^+\rangle,\quad \]

so $|\langle\Phi^+|P|\Phi^+\rangle|^2=1$ for $P\in\{XX,YY,ZZ\}$ and 0 for the remaining 12 non-identity two-qubit Paulis (they map $|\Phi^+\rangle$ to some orthogonal Bell state, e.g., $ZI|\Phi^+\rangle=|\Phi^-\rangle$). Hence,

\[ =1-\frac{12}{15}\,p \]

(The three Paulis XX, YY, ZZ leave the pure density matrix unchanged, since $P|\Phi^+\rangle\langle\Phi^+|P=|\Phi^+\rangle\langle\Phi^+|$ even when the eigenvalue is −1.)

Four-qubit physical-state fidelity
The target is a product state $|\Psi_{\mathrm{ideal}}\rangle=|\Phi^+\rangle_{21}\otimes|\Phi^+\rangle_{03}$ and the noisy state factors: $\rho_{\text{final}}=\rho_{21}\otimes\rho_{03}$. Using $\mathrm{Tr}[(A\otimes B)(C\otimes D)]=\mathrm{Tr}[AC]\mathrm{Tr}[BD]$,

\[ =\langle\Phi^+|\rho_{21}|\Phi^+\rangle\;\langle\Phi^+|\rho_{03}|\Phi^+\rangle =\left(F_{\text{pair}}(p)\right)^2 \]

2) Mathematical details used - Depolarizing/Pauli channel acting uniformly on non-identity n-qubit Paulis (here n=2) and the standard H–CNOT Bell-state preparation. (qiskit.github.io, docs.quantum.ibm.com, en.wikipedia.org)

3) Conventions and units - Qubits labeled 0,1,2,3 as in the prompt. Gates are ideal except for the specified post-CNOT two-qubit depolarizing channels of error probability p. Fidelity is the state overlap with the ideal pure state, $F=\langle\psi|\rho|\psi\rangle$.

Final Answer:

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

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