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: produces (|0⟩+|1⟩)/√2 on qubit 2.
Apply CNOT21: entangles qubits 2 (control) and 1 (target) into a Bell pair

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

Apply H0 and then CNOT03: analogously prepares a Bell pair on qubits 0 and 3,

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

Hence the ideal final 4-qubit state is

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

Noise model
After each CNOTij, a two-qubit depolarizing channel acts on the same two qubits:

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

where $\mathcal{P}_2=\{I,X,Y,Z\}^{\otimes 2}$ and the sum runs over the 15 non-identity two-qubit Paulis.

There are two such channels: one on qubits (2,1) after CNOT21 and one on (0,3) after CNOT03. No subsequent gates touch those qubits, so the two channels act independently on disjoint subsystems. The total channel is $\mathcal{E}_p^{(03)}\otimes \mathcal{E}_p^{(21)}$.
Fidelity for a single Bell pair under two-qubit depolarizing noise
For a pure target $|\phi\rangle$, the output fidelity is

\[ F=\langle\phi|\mathcal{E}_p(|\phi\rangle\langle\phi|)|\phi\rangle=(1-p)+\frac{p}{15}\sum_{P\neq I}\left|\langle\phi|P|\phi\rangle\right|^2. \]

For $|\phi\rangle=|\Phi^+\rangle$, the only two-qubit Paulis with nonzero expectation are $I, XX, YY, ZZ$, with

\[ \langle\Phi^+|XX|\Phi^+\rangle=+1,\quad \langle\Phi^+|ZZ|\Phi^+\rangle=+1,\quad \langle\Phi^+|YY|\Phi^+\rangle=-1. \]

Hence for the 15 non-identity Paulis, exactly 3 contribute $|\langle\cdot\rangle|^2=1$ and the remaining 12 contribute 0. Therefore,

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

Total 4-qubit fidelity
Since the ideal state factorizes as $|\Psi\rangle=|\Phi^+\rangle_{03}\otimes|\Phi^+\rangle_{21}$ and the noise channels act independently on the two disjoint pairs, the total fidelity factorizes:

\[ F_{\text{phys}}(p)=F_{03}(p)\,F_{21}(p)=\left(F_{\text{pair}}(p)\right)^2=\left(1-\frac{4p}{5}\right)^2. \]

Mathematical Typesetting

All equations are provided above in LaTeX.

Conventions and Units

No special units; standard quantum information conventions for Pauli operators and fidelities are used.

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

Setup and noise model
The circuit prepares the ideal state

\[ |\Psi_{\text{ideal}}\rangle = |\Phi^+\rangle_{03}\otimes|\Phi^+\rangle_{21},\quad |\Phi^+\rangle=\frac{|00\rangle+|11\rangle}{\sqrt{2}}. \]

After each CNOT, a two-qubit depolarizing channel acts on that pair:

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

There are two such channels, on pairs (0,3) and (2,1), acting independently.
Post-selection on the code’s stabilizers
The [[4,2,2]] code has stabilizers $g_X=XXXX$ and $g_Z=ZZZZ$. We post-select on the +1 eigenspace of both stabilizers (detectable errors are discarded).
For a two-qubit Pauli error $E$ acting on a given pair, define the commutation parities with $g_X$ and $g_Z$:

\[ v(E)=(s_X(E),s_Z(E))\in\{(0,0),(0,1),(1,0),(1,1)\}, \]

where $s_X(E)=1$ if $E$ anticommutes with $g_X$ and 0 otherwise, and similarly for $s_Z$.

For a single pair, among the 16 two-qubit Paulis (including identity):
- Class $v=(0,0)$ has 4 elements: $\{II,XX,YY,ZZ\}$.
- Each of the other three classes has 4 elements: e.g., $v=(0,1)$ is $\{IX,XI,YZ,ZY\}$, etc.
The probability per pair to land in each class is

\[ \Pr[v=(0,0)]=(1-p)+3\frac{p}{15}=1-\frac{12}{15}p=1-\frac{4p}{5}, \]

\[ \Pr[v=(0,1)]=\Pr[v=(1,0)]=\Pr[v=(1,1)]=4\frac{p}{15}. \]

Acceptance condition for the two-gate error pattern
Let $v_{03}$ and $v_{21}$ be the class labels for the errors on pairs (0,3) and (2,1), respectively. The total error $E=E_{03}E_{21}$ commutes with both stabilizers (is accepted) iff

\[ v_{03}=v_{21}. \]

Therefore the total acceptance probability is

\[ P_{\text{acc}}=\sum_{v}\Pr[v]^2=\left(1-\frac{4p}{5}\right)^2+3\left(\frac{4p}{15}\right)^2. \]

Logical action of accepted errors
For $v=(0,0)$ on a pair, the allowed errors are $II,XX,YY,ZZ$. Using the given logical representatives

\[ X_A=X_0X_2,\quad X_B=X_0X_1,\quad Z_A=Z_0Z_1,\quad Z_B=Z_0Z_2, \]

and stabilizers $XXXX, ZZZZ$, one finds on either pair that

\[ XX \mapsto X_AX_B,\quad ZZ \mapsto Z_AZ_B,\quad YY \mapsto Y_AY_B. \]

Thus when $v_{03}=v_{21}=(0,0)$, the total logical operator is in

\[ H=\{I,\;X_AX_B,\;Z_AZ_B,\;Y_AY_B\}, \]

which is exactly the stabilizer set of the logical Bell state $|\Phi^+\rangle_{AB}$. Hence all such accepted events have logical fidelity 1.

For $v\neq(0,0)$, each pair error is detectable by itself, but if $v_{03}=v_{21}$ the combined error is accepted. Direct calculation (examples below) shows these map to single-logical-qubit Paulis:
- For $v=(0,1)$: products yield $X_A$ or $X_B$ (e.g., $(I_0X_3)(I_2X_1)=X_1X_3\equiv X_A$).
- For $v=(1,0)$: products yield $Z_A$ or $Z_B$.
- For $v=(1,1)$: products yield $Y_A$ or $Y_B$. None of these lie in $H$, so they map $|\Phi^+\rangle_{AB}$ to an orthogonal logical Bell state, contributing zero to the target-state fidelity.
Conditional logical fidelity
The only accepted events with nonzero fidelity are those with $v_{03}=v_{21}=(0,0)$. Therefore

\[ P_{\text{good}}=\Pr[v=(0,0)]^2=\left(1-\frac{4p}{5}\right)^2. \]

Conditioned on acceptance,

\[ \]

Small-$p$ expansion: $F_{\text{logical}}(p)=1-\frac{16}{75}p^2+O(p^3)$, showing first-order errors are fully post-selected away, as expected for a distance-2 detection code.

Mathematical Typesetting

All expressions above are typeset in LaTeX.

Conventions and Units

Standard stabilizer-code and depolarizing-channel conventions; no physical units involved.

Final Answer: $F_{\rm logical}(p)=\dfrac{\left(1-\frac{12}{15}p\right)^2}{\left(1-\frac{12}{15}p\right)^2+3\left(\frac{4}{15}p\right)^2} =\dfrac{\left(1-\frac{4p}{5}\right)^2}{\left(1-\frac{4p}{5}\right)^2+\frac{16}{75}p^2}.$

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(12, 15)*p)**2) / ((1 - sp.Rational(12, 15)*p)**2 + 3*(sp.Rational(4, 15)*p)**2)
    # ---------------------------------------------------------------

    return F_logical