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

To solve this problem, we will first describe the state preparation circuit and analyze how errors affect the logical state. We start by understanding the sequence of operations in the circuit.

Step-by-Step Derivation

1. Circuit Analysis

The given state preparation circuit in matrix multiplication order is:

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

which corresponds to the following sequence when executed in a quantum circuit from right to left:

Apply a Hadamard gate $H$ to qubit 2.
Apply a CNOT gate with control qubit 2 and target qubit 1.
Apply a Hadamard gate $H$ to qubit 0.
Apply a CNOT gate with control qubit 0 and target qubit 3.

2. State Initialization

Initially, the physical qubits are in the state $|0000\rangle$. Applying each operation sequentially:

Step 1: $H_2$ on $|0000\rangle \rightarrow \frac{1}{\sqrt{2}}(|0000\rangle + |0010\rangle)$
Step 2: $CNOT_{21}$ flips qubit 1 if qubit 2 is 1 $\rightarrow \frac{1}{\sqrt{2}}(|0000\rangle + |0011\rangle)$
Step 3: $H_0$ creates superpositions $\rightarrow \frac{1}{2} (|0000\rangle + |0001\rangle + |1000\rangle + |1001\rangle)$
Step 4: $CNOT_{03}$ flips qubit 3 if qubit 0 is 1 $\rightarrow \frac{1}{2} (|0000\rangle + |0001\rangle + |1001\rangle + |1000\rangle)$.

The final state ideally becomes $ \frac{1}{2} \big(|0000\rangle + |0001\rangle + |1001\rangle + |1000\rangle \big)$, which corresponds to the logical GHZ state $(|00\rangle_{AB} + |11\rangle_{AB})/\sqrt{2}$.

3. Introducing Errors

Considering the depolarizing errors, each two-qubit gate has a chance $p$ to introduce an error, which transforms the gate operation to any of the 15 non-identity Pauli operations.

Error Model: After each CNOT, the joint state $\rho$ is depolarized:

\[ \rho \to (1-p)\rho + \frac{p}{15} \sum_{P \in \{II, IX, \ldots, ZZ\}} P\rho P, \]

where the sum is over the 15 non-identity Pauli operators acting on two qubits.

4. Calculating the Fidelity

To find the fidelity, we calculate how the final error channel affects the ideal state:

For one $CNOT$, the fidelity $F$ of the state after a depolarizing error is:

\[ F = 1 - \frac{16}{15}p \quad (1 - \text{total error probability}) \]

With two CNOT gates, assuming independence:

\[ F = (1 - \frac{16}{15}p)^2 \]

This results from two independent depolarizing errors acting on adjacent pairs. Since each CNOT can introduce errors independent of others, the accumulated effect with probabilities composes multiplicatively.

Final Answer: The physical state fidelity $F$ of the final 4-qubit state as a function of the two-qubit gate error rate $p$ is approximately: [ F = \left(1 - \frac{16}{15}p\right)^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

To determine the logical state fidelity of the prepared GHZ state conditioned on the detection and post-selection of all discernible errors, we need to take into account the protective structure of the [[4,2,2]] quantum code. This type of code can detect single-qubit errors, and with appropriate post-selection, erroneous states are discarded, ideally increasing logical state fidelity.

Step-by-Step Derivation

1. Circuit Analysis and Post-selection

We’ve already established the circuit operations in the order:

$H_2$
$CNOT_{21}$
$H_0$
$CNOT_{03}$

The initial state of the 4 physical qubits is $|0000\rangle$, evolving to an ideal GHZ state: $ \frac{1}{2} (|0000\rangle + |0001\rangle + |1000\rangle + |1001\rangle).$

Upon considering errors, after the CNOTs (the only gates susceptible to depolarizing errors), each may independently introduce any of the 15 possible Pauli pair errors.

2. Logical Error Robustness with Post-selection

Error Model and Detection: - Each CNOT is followed by an error channel, $\mathcal{E}$, that picks a two-qubit Pauli error with probability $p/15$. - The [[4,2,2]] code allows for post-selection that detects single-qubit errors (via parity check operators).
Post-selection Process: - Measure stabilizers $XXXX$ and $ZZZZ$. - Errors detectable: single-qubit errors, those flipping parity in stabilizer checks. - Two-qubit error can escape detection if error changes both qubits but remains a detectable single-bit logical error equivalence (like $XZ$).
Logical State Fidelity Calculation: - The majority of depolarizing errors disturb the stabilizer checks, making them detectable. - Errors only lead to the loss of computational paths for logical qubit entanglement.

Fidelity, therefore, accounts for: - Undetectable Error Paths: These are errors that do not trigger stabilizers (i.e., full stabilizer-flip pairs/errors are undetectable if act on both qubits). - Error-free Paths: Remaining pathways (error-free or logically equivalent detected to state).

Considering undetectable errors happen with $\frac{3}{15}p$ probability per CNOT (as no Pauli error acts trivially across both qubits without affecting logical states appreciably when detected), the effective error-free preparations as a ratio of $1-p$ become:

\[ F_{\text{logical}} = (1 - \frac{3}{15}p)^2, \]

due to $3$ undetectable nontrivial Pauli pairs that preserve all stabilizers but affect individual specifics/trivial pairs that negate mispair parity.

This fidelity is conditional upon successful error detection and discarding of unstable states which do not align/deparity-stabilize.

Final Answer: The logical fidelity of the final 2-qubit logical state at the end of the circuit, assuming an effective post-selection on detectable errors, is: [ F_{\text{logical}} = \left(1 - \frac{3}{15}p\right)^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 - 3/15 * p)**2
    # ---------------------------------------------------------------

    return F_logical