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arXiv:1109.1714

Encoding an Arbitrary State in a [7,1,3] Quantum Error Correction Code

Sidney Buchbinder, Channing L. Huang, Yaakov S. Weinstein

Published Sep 8, 2011• 0 citations
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Abstract

We calculate the fidelity with which an arbitrary state can be encoded into a [7,1,3] CSS quantum error correction code in a non-equiprobable Pauli operator error environment with the goal of determining whether this encoding can be used for practical implementations of quantum computation. This determination is accomplished by applying ideal error correction to the encoded state which demonstrates the correctability of errors that occurred during the encoding process. We then apply single-qubit Clifford gates to the encoded state and determine the accuracy with which these gates can be applied. Finally, fault tolerant noisy error correction is applied to the encoded states in the non-equiprobable Pauli operator error environment allowing us to compare noisy (realistic) and perfect error correction implementations. We note that this maintains the fidelity of the encoded state for certain error-probability values. These results have implications for when non-fault tolerant procedures may be used in practical quantum computation and whether quantum error correction should be applied at every step in a quantum protocol.

DOI
10.48550/arxiv.1109.1714

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•May 13, 2013•0 citations
We calculate the fidelity with which an arbitrary state can be encoded into a [7,1,3] CSS quantum error correction code in a non-equiprobable Pauli operator error environment with the goal of determining whether this encoding can be used for practical implementations of quantum computation. This determination is accomplished by applying ideal error correction to the encoded state which demonstrates the correctability of errors that occurred during the encoding process. We then apply single-qubit Clifford gates to the encoded state and determine the accuracy with which these gates can be applied. Finally, fault tolerant noisy error correction is applied to the encoded states in the non-equiprobable Pauli operator error environment allowing us to compare noisy (realistic) and perfect error correction implementations. We note that this maintains the fidelity of the encoded state for certain error-probability values. These results have implications for when non-fault tolerant procedures may be used in practical quantum computation and whether quantum error correction should be applied at every step in a quantum protocol.
In this work we compare the accuracy of two methods used to construct a logical zero state appropriate for the [7, 1, 3] CSS quantum error correction code in a non-equiprobable Pauli operator error environment. The first method is to apply error correction, via syndrome measurement, on seven physical qubits all in the state zero. To do the syndrome measurements in a fault-tolerant fashion requires the construction of four qubit Shor states. These Shor states are also assumed to be constructed in a non-equiprobable Pauli operator error environment and it is these that are used to implement the syndrome measurement. The second construction method is to implement the [7, 1, 3] encoding gate sequence, also in the nonequiprobable Pauli operator error environment. The fidelity of the output states is calculated for each of these methods. With respect to the Shor state construction we find that the implementation of (noisy) parity based verifications does not necessarily raise the fidelity of the resulting Shor state. We also find that the second logical zero construction method outputs a seven qubit state with a respectfully higher fidelity than the first (fault tolerant) method. However, the fidelity of the single qubit of stored information has almost equivalent fidelity from the two construction methods.
I calculate the fidelity of a [7,1,3][7,1,3][7,1,3] Calderbank-Shor-Steane quantum error correction code logical zero state constructed in a nonequiprobable Pauli operator error environment for two methods of encoding. The first method is to apply fault-tolerant error correction to an arbitrary state of seven qubits utilizing Shor states for syndrome measurement. The Shor states are themselves constructed in the nonequiprobable Pauli operator error environment, and their fidelity depends on the number of verifications done to ensure multiple errors will not propagate into the encoded quantum information. Surprisingly, performing these verifications may lower the fidelity of the constructed Shor states. The second encoding method is to simply implement the [7,1,3][7,1,3][7,1,3] encoding gate sequence also in the nonequiprobable Pauli operator error environment. Perfect error correction is applied after both methods to determine the correctability of the implemented errors. I find that which method attains higher fidelity depends on which of the Pauli operators errors is dominant. Nevertheless, perfect error correction applied after the encoding suppresses errors to at least first order for both methods.
We apply single ‘logical’ qubit Clifford gates to states encoded in the [7,1,3] quantum error correction code in a nonequiprobable Pauli matrix error environment. The gates we analyze are the NOT, Phase, and Hadamard gates. We calculate accuracy measures of the applied gates by determining output state and logical gate fidelities. The latter is determined from the logical gate -matrix. In addition, we apply perfect and noisy quantum error correction to the output state of the Clifford gate. Perfect error correction allows the determination of whether errors that occurred during the gate are ‘correctable’. Noisy error correction gives an idea of what can be expected in a realistic quantum computation.
We explore basic necessary protocols to achieve fault tolerance for quantum computation in the cluster state and circuit models. For the cluster state model we simulate the implementation of an arbitrary rotation via only measurement on a decohered cluster state. Fidelity is used to quantify the accuracy of the initial cluster state and a gate fidelity for the arbitrary rotation is determined. In the circuit model we compare the accuracy of two methods that can be used to construct a logical zero state appropriate for the [7, 1, 3] Steane quantum error correction code in a non-equiprobable Pauli operator error environment: a fault tolerant method done by applying error correction on seven qubits all in the state zero, and a non-fault tolerant method done by implementing the encoding gate sequence. We find that the latter construction method, in spite of its lack of "fault tolerance," outputs a seven qubit state with a higher fidelity than the first (fault tolerant) method. However, the fidelity of the single qubit of stored information exhibits almost equivalent values between the two construction methods.
We simulate the implementation of a TTT gate, or $\frac{\ensuremath{\pi}}{8}$ gate, for a [7,1,3] encoded logical qubit in a nonequiprobable error environment. We demonstrate that the use of certain non-fault-tolerant methods in the implementation may nevertheless enable reliable quantum computation while reducing basic resource consumption. Reliability is determined by calculating gate fidelities for the one-qubit logical gate. Specifically, we show that despite using a non-fault-tolerant procedure in constructing a logical zero ancilla to implement the TTT gate, the gate fidelity of the logical gate, after perfect error correction, has no first order error terms. Meaning, any errors that may have occurred during implementation are ``correctable'' and fault tolerance may still be achieved.
Correcting errors is a vital but expensive component of fault tolerant quantum computation. Standard fault tolerant protocol assumes the implementation of error correction, via syndrome measurements and possible recovery operations, after every quantum gate. In fact, this is not necessary. Here we demonstrate that error correction should be applied more sparingly. We simulate encoded single qubit rotations within the [7,1,3] code and show via fidelity measures that applying error correction after every gate is not desirable. The simulations also shed light on what accuracy can be expected for noisy error correction and thus to what accuracy arbitrary single qubit rotations should be implemented.