# Dual-rail quantum code[1]

## Description

Two-mode bosonic code encoding a logical qubit in Fock states with one excitation. The logical-zero state is represented by \(|10\rangle\), while the logical-one state is represented by \(|01\rangle\). This encoding is often realized in temporal or spatial modes, corresponding to a time-bin or frequency-bin encoding. Two different types of photon polarization can also be used.

## Protection

This is an error-detecting code against one photon loss event; it is often used in photonic quantum devices because of its ease of realization. A single loss event can be detected because, after the loss occurs, the output state \(|00\rangle\) is orthogonal to the codespace. Recovery is not possible, so a successful run of a quantum circuit is conditioned on not losing a photon during the circuit.

For Deutsch''s problem specifically, this code protects against errors resulting in states that have the correct photon number, but in the wrong modes [1].

## Gates

General gates are performed using beamsplitters [2] and Kerr nonlinearities. Universal quantum computing can be achieved using the KLM protocol [3] with only linear optical elements and photon detectors.A probabilistic CZ gate via a non-linear sign-shift gate, which transforms the Fock states \( \alpha|0\rangle+\beta|1\rangle+\gamma|2\rangle\) into \(\alpha|0\rangle+\beta|1\rangle-\gamma|2\rangle \), followed by measurement [4].Error-detecting \(CCZ\) and \(cSWAP\) gates using three-level ancilla [5].

## Fault Tolerance

Dual-rail qubits can be used to convert leakage and amplitude damping noise into erasure noise [6].

## Threshold

Between \(1.78\%\) and \(11.5\%\) with faulty photon detectors when repeatedly concatenating with the Steane code [7].

## Realizations

The dual-rail code is ubiquitous in linear-optical quantum devices and is behind the KLM protocol, one of the first proposals for fault-tolerant computation. See reviews [8,9] for more details.Superconducting circuit devices: Gates have been demonstrated in Rob Schoelkopf's group at Yale University [2]. Error detection has been demonstrated in 3D cavities in Michel Devoret's group at Yale University [10] and Amazon Web Services [11] using transmon qubits, following earlier theoretical proposals [6,12]. Logical readout in 3D cavities has been demonstrated by Quantum Circuits Inc. [13].

## Notes

For Deutsch's problem [14] with optical qubits, error correction using photon number detection reduces the error probability from \(\frac{1}{4} (1+e^{-\gamma}-2e^{-3\gamma/2}) \) to \(\frac{1}{2} (1- \text{sech} \gamma/2)\).

## Parent

## Cousins

- Concatenated quantum code — The KLM protocol, one of the first protocols for fault-tolerant quantum computation, utilizes concatenations of the dual-rail with a stabilizer code [3]. Concatenating the dual-rail code with an \([[n,k,d]]\) stabilizer code yields an \([[2n,k,d]]\) constant-excitation code [15].
- Constant-excitation (CE) code — An \(((8,1))\) constant-excitation code correcting a single amplitude damping error can be obtained from concatenating the \(\{|\overline{01}\rangle,|\overline{11}\rangle\}\) (constant-excitation) subcode with the dual-rail code. More generally, concatenating the dual-rail code with an \([[n,k,d]]\) stabilizer code yields an \([[2n,k,d]]\) constant-excitation code [15].
- \([[4,2,2]]\) CSS code — An \(((8,1))\) constant-excitation code correcting a single amplitude damping error can be obtained from concatenating the \(\{|\overline{01}\rangle,|\overline{11}\rangle\}\) [16] subcode with the dual-rail code [15].
- Cluster-state code — MBQC can be achieved with dual-rail codes using linear optical elements and photon detectors [17].
- Fusion-based quantum computing (FBQC) code — FBQC resource states are concatenated with dual-rail codes to increase loss detection.

## References

- [1]
- I. L. Chuang and Y. Yamamoto, “Simple quantum computer”, Physical Review A 52, 3489 (1995) DOI
- [2]
- Y. Lu et al., “High-fidelity parametric beamsplitting with a parity-protected converter”, Nature Communications 14, (2023) DOI
- [3]
- E. Knill, R. Laflamme, and G. J. Milburn, “A scheme for efficient quantum computation with linear optics”, Nature 409, 46 (2001) DOI
- [4]
- E. Knill, “Bounds on the probability of success of postselected nonlinear sign shifts implemented with linear optics”, Physical Review A 68, (2003) arXiv:quant-ph/0307015 DOI
- [5]
- T. Tsunoda et al., “Error-detectable bosonic entangling gates with a noisy ancilla”, (2022) arXiv:2212.11196
- [6]
- A. Kubica et al., “Erasure qubits: Overcoming the \(T_1\) limit in superconducting circuits”, (2022) arXiv:2208.05461
- [7]
- M. Silva, M. Rötteler, and C. Zalka, “Thresholds for linear optics quantum computing with photon loss at the detectors”, Physical Review A 72, (2005) arXiv:quant-ph/0502101 DOI
- [8]
- C. R. Myers and R. Laflamme, “Linear Optics Quantum Computation: an Overview”, (2005) arXiv:quant-ph/0512104
- [9]
- P. Kok et al., “Linear optical quantum computing with photonic qubits”, Reviews of Modern Physics 79, 135 (2007) arXiv:quant-ph/0512071 DOI
- [10]
- A. Koottandavida et al., “Erasure detection of a dual-rail qubit encoded in a double-post superconducting cavity”, (2023) arXiv:2311.04423
- [11]
- H. Levine et al., “Demonstrating a Long-Coherence Dual-Rail Erasure Qubit Using Tunable Transmons”, Physical Review X 14, (2024) arXiv:2307.08737 DOI
- [12]
- J. D. Teoh et al., “Dual-rail encoding with superconducting cavities”, Proceedings of the National Academy of Sciences 120, (2023) arXiv:2212.12077 DOI
- [13]
- K. S. Chou et al., “Demonstrating a superconducting dual-rail cavity qubit with erasure-detected logical measurements”, (2023) arXiv:2307.03169
- [14]
- “Rapid solution of problems by quantum computation”, Proceedings of the Royal Society of London. Series A: Mathematical and Physical Sciences 439, 553 (1992) DOI
- [15]
- Y. Ouyang, “Avoiding coherent errors with rotated concatenated stabilizer codes”, npj Quantum Information 7, (2021) arXiv:2010.00538 DOI
- [16]
- G. Alber et al., “Stabilizing Distinguishable Qubits against Spontaneous Decay by Detected-Jump Correcting Quantum Codes”, Physical Review Letters 86, 4402 (2001) arXiv:quant-ph/0103042 DOI
- [17]
- D. E. Browne and T. Rudolph, “Resource-Efficient Linear Optical Quantum Computation”, Physical Review Letters 95, (2005) arXiv:quant-ph/0405157 DOI

## Page edit log

- Yinchen Liu (2024-03-15) — most recent
- Esha Swaroop (2024-03-14)
- Victor V. Albert (2021-12-18)
- Dhruv Devulapalli (2021-12-17)
- Aniket Maiti (2024-02-08)

## Cite as:

“Dual-rail quantum code”, The Error Correction Zoo (V. V. Albert & P. Faist, eds.), 2024. https://errorcorrectionzoo.org/c/dual_rail