# Gottesman-Kitaev-Preskill (GKP) code[1]

## Description

Bosonic qudit-into-oscillator code whose stabilizers are oscillator displacement operators \(\hat{S}_q(2\alpha)=e^{-2i\alpha \hat{p}}\) and \(\hat{S}_p(2\beta)=e^{2i\beta \hat{x}}\). To ensure \(\hat{S}_q(2\alpha)\) and \(\hat{S}_p(2\beta)\) generate a stabilizer group that is Abelian, there is another constraint that \(\alpha\beta=2k\pi\) where \(k\) is an integer. Codewords can be expressed as equal weight superpositions of coherent states on an infinite lattice, such as a square lattice in phase space with spatial period \(2\sqrt{\pi}\). The exact GKP state is non-normalizable, so approximate constructs have to be considered.

## Protection

For stabilizer \(\hat{S}_q(2\alpha),\hat{S}_p(2\beta)\), code can correct displacement errors up to \(\frac{\alpha}{2}\) in the \(q\)-direction and \(\frac{\beta}{2}\) at \(p\)-direction. Approximately protects against photon loss errors [2], outperforming most other codes designed to explicitly protect against loss [3]. Very sensitive to dephasing errors [4]. A biased-noise GKP error correcting code can be prepared by choosing \(\alpha\neq \beta\).

## Encoding

Preparation of approximate GKP states is studied both theoretically and experimentally on circuit-QED platforms, by putting the GKP lattice inside a Gaussian envelope [2][5][6].Dissipative stabilization of finite-energy GKP states using stabilizers conjugated by a damping operator, i.e., a damped exponential of the total occupation number [7][8].

## Gates

By applying GKP error correction to Gaussian input states, universality can be achieved without non-Gaussian elements [9].

## Decoding

Syndrome measruement of displacement error can be done by applying a controlled-displacement controlled by an ancilla qubit. The syndrome information can be obtained by measuring the ancilla qubit after controlled-displacement opearation. See Section. 2D in [4].Pauli \(X\),\(Y\) and \(Z\) measurements can be performed by measuring \(-\hat{p},\hat{x}-\hat{p}\) and \(\hat{x}\) repectively. If the measurement outcome is closed to an even multiple of \(\sqrt{\pi}\), then the outcome is +1. If the measurement outcome is closed to an odd multiple of \(\sqrt{\pi}\), then the outcome is -1. See Section. 2D in [4].

## Fault Tolerance

Clifford gates can be realized by performing linear-optical operations, sympletic transformations and displacements, all of which are Gaussian operations. Pauli gates can be performed using displacement operators. Clifford gates are fault tolerant in the sense that they map bounded-size errors to bounded-size errors [1].Error correction scheme is fault-tolerant to displacement noise as long as all input states have displacement errors less than \(\sqrt{\pi}/6\) [10].

## Realizations

GKP encoding realized in the motional degree of freedom of a trapped ion [11], followed by realization of dissipative stabilization scheme [8].A reduced form of GKP error correction, consisting of measuring only the direction of a displacement error with an ancillary transmon, realized in a microwave cavity coupled to superconducting circuits [12].

## Notes

GKP codes were obtained after iterative numerical optimization of encoding and recovery against photon loss, starting with Haar-random states [13].Pedagogical introduction into GKP codes presented in Ref. [4].

## Parent

## Cousins

- Approximate quantum error-correcting code (AQECC) — GKP codes approximately protect against photon loss [2][3][13].
- Bosonic rotation code — GKP codes utilize the translational symmetry in phase space, while rotation codes utilize rotational symmetry.
- Rotor GKP code — GKP (rotor GKP) codes protect against shifts in linear (angular) degrees of freedom.

## Zoo code information

## References

- [1]
- D. Gottesman, A. Kitaev, and J. Preskill, “Encoding a qubit in an oscillator”, Physical Review A 64, (2001). DOI; quant-ph/0008040
- [2]
- B. M. Terhal and D. Weigand, “Encoding a qubit into a cavity mode in circuit QED using phase estimation”, Physical Review A 93, (2016). DOI; 1506.05033
- [3]
- V. V. Albert et al., “Performance and structure of single-mode bosonic codes”, Physical Review A 97, (2018). DOI; 1708.05010
- [4]
- A. L. Grimsmo and S. Puri, “Quantum Error Correction with the Gottesman-Kitaev-Preskill Code”, PRX Quantum 2, (2021). DOI; 2106.12989
- [5]
- D. J. Weigand and B. M. Terhal, “Generating grid states from Schrödinger-cat states without postselection”, Physical Review A 97, (2018). DOI
- [6]
- P. Campagne-Ibarcq et al., “Quantum error correction of a qubit encoded in grid states of an oscillator”, Nature 584, 368 (2020). DOI
- [7]
- B. Royer, S. Singh, and S. M. Girvin, “Stabilization of Finite-Energy Gottesman-Kitaev-Preskill States”, Physical Review Letters 125, (2020). DOI; 2009.07941
- [8]
- Brennan de Neeve et al., “Error correction of a logical grid state qubit by dissipative pumping”. 2010.09681
- [9]
- B. Q. Baragiola et al., “All-Gaussian Universality and Fault Tolerance with the Gottesman-Kitaev-Preskill Code”, Physical Review Letters 123, (2019). DOI; 1903.00012
- [10]
- S. Glancy and E. Knill, “Error analysis for encoding a qubit in an oscillator”, Physical Review A 73, (2006). DOI; quant-ph/0510107
- [11]
- C. Flühmann et al., “Encoding a qubit in a trapped-ion mechanical oscillator”, Nature 566, 513 (2019). DOI; 1807.01033
- [12]
- P. Campagne-Ibarcq et al., “Quantum error correction of a qubit encoded in grid states of an oscillator”. 1907.12487
- [13]
- K. Noh, V. V. Albert, and L. Jiang, “Quantum Capacity Bounds of Gaussian Thermal Loss Channels and Achievable Rates With Gottesman-Kitaev-Preskill Codes”, IEEE Transactions on Information Theory 65, 2563 (2019). DOI; 1801.07271

## Cite as:

“Gottesman-Kitaev-Preskill (GKP) code”, The Error Correction Zoo (V. V. Albert & P. Faist, eds.), 2022. https://errorcorrectionzoo.org/c/gkp

Github: https://github.com/errorcorrectionzoo/eczoo_data/tree/main/codes/quantum/oscillators/gkp.yml.