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arXiv:2509.18204

Generalized Gottesman-Kitaev-Preskill States on a Quantum Torus

Sijo K. Joseph, Sudhir Kumar Singh

Published Sep 20, 2025• 0 citations
arXivHTMLPDF

Abstract

We introduce a novel formulation of a Generalized Gottesman-Kitaev-Preskill (GKP) state that resolves all of its foundational pathologies, such as infinite energy, non-normalizability, and orthogonality. We demonstrate that these issues are artifacts of defining the code on an unbounded phase-space. By considering the compact quantum-phase-space intrinsic to systems like coupled quantum harmonic oscillators, we have obtained a Generalized GKP (GGKP) state that is both exact and physically realizable. This is achieved by applying an obtained Quantum Zak Transform (QZT) to Squeezed Coherent States, which reveals that Riemann-Theta functions are the natural representation of these states on the quantum torus phase-space. This framework not only provides a well-behaved quantum state, but also reveals a deep connection between quantum error correction, non-commutative geometry, and the theory of generalized Theta functions. This opens a new avenue for fault-tolerant photonic quantum computing.

DOI
10.48550/arxiv.2509.18204

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We introduce a novel formulation of a Generalized Gottesman-Kitaev-Preskill (GKP) state that resolves all of its foundational pathologies, such as infinite energy, non-normalizability, and orthogonality. We demonstrate that these issues are artifacts of defining the code on an unbounded phase-space. By considering the compact quantum-phase-space intrinsic to systems like coupled quantum harmonic oscillators, we have obtained a Generalized GKP (GGKP) state that is both exact and physically realizable. This is achieved by applying an obtained Quantum Zak Transform (QZT) to Squeezed Coherent States, which reveals that Riemann-Theta functions are the natural representation of these states on the quantum torus phase-space. This framework not only provides a well-behaved quantum state, but also reveals a deep connection between quantum error correction, non-commutative geometry, and the theory of generalized Theta functions. This opens a new avenue for fault-tolerant photonic quantum computing.
The Gottesman-Kitaev-Preskill (gkp) quantum error-correcting code attracts much attention in continuous variable (CV) quantum computation and CV quantum communication due to the simplicity of error-correcting routines and the high tolerance against Gaussian errors. Since the gkp code state should be regarded as a limit of physically meaningful approximate ones, various approximations have been developed until today, but explicit relations among them are still unclear. In this paper, we rigorously prove the equivalence of these approximate gkp codes with an explicit correspondence of the parameters. We also propose a standard form of the approximate code states in the position representation, which enables us to derive closed-form expressions for the Wigner function, inner products, and the average photon number in terms of the theta functions. Our results serve as fundamental tools for further analyses of fault-tolerant quantum computation and channel coding using approximate gkp codes.
The Gottesman-Kitaev-Preskill (GKP) encoding of a qubit into a bosonic mode is a promising bosonic code for quantum computation due to its tolerance for noise and all-Gaussian gate set. We present a toolkit for phase-space description and manipulation of GKP encodings that includes Wigner functions for ideal and approximate GKP states, for various types of mixed GKP states, and for GKP-encoded operators. One advantage of a phase-space approach is that Gaussian unitaries, required for computation with GKP codes, correspond to simple transformations on the arguments of Wigner functions. We use this fact and our toolkit to describe GKP error correction, including magic-state preparation, entirely in phase space using operations on Wigner functions. While our focus here is on the square-lattice GKP code, we provide a general framework for GKP codes defined on any lattice.
The Gottesman-Kitaev-Preskill (GKP) encoding of a qubit within an oscillator is particularly appealing for fault-tolerant quantum computing with bosons because Gaussian operations on encoded Pauli eigenstates enable Clifford quantum computing with error correction. We show that applying GKP error correction to Gaussian input states, such as vacuum, produces distillable magic states, achieving universality without additional non-Gaussian elements. Fault tolerance is possible with sufficient squeezing and low enough external noise. Thus, Gaussian operations are sufficient for fault-tolerant, universal quantum computing given a supply of GKP-encoded Pauli eigenstates.
Physical Gottesman-Kitaev-Preskill (GKP) states are inherently noisy as ideal ones would require infinite energy. While this is typically considered as a deficiency to be actively corrected, this work demonstrates that imperfect GKP stabilizer states can be leveraged in order to apply non-Clifford gates using only linear optical elements. In particular, Gaussian operations on normalizable GKP states, combined with homodyne measurements, permit two key primitives: clean projection onto Pauli eigenstates in the normalizable GKP codespace, thereby implementing Clifford gates with high fidelity; and probabilistic projection of unmeasured modes onto non-Pauli eigenstates. These results demonstrate that normalizable GKP stabilizer states combined with Gaussian operations provide a practical framework for computational universality within the measurement-based model of quantum computation in a realistic continuous-variable setting.
Covariant integral quantization is developed on the double discrete cylinder phase space, with the torus as configuration space, and yields the Wigner distribution and the stellar representation. This phase space is particularly well suited to translationally invariant states, such as the Gottesman Kitaev Preskill (GKP) states. These states are central to fault-tolerant quantum computing architectures, yet their quantum resources are still under investigation. The stellar representations of ideal GKP logical states, which behave as phase eigenstates of the double discrete cylinder, have no zeros on this phase space, whereas those of their superpositions have infinitely many. The vacuum, although Gaussian, has zeros on this phase space, and the zeros of finitely squeezed GKP states move with the widths of the peaks and of the envelope, and reach this phase space for fine-tuned widths. These features echo the role of the vacuum as the resource that makes GKP-based architectures hard to simulate.
The Gottesman-Kitaev-Preskill (GKP) code encodes a qubit into a bosonic mode using periodic wave functions. This periodicity makes the GKP code a natural setting for the Zak transform, which is tailor-made to provide a simple description for periodic functions. We review the Zak transform and its connection to a Zak basis of states in Hilbert space, decompose the shift operators that underpin the stabilizers and the correctable errors, and find that Zak transforms of the position wave function appear naturally in GKP error correction. We construct a bosonic subsystem decomposition (SSD)---the modular variable SSD---by dividing a mode's Hilbert space, expressed in the Zak basis, into that of a virtual qubit and a virtual gauge mode. Tracing over the gauge mode gives a logical-qubit state, and preceding the trace with a particular logical-gauge interaction gives a different logical state---that associated to GKP error correction.