Hardware & Errors
GKP Qubit
Gottesman-Kitaev-Preskill qubit — an error-resistant way of encoding a qubit using continuous-variable photonic states.
Definition
A GKP qubit is a method of encoding a single, error-resistant logical qubit within the continuous-variable states of a photonic (light-based) system, named after the physicists who proposed the encoding. It's a specific approach to achieving error correction within photonic quantum computing platforms.
In depth
The Gottesman-Kitaev-Preskill qubit, or GKP qubit, stores a qubit in the state of a harmonic oscillator, such as a mode of light or a vibration of an ion. The oscillator has a continuous range of position and momentum, and a GKP state is a comb-like pattern in that phase space: a superposition of sharply peaked spikes spaced evenly along both axes. Small shifts in position or momentum move the state off the grid points by a small amount, and measuring the shift modulo the grid spacing reveals the error so it can be undone.
This turns the main noise processes of an oscillator, which are small random displacements, into correctable errors. GKP codes, proposed in 2001, are attractive because they can be combined with an outer error-correcting code, such as the surface code, and the combination lowers the number of oscillators needed. The quality of a GKP state is measured by its squeezing, the sharpness of the spikes, and values of about 10 to 12 decibels are needed for useful error correction. Real states have spikes with finite width, and gates and measurements are harder than for conventional qubits.
Experiments have steadily advanced. ETH Zurich created GKP states of a trapped ion's motion in 2019, groups at Yale and in Paris produced them in superconducting cavities in 2020, and in 2023 a Yale team reached the break-even point where a GKP-encoded qubit outlived the best qubit available in the same system. Xanadu demonstrated GKP states on photonic chips as part of its Aurora system in 2025, and companies including Nord Quantique build GKP-based superconducting machines.
Technical Definition
GKP encoding represents qubit states as specific grid-like patterns in a continuous phase space, designed so that small errors (like photon loss or phase noise) shift the state only slightly within a detectable and correctable region of that grid.
Visual Explanation: An Analogy
Think of GKP encoding like writing a message using an unusually wide, bold font with built-in checksums — small smudges or noise don't prevent the message from being read correctly, unlike writing in tiny, fragile handwriting.
Key facts
| Item | Detail |
|---|---|
| Encoding | A grid of peaks in the position and momentum phase space of an oscillator |
| Corrects | Small displacements in position and momentum |
| Required squeezing | About 10 to 12 decibels for useful error correction |
| Break-even | Demonstrated in a superconducting cavity at Yale in 2023 |
| Photonic GKP | Xanadu reported on-chip GKP states in 2025 |
Where the idea comes from
Daniel Gottesman, Alexei Kitaev, and John Preskill proposed the code in a 2001 paper. First experimental GKP states appeared in 2019 and 2020, and error-corrected GKP qubits followed in 2023 and 2025.
Real-World Use Cases
- Demonstrated by Xanadu on its Aurora photonic system as a step toward fault-tolerant photonic quantum computing
- An active research approach for achieving error correction specifically within continuous-variable photonic architectures
How it differs from related ideas
- A GKP qubit protects against small shifts, while a cat qubit protects against bit flips.
- Both are bosonic codes that use an oscillator instead of many two-level qubits.
- GKP states are sensitive to loss and need high squeezing, while cat states are sensitive to phase-flip errors.
Frequently asked questions
What is a GKP qubit?
A qubit encoded in a grid-like state of an oscillator so that small displacement errors can be detected and corrected.
Why are GKP qubits useful?
They fix the most common oscillator errors at the hardware level and can be combined with another code to cut overhead.
Who works on GKP qubits?
Yale, ETH Zurich, Xanadu, Nord Quantique, and other academic and company groups.
Related Terms
Keep exploring
- Algorithm: Quantum Error-Correcting Codes (Shor Code, Surface Code)
- Hardware: Xanadu Borealis