By Sinan Utku
Example 2: Implementing a Logical Qubit in a Surface Code
Surface codes are a promising class of quantum error correction codes that will likely be used in fault-tolerant quantum computing. In a surface code, many physical data qubits collectively encode a logical qubit in a nonlocal, topologically protected manner, making it robust against physical errors. The qubits are arranged on a surface lattice, such as a plane, in which data qubits are interleaved with ancilla (measurement) qubits. These measurement qubits interact with neighboring data qubits to perform stabilizer measurements, extracting error syndromes without collapsing the encoded logical state. The syndromes reveal the presence and locations of errors, which can then be inferred and corrected using classical decoding algorithms.
In particular, a quantum computer might carry out the following functionality to construct a logical qubit and maintain it without errors:
- configure a plurality of data qubits on a two-dimensional lattice structure;
- interleave ancilla qubits with the data qubits, an configure the ancilla qubits to measure stabilizers of neighboring data qubits;
- periodically perform stabilizer measurements on the ancilla qubits to detect errors in the data qubits;
- determine error syndromes based on the stabilizer measurement results; and
- apply corrective operations to the data qubits based on the error syndromes.
The patent eligibility of such an invention could be challenged based on at least two grounds. First, the invention is largely directed to an error correction scheme that involves redundant bits. This concept has been known and broadly used in communication and classical computational technologies for a long time. Accordingly, the invention might be found to be directed to the abstract idea of error correction using redundant bits. The likelihood of this happening will be greater if the invention is not particularized to a specific quantum computer hardware platform or set of components; generally, inclusion of hardware elements increases the likelihood that the invention will be found to be patent eligible.[i] Finally, just based on the specification of the invention above, there is likely no good argument based on step 2 of the Mayo/Alice framework that there are inventive elements that transform the invention into eligible subject matter despite its being directed to an abstract idea. The invention specified above arguably has a foundation in error correction concepts that have long been used in classical computing.
In response to a challenge to the patent eligibility of the invention based on abstractness, the patentee could argue that the surface code improves the quantum computer of the invention. Generally, showing that the invention improves an existing technological process, or improves the device that carries out the invention, is helpful in demonstrating patent eligibility.[ii] The invention in this example is broadly drawn to a “quantum computing system”. The patentee could reasonably argue that that the surface code and the resultant logical qubit improves the functioning of quantum computing systems.[iii] The invention could be found patent eligible based on such arguments.
[i] This is not a guarantee, however. Recall that the invention in Symantec included only mechanical, i.e., hardware, elements and was nevertheless found to be patent ineligible.
[ii] See, e.g., MPEP § 2106.04(d)(1) (U.S. Pat. & Trademark Off. Nov. 2024) (recognizing that improving “the functioning of a computer” or “another technology or technical field” constitute a patent-eligible practical application.)
[iii] If the invention is just directed to “a logical qubit”, one could not easily make an analogous argument. A logical qubit is not a device that itself is capable of improvement and is arguably itself an abstraction. Here, specifying details that particularize the invention should increase the odds of a finding of patent eligibility.
Sinan Utku is a Special Counsel, Covington and Burling LLP; Instructor, Bilkent University Law School. Nothing in this article should be construed as reflecting the official views, opinions, or positions of any organisation or institution with which the author is affiliated. The author writes in a personal capacity only.
September 24, 2026
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