# Nicolas Delfosse

Principal Researcher at Microsoft

Update: Why I am joining IonQ.

It is exciting to work on the first generation of quantum computers. Unfortunately, current quantum hardware is not sufficiently reliable for industrial applications. The primary challenge for the quantum computing community is to transition from small noisy devices to large-scale fault-tolerant quantum computers capable of solving industrial problems. This requires designing a quantum computer around quantum error correction and fault-tolerance. Most of what a large-scale quantum computer will do is correcting faults!

I am an expert in quantum error correction and fault tolerance. The goal of my research is to use these techniques to make quantum bits and quantum operations good enough to run large-scale quantum algorithms and to solve practical problems.

More precisely, I work on different kinds of quantum error correction codes including surface codes, color codes, Floquet codes, block codes and LDPC codes. One of my favorite research problems is the design of fast decoders for these codes. I designed several decoding algorithms such as the Union-Find decoder, the projection decoder for color codes or the peeling decoder capable of correcting erasures. My goal is to make quantum error correction practical to enable large-scale fault-tolerant quantum computing.

## Recent News

Feb 2024 - Workshop: With Nikolas Breuckmann and Anirudth Krishna, we are organizing a workshop on quantum LDPC codes at the Simons Institute at UC Berkeley.

Oct. 23rd 2023 - New preprint: How to choose a decoder for a fault-tolerant quantum computer?

Sept. 28th 2023 - New preprint: Simulation of noisy Clifford circuits without fault propagation

Sept. 28th 2023 - New preprint: Splitting decoders for correcting hypergraph faults

Sept. 2023 - Conference Talk: I will be speaking at the IEEE Quantum Week.

Sept. 2023 - Lectures: I will be giving a tutorial on quantum error correction at UCLA for the workshop Mathematical and Computational Challenges in Quantum Computing.

My slides are here.

July 2023 - Lectures: I will be giving lectures on quantum LDPC codes at the PCMI summer school.

My slides are here.

July 10th 2023 - Seminar talk: I will be speaking at the virtual QCDA seminar organized by Christophe Vuillot. Register here to this seminar series.

June 7th 2023 - Seminar talk: I will be speaking at the QEC seminar of Duke University.

April 2023 - New preprint: Spacetime codes of Clifford circuits

April 2023 - Conference Talk: I will be speaking at the QCTIP conference.

March 2023 - Conference Talk: I will be speaking at the APS March meeting.

Feb. 2023 - Seminar Talk: I will be speaking at the Caltech IQIM seminar.

Jan. 2023 - Publication: Our paper Performance of Planar Floquet Codes with Majorana-Based Qubit has been published in PRX Quantum.

## Publications

Selected papers:

Constant-Overhead Quantum Error Correction with Thin Planar Connectivity

M Tremblay, N Delfosse, M Beverland

Physical Review Letters 129, 050504 (2022).

Almost-linear time decoding algorithm for topological codes

N Delfosse, NH Nickerson

Quantum 5, 595

Optimizing quantum error correction codes with reinforcement learning

HP Nautrup, N Delfosse, V Dunjko, HJ Briegel, N Friis

Quantum 3, 215

Linear-time maximum likelihood decoding of surface codes over the quantum erasure channel

N Delfosse, G Zémor

Physical Review Research 2 (3), 033042

Wigner function negativity and contextuality in quantum computation on rebits

N Delfosse, PA Guerin, J Bian, R Raussendorf

Physical Review X 5 (2), 021003

Decoding color codes by projection onto surface codes

N Delfosse

Phys. Rev. A 89, 012317

## Short resume

Background:

I obtained my PhD degree in pure mathematics from Bordeaux University in 2012, under the supervision of Gilles Zémor. My PhD work was about quantum error correction and quantum LDPC codes. Then, I joined Ecole polytechnique supported by the LIX-Qualcomm fellowship, where I worked with Alain Couvreur. Prior to joining Microsoft in 2017, I held a postdoc position at Sherbrooke University in the group of David Poulin and a postdoc position shared between UCR and Caltech in the groups of Leonid Pryadko and John Preskill.

Intern supervision:

I supervised the following PhD students for a three-month internship:

- Rui Chao, summer 2019 (Duke University)

- Poulami Das, summer 2019 (Georgia Tech)

- Christopher Pattison, Fall 2020 (Caltech)

- Maxime Tremblay, Fall 2020 (Sherbrooke University)

- Shilin Huang, Summer 2021 (Duke University)

- Yue Wu, Summer 2023 (Yale University)

Postdoc supervision:

In collaboration with Vivien Londe and Anthony Leverrier, I supervised Nicholas Connolly for his postdoc working on quantum LDPC codes from Sept 2021 to Aug 2023.

Teaching:

In 2008, I obtained the agrégation de Mathématiques (French competitive examination for the recruitment of math teachers).

Between 2009 and 2013, I have been teaching mathematics (Algebra, Calculus, Discrete Math, Numerical Analysis and Cryptography) and computer science (Java, Python) classes at classes préparatoire Camille Jullian, Bordeaux University, Enseirb-Matmeca engineering school and Ecole polytechnique.

Quantum computing lectures:

More recently, I have been teaching lectures for grad students at the IBM summer school in 2022 and at the PCMI summer school in 2023. I gave tutorials on quantum error correction at UCLA for the IPAM quantum computing program.

In 2023, I taught a series of lectures on Shor's algorithm for a non-profit teaching science to undergrad in the Seattle area (Northwest academy of sciences).

Lecture slides and recordings:

IPAM Quantum computing program 2023 at UCLA:

Tutorials on quantum error correction.

PCMI Summer School 2023:

Lectures on quantum LDPC codes.

IBM Summer School 2022:

Lectures on quantum LDPC codes.