# Nicolas Delfosse

Principal Researcher at IonQ

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.

In the past, I worked 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 a peeling decoder capable of correcting erasures.

Today, I lead the quantum error correction team at IonQ. My goal is to make quantum error correction practical to enable large-scale fault-tolerant quantum computing. To learn more about the work of my team check this blogpost. If you are interested in an overview of quantum error correction, I recently gave a tutorial on the topic at UCLA. The videos are available here.

## Recent News

Nov 3rd 2024 - Conference Talk: I will speak at the NSF Workshop on Quantum Operating Systems and Real-Time Control in Austin, Texas.

Oct. 2024 - QIP conference: I will be on the Program Committee of QIP2025.

Sept. 26th 2024 - Conference Talk: I will speak at the ML4Q summer school on quantum error correction in Bonn, Germany.

Sept. 17th 2024 - Seminar talk: I will give a seminar at the CS department of the university of Bordeaux, France.

Aug 27th 2024 - Publication: Our erasure decoder for hypergraph product codes was published in Quantum Journal.

Aug. 14th 2024 - Blogpost: We wrote a blogpost about our CliNR scheme.

Aug. 4th - Aug. 17th 2024 - I will be attending the FTQT2024 conference.

July 9th 2024 - New preprint: Low-cost noise reduction for Clifford circuits.

June 2nd 2024 - Math Hour Olympiad: I will be a judge for the Math Hour Olympiad at the University of Washington.

March 28th 2024 - Seminar: I will speak at the Joint Center for Quantum Information Science and Computer Science at the university of Maryland.

March 2024 - TQC conference: I will be on the Program Committee of TQC 2024.

March 2024 - PhD examination - I will be an examiner for the PhD thesis of Lawrence Cohen in Sydney University.

Feb 2024 - Simon's workshop on QEC: With Nikolas Breuckmann and Anirudth Krishna, we are organizing a workshop on quantum error correction at the Simons Institute at UC Berkeley.

## 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. I held a postdoc position at Sherbrooke University in the group of David Poulin and a postdoc position shared between Caltech and UCR in the groups of John Preskill. and Leonid Pryadko. Before joining IonQ, I was as a principal researcher at Microsoft, where I spent 6 years, working on fault-tolerant quantum computing. I am currently leading the quantum error correction team of IonQ.

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:

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

Teaching:

In 2008, I obtained the agrégation de Mathématiques. 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.

Math circle:

I am regularly teaching for a math circle in the Seattle area (the northwest academy of sciences). In 2023, I taught a series of lectures on Shor's algorithm and in 2024, I gave lectures on fault-tolerant quantum computing.

Recent lectures:

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.