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Exotic superconducting CIrcuits to Probe and protect quantum States of light and mattEr

Periodic Reporting for period 1 - ECLIPSE (Exotic superconducting CIrcuits to Probe and protect quantum States of light and mattEr)

Reporting period: 2020-03-01 to 2021-08-31

Quantum systems can occupy peculiar states, such as superposition or entangled states. These states are intrinsically fragile and eventually get wiped out by inevitable interactions with the environment. Protecting quantum states against decoherence is a formidable and fundamental problem in physics, which is pivotal for the future of quantum computing. The theory of quantum error correction provides a solution, but its current envisioned implementations require daunting resources: a single bit of information is protected by encoding it across tens of thousands of physical qubits.
Project ECLIPSE aims to protect quantum information in an entirely new type of qubit with two key specificities. First, it will be encoded in a single superconducting circuit resonator whose infinite dimensional Hilbert space can replace large registers of physical qubits. Second, this qubit will be rf-powered, continuously exchanging photons with a reservoir. This approach challenges the intuition that a qubit must be isolated from its environment. Instead, the reservoir acts as a feedback loop which continuously and autonomously corrects against errors. This correction takes place at the level of the quantum hardware, and reduces the need for error syndrome measurements which are resource intensive.
Since the beginning of the project in March 2020, and despite the pandemic outbreak two weeks later, we have:
- Built a new lab space: We have built a new lab space at ENS Paris in order to host the experiments from project ECLIPSE;
- Installed a new dilution refrigerator: Using funds from project ECLIPSE, we have ordered a new dilution refrigerator from Bluefors, wired it and it is up and running;
- Hired a team of scientists: We have hired a great team of students and postdocs working on this project. Some of them are paid by the ECLIPSE funds, and some have complementary funding;
- Made steady progress on various project tasks.
Beyond state of the art: we have observed the magnification of quantum phase fluctuations with Cooper-pair pairing. This work is currently on the arXiv:2010.15488 and is in review. We have demonstrated that a new type of junction – that allows only pairs of Cooper pairs to tunnel – significantly increases the spread of the circuit wavefunction across multiple Josephson wells. This has important implications for (i) performing fault-tolerant error syndrome measurements for cat-qubits and (ii) building a new type of qubit, entirely protected from decoherence at the Hamiltonian level.

Expected results: by conceiving exotic circuits to first protect quantum information and then probe fundamental properties of matter, project ECLIPSE addresses hard problems at the boundaries of quantum engineering and fundamental physics. It aims at overcoming the outstanding difficulty of hardware overhead for fault-tolerant quantum computing, and opens the path to probe complex mesoscopic phenomena at the quantum level.
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