Pierre Desjardins is the cofounder of C12, a Paris-based quantum computing hardware startup that specializes in carbon nanotube-based spin qubits. Notably, Pierre founded the company alongside his twin brother, Mathieu, making them the only twin-led deep-tech startups that we know of! Pierre’s journey is unconventional—he is a rare founder in quantum hardware without a PhD, drawing instead on engineering and entrepreneurial experience. The episode dives into what drew him to quantum computing and the pivotal role COVID-19 played in catalyzing his career shift from consulting to quantum technology.

C12’s Technology and Unique Angle

C12 focuses on developing high-performance qubits using single-wall carbon nanotubes. Unlike companies centered on silicon or germanium spin qubits, C12 fabricates carbon nanotubes, tests them for impurities, and then assembles them on silicon chips as a final step. The team exclusively uses isotopically pure carbon-12 to minimize magnetic and nuclear spin noise, yielding a uniquely clean environment for electron confinement. This yields ultra-low charge noise and enables the company to build highly coherent qubits with remarkable material purity.

Key Technical Innovations

• Spin-Photon Coupling: C12’s system stands out for driving spin qubits using microwave photons, drawing inspiration from superconducting qubit architectures. This enables the implementation of a “quantum bus”—a superconducting interconnect that allows long-range coupling between distant qubits, sidestepping the scaling bottleneck of nearest-neighbor architectures.
• Addressable Qubits: Each carbon nanotube qubit can be tuned on or off the quantum bus by manipulating the double quantum dot confinement, providing flexible connectivity and the ability to maximize coherence in a memory mode.
• Stability and Purity: Pierre emphasizes that C12’s suspended architecture dramatically reduces charge noise and results in exceptional stability, with minimal calibration drift, over years-long measurement campaigns—a stark contrast with many superconducting platforms.

Recent Milestones

C12 celebrated its fifth anniversary and recently demonstrated the first qubit operation on their platform. The company achieved ultra-long coherence times for spin qubits coupled via a quantum bus, publishing these results in *Nature*. The next milestone is demonstrating two-qubit gates mediated by microwave photons—a development that could set a new benchmark for both C12 and the wider quantum computing industry.

Challenges and Outlook

C12’s current focus is scaling up from single-qubit demonstrations to multi-qubit gates with long-range connectivity, a crucial step toward error correction and practical algorithms. Pierre notes the rapid evolution of error-correcting codes, remarking that some codes they are now working on did not exist two years ago. The interview closes with an eye on the race to demonstrate long-distance quantum gates, with Pierre hoping C12 will make industry headlines before larger competitors like IBM.

Notable Quotes

• “The more you dig into this technology, the more you understand why this is just the way to build a quantum computer.”
• “We have the lowest charge noise compared to any kind of spin qubit—this is because of our suspended architecture.”
• “What we introduced is the concept of a quantum bus… really the only way to scale spin qubits.”

Episode Themes

• Entrepreneurship in deep tech without a traditional research background
• Technical deep dive on carbon nanotube spin qubits and quantum bus architecture
• Materials science as the foundation of scalable quantum hardware
• The importance of coherence, noise reduction, and tunable architectures in quantum system design
• The dynamic evolution of error correction and industry competition

Listeners interested in cutting-edge hardware, quantum startup journeys, or the science behind scalable qubit platforms will find this episode essential. Pierre provides unique clarity on why C12’s approach offers both conceptual and practical advantages for the future of quantum computing,

Dr. Eli Levenson-Falk joins Sebastian Hassinger, host of The New Quantum Era to discuss his group’s recent advances in quantum measurement and control, focusing on a new protocol that enables measurements more sensitive than the Ramsey limit. Published in Nature Communications in April 2025, this work demonstrates a coherence stabilized technique that not only enhances sensitivity for quantum sensing but also promises improvements in calibration speed and robustness for superconducting quantum devices and other platforms. The conversation travels from Eli’s origins in physics, through the conceptual challenges of decoherence, to experimental storytelling, and highlights the collaborative foundation underpinning this breakthrough.

Guest Bio
Eli Levenson-Falk is an Associate Professor at USC. He earned his PhD at UC Berkeley with Professor Irfan Siddiqui, and now leads an experimental physics research group working with superconducting devices for quantum information science.

Key Topics

• The new protocol described in the paper: “Beating the Ramsey Limit on Sensing with Deterministic Qubit Control."
• Beyond the Ramsey measurement: How the team’s technique stabilizes part of the quantum state for enhanced sensitivity—especially for energy level splittings—using continuous, slowly varying microwave control, applicable beyond just superconducting platforms.
• From playground swings to qubits: Eli explains how the physics of a playground swing inspired his passion for the field and lead to his understanding of the transmon qubit, and why analogies matter for intuition.
• Quantum decoherence and stabilization: How the method controls the “vector” of a quantum state on the Bloch sphere, dumping decoherence into directions that can be tracked or stabilized, markedly increasing measurement fidelity.
• Calibration and practical speedup: The protocol achieves greater measurement accuracy in less time or greater accuracy for a given time investment. This has implications for both calibration routines in quantum computers and for direct quantum measurements of fields (e.g., magnetic) or material properties.
• Applicability: While demonstrated on superconducting transmons, the protocol’s generality means it may bring improved sensitivity to a variety of platforms—though the greatest benefits will be seen where relaxation processes dominate decoherence over dephasing.
• Collaboration and credit: The protocol was the product of a collaborative effort with theorist Daniel Lidar and his group, also at USC. In Eli's group, Malida Hecht conducted the experiment.

Why It Matters
By breaking through the Ramsey sensitivity limit, this work provides a new tool for both quantum device calibration and quantum sensing. It allows for more accurate and faster frequency calibration within quantum processors, as well as finer detection of small environmental changes—a dual-use development crucial for both scalable quantum computing and sensitive quantum detection technologies.

Episode Highlights

• Explanation of the “Ramsey limit” in quantum measurement and why surpassing it is significant.
• Visualization of quantum states using the Bloch sphere, and the importance of stabilizing the equatorial (phase) components for sensitivity.
• Experimental journey from “plumber” lab work to analytic insights, showing the back-and-forth of theory confronting experiment.
• Immediate and future impacts, from more efficient calibration in quantum computers to potentially new standards for quantum sensing.
• Discussion of related and ongoing work, such as improvements to deterministic benchmarking for gate calibration, and the broader applicability to various quantum platforms.

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Assistant Professor Mohammad Mirhosseini (Caltech EE/APh) explains how his group built a mechanical quantum memory that stores microwave-photon quantum states far longer than typical superconducting qubits, and why that matters for hybrid quantum architectures. The discussion covers microwave photons, phonons, optomechanics, coherence versus lifetime (T2 vs. T1), current speed bottlenecks, and implications for quantum transduction and error mechanisms. The discussion centers on a paper from Mirhosseini's paper from December of 2024 titled, “A mechanical quantum memory for microwave photons,” detailing strong coupling between a transmon and a long‑lived nanomechanical oscillator for storage and retrieval of nonclassical states.

Guest

Mohammad Mirhosseini is an Assistant Professor of Electrical Engineering and Applied Physics at Caltech, where his group engineers hybrid superconducting–phononic–photonic systems at millikelvin temperatures for computing, communication, and sensing. He completed his PhD at the University of Rochester’s Institute of Optics and was a postdoc in Oscar Painter’s group at Caltech before starting his lab. His recent team effort demonstrates mechanical oscillators as compact, long‑lived quantum memories integrated with superconducting circuits.

Key topics

• What “microwave photons” are and how qubits emit/absorb single microwave photons in circuit QED analogously to atoms and optical photons.
• Why “memory” is missing in today’s quantum processors and how a dedicated long‑lived storage element can complement fast but dissipative superconducting qubits.
• Optomechanics 101: mapping quantum states between electrical and mechanical degrees of freedom, with phonons as the quantized vibrational excitations.
• T1 vs. T2: demonstrated order‑of‑magnitude gains in lifetime (T1) and more modest current gains in coherence (T2), plus paths to mitigate dephasing.
• Present bottleneck: state conversion between qubit and oscillator is about 100× slower than native superconducting operations, with clear engineering avenues to speed up.
• Quantum transduction: leveraging the same mechanical intermediary to bridge microwave and optical domains for interconnects and networking.
• Two‑level system (TLS) defects: shared decoherence mechanisms across mechanical oscillators and superconducting circuits and why comparing both can illuminate materials limits.

Why it matters

Hybrid architectures that pair fast processors with long‑lived memories are a natural route to scaling, and mechanical oscillators offer lifetimes far exceeding conventional superconducting storage elements while remaining chip‑integrable.. Demonstrating nonclassical state storage and retrieval with strong qubit–mechanics coupling validates mechanical oscillators as practical quantum memories and sets the stage for on‑chip transduction. Overcoming current speed limits and dephasing would lower the overhead for synchronization, buffering, and possibly future fault‑tolerant protocols in superconducting platforms.

Episode highlights

• A clear explanation of microwave photons and how circuit QED lets qubits create and absorb them one by one.
• Mechanical memory concept: store quantum states as phonons in a gigahertz‑frequency nanomechanical oscillator and read them back later.
• Performance today: roughly 10–30× longer T1 than typical superconducting qubits with current T2 gains of a few×, alongside concrete strategies to extend T2.
• Speed trade‑off: present qubit–mechanics state transfer is ~100× slower than native superconducting gates, but device design and coupling improvements are underway.
• Roadmap: tighter coupling for in‑oscillator gates, microwave‑to‑optical conversion via the same mechanics, and probing TLS defects to inform both mechanical and superconducting coherence.