Insider Brief
- Researchers demonstrated microsecond-scale coherence times in a carbon nanotube quantum circuit, surpassing previous records for carbon- and silicon-based quantum dots.
- The carbon device integrated into a microwave cavity maintained quantum coherence for up to 1.3 microseconds at 300 millikelvin without external magnetic fields.
- Analysis suggests cotunneling is the main source of decoherence, while charge noise and phonons played a lesser role due to the suspended, oxide-free design.
A team of researchers report they have demonstrated microsecond-scale coherence times in a carbon nanotube circuit driven by cavity photons—showing longer-lived quantum states than any previously recorded for carbon quantum dots and surpassing similar systems built with silicon.
The study, published in Nature Communications by a team that included scientists from C12 Quantum Electronics and several French research institutions, reports coherence times of 1.3 microseconds in a suspended carbon nanotube double quantum dot setup integrated within a microwave cavity. That figure represents a roughly 100-fold improvement over previous carbon-based implementations and a tenfold improvement over similar silicon quantum dot devices.
The result not only revives interest in carbon nanotubes as a quantum material, but also sets a new benchmark for hybrid quantum electrodynamics systems, where spin qubits interact with microwave photons—a configuration known as circuit quantum electrodynamics (cQED).
Carbon, Cleaned Up and Suspended
Carbon nanotubes have long been theorized to offer ideal conditions for spin qubits. Their structure allows for well-defined, tunable quantum dots and, when grown using pure carbon-12, eliminates nuclear spin noise, which is a frequent source of calculation-hampering decoherence. But until now, their short coherence times, typically on the order of 10 nanoseconds, have limited practical progress.
The new device changes that, according to the team. It suspends a carbon nanotube between ferromagnetic electrodes inside a high-quality microwave cavity, while using five electrostatic gates to tune the quantum dot configuration. One of the gates connects directly to the cavity’s central conductor, enabling photon-mediated coupling to the quantum system.
Importantly, the entire system operates at a relatively elevated temperature of 300 millikelvin, significantly higher than typical dilution refrigerator setups. The absence of external magnetic fields and reliance solely on cavity photons for control simplify operation and hint at future scalability.
Stability, Precision and Multi-Level Dynamics
According to the paper, measurements showed that quantum states could be manipulated and read out via cavity photons. The team used Rabi and Ramsey sequences to characterize the system. Yoy could think of these sequences special ways to poke and prod a quantum system and watch how it responds, something like tapping a top that is spinning and seeing how long it keeps spinning. The team found a Rabi decay time of approximately 0.6 microseconds and a Ramsey dephasing time exceeding 1.2 microseconds. That might not sound like a long time for the classical world, but in the quantum world these times are strong signs of a robust and reliable quantum system.
The team also observed a Hahn echo coherence time — another type of test — of just over 2 microseconds, placing their system near the theoretical limit where decoherence is governed almost entirely by energy relaxation. These results were confirmed through simulations that matched both the system’s spectral features and time-domain behavior.
Unlike traditional qubits that toggle between two states, the carbon system displayed multi-level quantum dynamics — more like a quasi-harmonic ladder — offering access to a richer set of quantum states. This complexity could enable more advanced operations and open new possibilities for quantum computing beyond standard designs. Despite the added complexity, the system still exhibited well-defined Rabi oscillations, showing it can reliably perform basic quantum logic tasks.
Toward High-Fidelity Quantum Gates
The findings suggest that carbon nanotube circuits may become serious contenders in the race to scale quantum processors. The observed coherence times rival those of many silicon spin qubits but come with potential advantages. Carbon systems can be suspended, reducing sensitivity to charge noise and can be entirely free of oxide interfaces, which are known sources of decoherence in solid-state devices.
The use of high-impedance cavities can also boost charge-photon coupling strength, which is a key metric for implementing two-qubit gates between distant quantum dots via microwave photons. In theory, these design choices could enable high-fidelity, long-range quantum gates beyond the performance of today’s leading spin-based architectures.
Technical Hurdles Remain
The researchers indicate challenges remain and there’s room for future work. The team performed a detailed “decoherence budget” analysis, exploring sources such as charge noise, nuclear spins, phonons and cotunneling. Among these, cotunneling — where electrons virtually hop in and out of the dots via the leads — emerged as the likely dominant source of decoherence.
According to the team, estimates for decoherence from charge noise and hyperfine interactions were at least an order of magnitude below the measured values. Simulated phonon-induced relaxation times were shorter than experimental results, suggesting phonons were not the primary cause. That leaves cotunneling, which the researchers could not directly measure due to the device’s configuration but appears to be the most consistent with their data.
Outlook: A Clean Room Path to Better Qubits?
By limiting interaction with external circuitry and designing the device to minimize charge noise and magnetic fluctuations, the study offers a roadmap for even longer-lived qubits. The researchers estimate that further reductions in coupling to the leads could push the pure dephasing time to nearly 3 microseconds, which approaches or exceeds the performance of the best spin qubits in any platform.
Because carbon nanotubes can be synthesized without isotopic impurities and without oxide interfaces, they may offer a naturally cleaner environment for quantum information processing. Combined with cavity-based control, this could enable new approaches to scaling spin-based quantum processors.
The findings also reinvigorate interest in alternative material systems beyond the usual contenders like silicon and gallium arsenide. With long-lived quantum states now confirmed in carbon, the once-hyped nanotube may be poised for a comeback—this time not as a conductor or transistor channel, but as the beating heart of quantum information hardware.
Additional institutions involved in this research include the Laboratoire de Physique de l’École normale supérieure, ENS, Université PSL, CNRS, Sorbonne Université, Université Paris Cité, the Laboratoire de Physique et d’Étude des Matériaux at ESPCI Paris (also affiliated with Université PSL, CNRS, and Sorbonne Université) and the Institut universitaire de France (IUF), all based in Paris, France.
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