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Optimization of the magnetic-field distribution along a suspended carbon nanotube to enhance the addressability of spin qubits.

Sandrine Lopes

Published Nov 5, 2025• 0 citations
OpenAlex

Abstract

In this thesis, we investigate the use of an on-chip micromagnet to control the magnetic-field profile along a carbon nanotube for spin-qubit applications. The nanotube serves as a coherent conductor in which a single electron is confined in a gate-defined double quantum dot shaped by the electrostatic potential along the tube. Quantum information is encoded in the electron's spin state, which serves as the qubit. The quantum-dot circuit is embedded in a circuit quantum electrodynamics (QED) architecture that enables qubit manipulation, readout, and long-range coupling. Coupling of the spin qubit to photons in a microwave resonator is enabled through spin-charge hybridization engineered by controlling the field profile along the nanotube: the electron in the double well experiences different spin-quantization axes in the two dots. Here, magnetic-field control is implemented by integrating a micromagnet into the quantum-dot circuit. Optimal geometries for on-chip field gradients are identified through numerical simulations to maximize the magnetic-field difference between the two quantum dots. Thin-film magnetic characterization and local field sensing of the micromagnet - using NV-center magnetometry - are performed. The experimental data are matched with micromagnetic simulations to predict the magnetic field generated by the magnet at various external fields and positions, and hence to forecast spin-qubit performance. To further enhance spin-photon coupling, the spin-qubit architecture is integrated with a microwave circuit fabricated from niobium nitride (NbN), a high-kinetic-inductance, magnetic-field-resilient superconductor. The design and characterization of a high-impedance resonator are detailed. Finally, we demonstrate on-chip integration of the carbon-nanotube quantum-dot circuit, the micromagnet, and the NbN microwave circuitry.

Related papers

Charge and spin dynamics in a hybrid circuit quantum electrodynamics architecture

J. J. Viennot

This thesis investigates experimentally the coupling mechanism of the charge and spin degrees of freedom in double quantum dots to high finesse superconducting microwave cavities. We use carbon nanotubes as a coherent conductor to host our quantum dots.We conceived an experimental setup and developed new fabrication methods in order to achieve control over these devices. With these methods, we scrutinize the resonant coupling of the dot's charge electronic transitions to a microwave cavity. We drive the system out of equilibrium to characterise its dynamics and extract its relevant intrinsic parameters. We examine a possible coupling of single photons to a single electron spin, using non-collinear exchange fields induced from ferromagnetic interfaces as a key ingredient to engineer this coupling. The preliminary results in this circuit architecture are promising for future spin-based QED experiments with single spins.

•Jun 6, 2014•0 citations

Hybrid circuit quantum electrodynamics with a hole spin in silicon

Cécile Xinqing Yu

Coherent and strong coupling between photons and solid-state qubits, in the form of circuit quantum electrodynamics (QED), has been harnessed for two-qubit gates mediated by photons and high-fidelity quantum non-demolition readout, which are the building blocks of large-scale quantum computation. Recently, circuit QED has been extended to gate-defined semiconductor quantum dots. In this thesis, we develop a novel hybrid circuit QED architecture composed of a high-impedance superconducting microwave resonator and spins localized in silicon-MOS quantum dots. To control and measure the spin degrees of freedom, this hybrid system needs to operate at finite magnetic field. Consequently, we develop and characterize microwave resonators based on thin niobium nitride (NbN) films featuring a high kinetic inductance. We demonstrate high-impedance resonators with magnetic field resilience and low photon loss rates. We then co-integrate the NbN resonators on silicon spin qubit chips. With a hole confined in a double quantum dot (DQD), we report the first realization of a strong hole charge-photon interaction bordering the ultra-strong coupling regime. At finite magnetic field, putting the spin transition energy in resonance with the microwave cavity, we observe large vacuum Rabi mode splittings, signature of a strong spin-photon coupling. Our findings are well captured by the modelling of a hole DQD with different anisotropic Zeeman response in each dot and spin-orbit coupling dependent tunnel rates. We also find a sizeable spin-photon coupling when the hole is localized in the single quantum dot, in line with recent theoretical predictions. The different works presented in this manuscript pave the way for circuit QED with hole spins in gate-defined semiconductor quantum dots.

•Sep 15, 2022•0 citations

Hybrid circuit QED with spin ensembles and carbon-nanotube-based superconducting qubits

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Circuit quantum electrodynamics has received an increasing amount of attention over the last decade in light of its potential use in quantum information processing. The underlying principle of this architecture is the coherent coupling between microwave photons stored in superconducting resonators and superconducting qubits made from non-linear, non-dissipative electrical circuits. This architecture can be extended to other solid-state systems with transition frequencies in the microwave regime, such as nuclear and electron spins or Andreev bound states. These hybrid devices would allow the coherent exchange of information between such systems and superconducting qubits mediated by a microwave resonator. This could lead to advances in quantum information processing and new insights in the physics governing these interacting systems. In this dissertation experimental work with different types of hybrid quantum circuits is presented. A first experiment investigates the coherent coupling of microwave photons with a spin ensemble in a correlated and uncorrelated state. The coupling strength and magnetic resonance is studied as a function of temperature and magnetic field direction. A pronounced temperature dependent anisotropy of the magnetic resonance is observed, which can be attributed to the onset of antiferromagnetic correlations in the spin ensemble when cooled below T=4 K. A simple one-dimensional model gives a good description of this anisotropy. A next experiment studies the transport characteristics of carbon nanotubes electrically accessed with normal and superconducting contacts. These devices were characterised using both DC and RF reflectometry techniques. The carbon nanotube devices with superconducting contacts exhibit transport characteristics of Josephson junctions with critical currents of up to Ic~18 nA, as well as multiple Andreev reflections. These semiconductor-superconductor hybrid Josephson junctions are then used to realise a carbon-nanotube-based superconducting qubit with voltage tunable transition frequency using a local electrostatic gate. Strong coupling (g~100 MHz) to a coplanar waveguide resonator is demonstrated via a resonator frequency shift dependent on applied gate voltage. Qubit parameters are extracted from spectroscopy and correspond well to the DC measurements of the carbon nanotube Josephson junctions. Qubit relaxation and coherence times in the range 10-100 ns are observed. The hybrid devices investigated in this work present potential building blocks for more extensive hybrid architectures for quantum information processing.

•Jan 1, 2018•0 citations

3D integration of a hybrid quantum dot circuit-QED device for fast gate dispersive charge readout and coherent spin-photon coupling

Sebastien Granel, F. Gustavo, Jean-Luc Thomassin +8 more

Hybrid circuit quantum electrodynamics (cQED) aims at coupling various quantum degrees of freedom, among which are spin and charge degrees of freedom in gate defined quantum dots, phonons or magnons... with quantized electromagnetic fields in superconducting microwave cavities to investigate fundamental physics questions or for quantum computation and simulation. However, low microwave losses, key for many hybrid cQED experiments, are challenging to achieve given the often exotic and/or complex material stacks (e.g. semiconducting material, ferromagnets, or piezoelectric materials) required to host the various quantum degrees of freedom. In this work, we present a 3D-integration process to overcome this challenge for semi-industrial silicon MOS spin qubits. The process is based on dense indium bump interconnects at a pitch of 10 $\mathrmμ$m and superconducting thin films of Niobium Nitride (NbN). First, we report on DC and RF interconnect properties that demonstrate a high galvanic interconnection yield and internal quality factors above $10^{5}$ in the single photon regime for NbN resonators interrupted by a single indium bump interconnect. Eventually, we fabricated a 3D-integrated hybrid circuit quantum electrodynamics (cQED) device based on a semi-industrial MOS hole double quantum dot and a high impedance NbN resonator. For this device, we report a cavity internal quality factor above 10000 and demonstrate record sensitivity for gate-based dispersive readout of the charge degree of freedom with an SNR of 100 in 300 ns. Finally, we demonstrate strong spin-photon coupling of $g_{s}/2π$ = 75 MHz, which highlights the viability of 3D-integration for quantum dot based hybrid spin circuit quantum electrodynamics and opens to high-fidelity spin readout and microwave photon-based remote spin qubit entanglement.

arXiv:2604.25871•Apr 28, 2026•0 citations

Towards Single Spin Manipulation in a cQED Architecture

Filippo Vicentini

In this thesis we present a spin based QuBit Architecture that implements single-QuBit gates in the QuBit itself and is predicted to have a long coherence time. This architecture has been physically realized in a Ferromagnetic Double Quantum Dot hosted in a Single Wall Carbon Nanotube, and studied through a coupled Microwave cavity. We present a Linear Response Theory model that ties the transmitted amplitude and phase of the microwave signal through the cavity to the microscopic charge susceptibility of the Double Quantum Dot. We fit those two channels of experimental data to a simple 4 level master equation model that allows us to extract the inter-dot tunneling rate, the charge-photons coupling rate and the charge dephasing rate. We also present a numerical algorithm that allows to evolve in time the quantum state of the device also considering the effect of charge noise. This algorithm is used to study the effect of fast Landau-Zener style pulses on the QuBit, giving the ideal shape of the control pulses.

•Sep 1, 2016•0 citations

Highly coherent spin states in carbon nanotubes coupled to cavity photons

Tino Cubaynes, Matthieu R. Delbecq, Matthieu Dartiailh +8 more

Abstract Spins confined in quantum dots are considered as a promising platform for quantum information processing. While many advanced quantum operations have been demonstrated, experimental as well as theoretical efforts are now focusing on the development of scalable spin quantum bit architectures. One particularly promising method relies on the coupling of spin quantum bits to microwave cavity photons. This would enable the coupling of distant spins via the exchange of virtual photons for two qubit gate applications, which still remains to be demonstrated with spin qubits. Here, we use a circuit QED spin–photon interface to drive a single electronic spin in a carbon nanotube-based double quantum dot using cavity photons. The microwave spectroscopy allows us to identify an electrically controlled spin transition with a decoherence rate which can be tuned to be as low as 250 kHz. We show that this value is consistent with the expected hyperfine coupling in carbon nanotubes. These coherence properties, which can be attributed to the use of pristine carbon nanotubes stapled inside the cavity, should enable coherent spin–spin interaction via cavity photons and compare favorably to the ones recently demonstrated in Si-based circuit QED experiments. Our clean and controlled nano-assembly technique of carbon nanotubes in the cavity could be further improved by purified 12 C growth to get rid of the nuclear spins resulting in an even higher spin coherence.

arXiv:1903.05229•npj Quantum Information•Jul 8, 2019•58 citations
In this thesis, we investigate the use of an on-chip micromagnet to control the magnetic-field profile along a carbon nanotube for spin-qubit applications. The nanotube serves as a coherent conductor in which a single electron is confined in a gate-defined double quantum dot shaped by the electrostatic potential along the tube. Quantum information is encoded in the electron's spin state, which serves as the qubit. The quantum-dot circuit is embedded in a circuit quantum electrodynamics (QED) architecture that enables qubit manipulation, readout, and long-range coupling. Coupling of the spin qubit to photons in a microwave resonator is enabled through spin-charge hybridization engineered by controlling the field profile along the nanotube: the electron in the double well experiences different spin-quantization axes in the two dots. Here, magnetic-field control is implemented by integrating a micromagnet into the quantum-dot circuit. Optimal geometries for on-chip field gradients are identified through numerical simulations to maximize the magnetic-field difference between the two quantum dots. Thin-film magnetic characterization and local field sensing of the micromagnet - using NV-center magnetometry - are performed. The experimental data are matched with micromagnetic simulations to predict the magnetic field generated by the magnet at various external fields and positions, and hence to forecast spin-qubit performance. To further enhance spin-photon coupling, the spin-qubit architecture is integrated with a microwave circuit fabricated from niobium nitride (NbN), a high-kinetic-inductance, magnetic-field-resilient superconductor. The design and characterization of a high-impedance resonator are detailed. Finally, we demonstrate on-chip integration of the carbon-nanotube quantum-dot circuit, the micromagnet, and the NbN microwave circuitry.
This thesis investigates experimentally the coupling mechanism of the charge and spin degrees of freedom in double quantum dots to high finesse superconducting microwave cavities. We use carbon nanotubes as a coherent conductor to host our quantum dots.We conceived an experimental setup and developed new fabrication methods in order to achieve control over these devices. With these methods, we scrutinize the resonant coupling of the dot's charge electronic transitions to a microwave cavity. We drive the system out of equilibrium to characterise its dynamics and extract its relevant intrinsic parameters. We examine a possible coupling of single photons to a single electron spin, using non-collinear exchange fields induced from ferromagnetic interfaces as a key ingredient to engineer this coupling. The preliminary results in this circuit architecture are promising for future spin-based QED experiments with single spins.
Coherent and strong coupling between photons and solid-state qubits, in the form of circuit quantum electrodynamics (QED), has been harnessed for two-qubit gates mediated by photons and high-fidelity quantum non-demolition readout, which are the building blocks of large-scale quantum computation. Recently, circuit QED has been extended to gate-defined semiconductor quantum dots. In this thesis, we develop a novel hybrid circuit QED architecture composed of a high-impedance superconducting microwave resonator and spins localized in silicon-MOS quantum dots. To control and measure the spin degrees of freedom, this hybrid system needs to operate at finite magnetic field. Consequently, we develop and characterize microwave resonators based on thin niobium nitride (NbN) films featuring a high kinetic inductance. We demonstrate high-impedance resonators with magnetic field resilience and low photon loss rates. We then co-integrate the NbN resonators on silicon spin qubit chips. With a hole confined in a double quantum dot (DQD), we report the first realization of a strong hole charge-photon interaction bordering the ultra-strong coupling regime. At finite magnetic field, putting the spin transition energy in resonance with the microwave cavity, we observe large vacuum Rabi mode splittings, signature of a strong spin-photon coupling. Our findings are well captured by the modelling of a hole DQD with different anisotropic Zeeman response in each dot and spin-orbit coupling dependent tunnel rates. We also find a sizeable spin-photon coupling when the hole is localized in the single quantum dot, in line with recent theoretical predictions. The different works presented in this manuscript pave the way for circuit QED with hole spins in gate-defined semiconductor quantum dots.
Circuit quantum electrodynamics has received an increasing amount of attention over the last decade in light of its potential use in quantum information processing. The underlying principle of this architecture is the coherent coupling between microwave photons stored in superconducting resonators and superconducting qubits made from non-linear, non-dissipative electrical circuits. This architecture can be extended to other solid-state systems with transition frequencies in the microwave regime, such as nuclear and electron spins or Andreev bound states. These hybrid devices would allow the coherent exchange of information between such systems and superconducting qubits mediated by a microwave resonator. This could lead to advances in quantum information processing and new insights in the physics governing these interacting systems. In this dissertation experimental work with different types of hybrid quantum circuits is presented. A first experiment investigates the coherent coupling of microwave photons with a spin ensemble in a correlated and uncorrelated state. The coupling strength and magnetic resonance is studied as a function of temperature and magnetic field direction. A pronounced temperature dependent anisotropy of the magnetic resonance is observed, which can be attributed to the onset of antiferromagnetic correlations in the spin ensemble when cooled below T=4 K. A simple one-dimensional model gives a good description of this anisotropy. A next experiment studies the transport characteristics of carbon nanotubes electrically accessed with normal and superconducting contacts. These devices were characterised using both DC and RF reflectometry techniques. The carbon nanotube devices with superconducting contacts exhibit transport characteristics of Josephson junctions with critical currents of up to Ic~18 nA, as well as multiple Andreev reflections. These semiconductor-superconductor hybrid Josephson junctions are then used to realise a carbon-nanotube-based superconducting qubit with voltage tunable transition frequency using a local electrostatic gate. Strong coupling (g~100 MHz) to a coplanar waveguide resonator is demonstrated via a resonator frequency shift dependent on applied gate voltage. Qubit parameters are extracted from spectroscopy and correspond well to the DC measurements of the carbon nanotube Josephson junctions. Qubit relaxation and coherence times in the range 10-100 ns are observed. The hybrid devices investigated in this work present potential building blocks for more extensive hybrid architectures for quantum information processing.
Hybrid circuit quantum electrodynamics (cQED) aims at coupling various quantum degrees of freedom, among which are spin and charge degrees of freedom in gate defined quantum dots, phonons or magnons... with quantized electromagnetic fields in superconducting microwave cavities to investigate fundamental physics questions or for quantum computation and simulation. However, low microwave losses, key for many hybrid cQED experiments, are challenging to achieve given the often exotic and/or complex material stacks (e.g. semiconducting material, ferromagnets, or piezoelectric materials) required to host the various quantum degrees of freedom. In this work, we present a 3D-integration process to overcome this challenge for semi-industrial silicon MOS spin qubits. The process is based on dense indium bump interconnects at a pitch of 10 μ\mathrmμμm and superconducting thin films of Niobium Nitride (NbN). First, we report on DC and RF interconnect properties that demonstrate a high galvanic interconnection yield and internal quality factors above 10510^{5}105 in the single photon regime for NbN resonators interrupted by a single indium bump interconnect. Eventually, we fabricated a 3D-integrated hybrid circuit quantum electrodynamics (cQED) device based on a semi-industrial MOS hole double quantum dot and a high impedance NbN resonator. For this device, we report a cavity internal quality factor above 10000 and demonstrate record sensitivity for gate-based dispersive readout of the charge degree of freedom with an SNR of 100 in 300 ns. Finally, we demonstrate strong spin-photon coupling of gs/2πg_{s}/2πgs​/2π = 75 MHz, which highlights the viability of 3D-integration for quantum dot based hybrid spin circuit quantum electrodynamics and opens to high-fidelity spin readout and microwave photon-based remote spin qubit entanglement.
In this thesis we present a spin based QuBit Architecture that implements single-QuBit gates in the QuBit itself and is predicted to have a long coherence time. This architecture has been physically realized in a Ferromagnetic Double Quantum Dot hosted in a Single Wall Carbon Nanotube, and studied through a coupled Microwave cavity. We present a Linear Response Theory model that ties the transmitted amplitude and phase of the microwave signal through the cavity to the microscopic charge susceptibility of the Double Quantum Dot. We fit those two channels of experimental data to a simple 4 level master equation model that allows us to extract the inter-dot tunneling rate, the charge-photons coupling rate and the charge dephasing rate. We also present a numerical algorithm that allows to evolve in time the quantum state of the device also considering the effect of charge noise. This algorithm is used to study the effect of fast Landau-Zener style pulses on the QuBit, giving the ideal shape of the control pulses.
Abstract Spins confined in quantum dots are considered as a promising platform for quantum information processing. While many advanced quantum operations have been demonstrated, experimental as well as theoretical efforts are now focusing on the development of scalable spin quantum bit architectures. One particularly promising method relies on the coupling of spin quantum bits to microwave cavity photons. This would enable the coupling of distant spins via the exchange of virtual photons for two qubit gate applications, which still remains to be demonstrated with spin qubits. Here, we use a circuit QED spin–photon interface to drive a single electronic spin in a carbon nanotube-based double quantum dot using cavity photons. The microwave spectroscopy allows us to identify an electrically controlled spin transition with a decoherence rate which can be tuned to be as low as 250 kHz. We show that this value is consistent with the expected hyperfine coupling in carbon nanotubes. These coherence properties, which can be attributed to the use of pristine carbon nanotubes stapled inside the cavity, should enable coherent spin–spin interaction via cavity photons and compare favorably to the ones recently demonstrated in Si-based circuit QED experiments. Our clean and controlled nano-assembly technique of carbon nanotubes in the cavity could be further improved by purified 12 C growth to get rid of the nuclear spins resulting in an even higher spin coherence.