🔬 mesoscale physics

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

This paper presents a 3D-integration process using dense indium bump interconnects and Niobium Nitride thin films to successfully fabricate a hybrid circuit-QED device that achieves high-quality resonators, record-fast dispersive charge readout, and strong spin-photon coupling for silicon MOS spin qubits.

Original authors: Sebastien Granel, Frederic Gustavo, Jean-Luc Thomassin, Heimanu Niebojewski, Benoit Bertrand, Frederic Berger, Alain Gueugnot, Chafik Mhamdi, Etienne Dumur, Romain Maurand, Simon Zihlmann

Published 2026-04-29

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CC BY 4.0

Original authors: Sebastien Granel, Frederic Gustavo, Jean-Luc Thomassin, Heimanu Niebojewski, Benoit Bertrand, Frederic Berger, Alain Gueugnot, Chafik Mhamdi, Etienne Dumur, Romain Maurand, Simon Zihlmann

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine you are trying to build a super-fast, super-precise communication system between two very different types of neighbors. One neighbor lives in a high-tech, fragile glass house (the quantum chip, which holds the tiny "spin" particles that act as computer bits). The other neighbor lives in a sturdy, noise-free concrete bunker (the microwave chip, which sends and receives radio signals to talk to the quantum bits).

The problem is that these two neighbors don't get along well if they try to build their houses on the same plot of land. The materials needed for the fragile glass house (semiconductors) create too much "static" and "noise" for the concrete bunker's radio signals to work properly. It's like trying to listen to a whisper in a room full of construction drills.

The Solution: A 3D "Flip-Chip" Marriage
The researchers in this paper came up with a clever way to let these two neighbors live together without ruining each other's work. Instead of building them side-by-side, they built them on top of each other and glued them together.

Think of it like a high-tech sandwich:

The Bottom Bun: A sturdy sapphire wafer holding the super-conducting radio circuits (made of a material called Niobium Nitride).
The Top Bun: The fragile silicon chip holding the quantum dots (the "spin" qubits).
The Filling: Tiny, microscopic pillars made of Indium (a soft, silvery metal) that act as the bridges connecting the two layers.

The "Micro-Pillars" (Indium Bumps)
To connect the top and bottom, they used thousands of tiny indium pillars, each only 5 micrometers wide (about half the width of a human hair).

The Challenge: If these pillars are too big, they act like a heavy anchor, dragging down the speed and clarity of the radio signals. If they are too small or poorly made, the connection breaks.
The Achievement: The team made these pillars incredibly small and precise. They proved that these tiny bridges are almost perfect: 99.95% of them connected successfully, and they conduct electricity with almost zero resistance when cooled down to near absolute zero.

The Results: A Clear Conversation
Once the sandwich was assembled, they tested how well the two chips could talk to each other:

The "Quality" of the Signal: They measured how "clean" the radio signals were. Even with the extra layer of the quantum chip on top, the radio signals remained very clear (a high "quality factor"). This means the "concrete bunker" wasn't ruined by the "glass house" sitting on top of it.
Reading the Charge (The "Whisper"): They tested how well they could listen to the "charge" (the electrical state) of the quantum bits. They achieved a record-breaking speed and clarity. They could hear the "whisper" of the quantum bit in just 300 nanoseconds (that's 300 billionths of a second) with a signal so clear it was 100 times louder than the background noise.
The "Spin-Photon" Dance: Finally, they tried to make the quantum "spin" (the direction of the particle's magnetic needle) dance with the radio waves (photons). Usually, this is very hard to do because the spin is shy and doesn't like to interact with radio waves. But thanks to this new 3D setup, they got the spin and the photon to dance together strongly. The strength of this dance was measured at 75 MHz, which is a very high score in this field.

Why This Matters (According to the Paper)
The paper claims this is a major step forward because it proves you can take a "semi-industrial" silicon chip (the kind used to make regular computer chips) and stack it on top of a super-sensitive quantum radio circuit without ruining the radio.

By using these tiny indium bridges, they created a system that is:

Fast: It can read the state of the quantum bits incredibly quickly.
Clear: The signals are strong and not drowned out by noise.
Scalable: Because the connection method is so small and precise, it opens the door to building much larger, more complex quantum computers in the future.

In short, they built a perfect "elevator" (the 3D stack) that lets a fragile quantum particle and a powerful radio wave meet and talk clearly, without the noise of the building materials getting in the way.

1. Problem Statement

Hybrid circuit quantum electrodynamics (cQED) aims to couple quantum degrees of freedom (such as electron/hole spins in quantum dots) with quantized electromagnetic fields in superconducting microwave cavities. This coupling is essential for quantum computation, simulation, and remote qubit entanglement.

However, a significant bottleneck exists in integrating high-quality superconducting circuits with semiconductor quantum dot devices:

Material Incompatibility: Semiconductor platforms (like Si/SiGe or Ge/Si) often contain complex material stacks (dielectrics, normal metals, spurious charges) that introduce microwave losses, drastically reducing the quality factor ( $Q$ ) of superconducting resonators.
Performance Degradation: In standard 2D integration, quality factors for spin qubits are often limited to a few hundred, whereas high-impedance cavities require $Q > 10^4$ to achieve strong coupling and high-fidelity readout.
Parasitic Capacitance: Conventional 3D flip-chip bonding often uses large interconnect bumps, which add significant parasitic capacitance to ground. This reduces the zero-point voltage fluctuations ( $V_{zpf}$ ), thereby weakening the light-matter coupling strength required for strong spin-photon interaction.

2. Methodology

The authors propose a 3D-integration process using indium bump interconnects to separate the semiconductor quantum dot device from the superconducting microwave circuit.

Architecture:
- Bottom Chip (Microwave): A sapphire wafer hosting a high-impedance Niobium Nitride (NbN) superconducting circuit. NbN was chosen for its high kinetic inductance and magnetic field resilience.
- Top Chip (Qubit): A semi-industrial Silicon MOS (Si-MOS) chip hosting a hole spin double quantum dot (DQD). The fabrication stops at the pre-metal dielectric (PMD) step, with contacts made via tungsten vias.
- Interconnect: The chips are flip-chip bonded using 5 µm × 5 µm Indium (In) bumps with a minimal pitch of 10 µm. This small pitch minimizes stray capacitance to ground, preserving the high impedance of the resonator.
Fabrication Details:
- Both chips feature a 10 nm NbN layer patterned via electron-beam lithography and dry etching.
- Under-bump metallization (UBM) consists of a Ti/Pt/Au tri-layer.
- Bonding is achieved via thermomechanical reflow of the indium bumps, ensuring a planarity of < 0.16 mrad and an inter-chip distance of ~4 µm.
Characterization Strategy:
- DC: Daisy-chain structures to measure galvanic connection yield and resistance vs. temperature/magnetic field.
- RF: Three types of resonators were tested to isolate loss mechanisms: (i) reference resonators, (ii) resonators with a bump at the voltage anti-node (testing dielectric loss), and (iii) resonators with a bump at the voltage node (testing resistive loss).
- Device Operation: A Si-MOS double quantum dot was coupled to a 1.8 kΩ characteristic impedance NbN resonator to measure charge and spin-photon coupling.

3. Key Contributions

High-Yield 3D Integration Process: Demonstrated a reliable flip-chip process using 5 µm indium bumps with a 99.95% galvanic connection yield.
Low-Loss Interconnects: Proved that small indium bumps introduce negligible dielectric loss at GHz frequencies and that resistive losses are minimal when placed at voltage anti-nodes.
High-Impedance Hybrid Device: Successfully fabricated a hybrid cQED device combining a high-impedance NbN resonator with a semi-industrial Si-MOS hole spin qubit, maintaining a high internal quality factor despite the 3D architecture.
Record Charge Readout: Achieved gate-based dispersive charge readout with a Signal-to-Noise Ratio (SNR) of 100 in just 300 ns, significantly faster than previous implementations.
Strong Spin-Photon Coupling: Demonstrated coherent strong coupling between a hole spin and a microwave photon in a 3D-integrated geometry.

4. Key Results

Interconnect Properties:
- Superconductivity: The interconnects become fully superconducting below 3.4 K (In transition), with vanishing resistance ( $< 5$ mΩ).
- Magnetic Field Resilience: The interconnects remain superconducting up to 24 mT (critical field of In). At higher fields (1 T), the resistance saturates at ~300 mΩ, consistent with the normal state of the UBM.
- RF Quality: NbN resonators interrupted by a single indium bump achieved internal quality factors ( $Q_i$ ) above $10^5$ in the single-photon regime.
Device Performance (Si-MOS DQD + NbN Resonator):
- Quality Factor: The integrated device achieved an internal quality factor of $Q_i \approx 10,000$ (one order of magnitude lower than reference chips due to MOS chip losses and DC line photon leakage, but sufficient for operation).
- Charge-Photon Coupling: Measured coupling strength $g_c/2\pi = 350$ MHz. This is close to the theoretical limit, with a slight reduction attributed to the 2.5 fF parasitic capacitance added by the interconnect.
- Charge Readout: Achieved an SNR of 100 in 300 ns (and $10^3$ in 1 µs). This represents a record speed for gate-based dispersive readout, enabling rapid single-shot measurements.
- Spin-Photon Coupling: At a magnetic field of ~250 mT, the device exhibited an avoided crossing with a vacuum Rabi splitting of $2g_s/2\pi = 150$ MHz, corresponding to a spin-photon coupling strength of $g_s/2\pi = 75$ MHz.
- Strong Coupling Regime: Since $g_s \gg \kappa$ (cavity loss rate) and $g_s \gg \gamma_s$ (spin decoherence rate), the system operates in the strong coupling regime.

5. Significance and Impact

Scalability: This work validates that semi-industrial CMOS fabrication processes can be successfully integrated with high-performance superconducting cQED architectures via 3D flip-chip bonding.
High-Fidelity Readout: The demonstrated fast charge readout (300 ns) provides a pathway for high-fidelity, single-shot spin readout, which is a critical requirement for error correction in quantum computing.
Remote Entanglement: The demonstration of strong spin-photon coupling ( $g_s = 75$ MHz) in a 3D-integrated device proves the viability of using microwave photons to mediate entanglement between distant spin qubits, a key step toward modular quantum processors.
Platform Versatility: The approach is not limited to Si-MOS; it is adaptable to other semiconductor platforms (Ge/SiGe, III-V) and hybrid systems (magnons, phonons), offering a general solution to the material incompatibility problem in hybrid quantum systems.

In conclusion, this paper establishes a robust 3D-integration framework that overcomes material-induced losses, enabling high-impedance cQED experiments with semiconductor spin qubits and paving the way for large-scale, high-fidelity quantum computing architectures.

1. Problem Statement

2. Methodology

3. Key Contributions

4. Key Results

5. Significance and Impact

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