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Development of high-coherence superconducting devices for quantum computing: Fabrication process development, materials analysis, and device characterization
Doktorsavhandling, 2024

One of the main challenges in quantum computing hardware is the limited time over which we can sustain the information encoded in a quantum state: the state is easily perturbed by its environment in a process known as decoherence, leading to a loss of the information stored within.

A major source of decoherence in superconducting circuits are parasitic two-level systems (TLSs), which can couple to the device and act as a source of dielectric loss.

In this thesis, I study the impact of device design, fabrication procedure, and materials properties on the TLS loss of our devices through materials analysis, cryogenic microwave measurements, and simulations.

The devices I study include coplanar waveguide (CPW) resonators, 3D cavity resonators, and aluminium-on-silicon transmon qubits.

In our quantum processor architecture, the CPW resonators used for qubit readout face similar loss sources as the qubits; however, resonators have a faster fabrication and characterization turnaround. Therefore, we use resonators as proxies to investigate the decoherence mechanisms in quantum circuits, and we show how the extracted information can be applied to improve coherence in transmon qubits.

Having identified a dominant source of TLS loss at the substrate-metal interface of our devices, we find that by increasing the grain size of the superconducting films through increasing the film thickness, we can decrease the contribution of intergranular oxide to the TLS loss at this interface. We show that this approach can yield time-averaged energy relaxation times T1 > 200 μs, with the best qubit reaching an average T1 = 270 μs and a highest observed T1 = 501 μs, improving on our previously standard T1 ~ 100 μs.

Studying the performance of our 3D cavity resonators as a function of design, material quality, and fabrication procedure, our improved cavities can reproducibly yield resonance quality factors above 80 million.

We also integrate our quantum processors into a flip-chip architecture to improve the scalability of our devices. We find that our approach does not measurably degrade the performance of the integrated qubits.

Additionally, we study the nonlinearity of the dielectric susceptibility of TLSs through the intermodulation products generated in a resonator driven by two detuned tones. Our analysis method can reconstruct the standard TLS model parameters from a single spectrum measured at relatively high drive powers corresponding to ~1000 photons.

superconducting circuits

Superconducting qubits

TLSs

coherence

two-level system loss

quantum computing

transmon

## Författare

### Janka Biznárová

Chalmers, Mikroteknologi och nanovetenskap, Kvantteknologi

### Mitigation of interfacial dielectric loss in aluminum-on-silicon superconducting qubits

npj Quantum Information,; Vol. 10(2024)

**Artikel i vetenskaplig tidskrift**

### Characterization of process-related interfacial dielectric loss in aluminum-on-silicon by resonator microwave measurements, materials analysis, and imaging

APL Quantum,; Vol. 1(2024)

**Artikel i vetenskaplig tidskrift**

### Intermodulation spectroscopy and the nonlinear response of two-level systems in superconducting coplanar-waveguide resonators

Physical Review Applied,; Vol. 22(2024)

**Artikel i vetenskaplig tidskrift**

### High quality three-dimensional aluminum microwave cavities

Applied Physics Letters,; Vol. 117(2020)

**Artikel i vetenskaplig tidskrift**

### Building blocks of a flip-chip integrated superconducting quantum processor

Quantum Science and Technology,; Vol. 7(2022)

**Artikel i vetenskaplig tidskrift**

We typically associate quantum mechanical phenomena with the minuscule elemental particles. However, quantum mechanics also explains macroscopic phenomena such as superconductivity, where a big chunk of a superconductor consisting of quadrillions of particles can still display quantum mechanical properties. We can also fabricate superconducting devices that enable us to engineer quantum mechanical states through light-matter interactions, which lays the foundation for superconducting quantum computing.

One of the main challenges in quantum computing is that the quantum states used to encode information are very delicate. The energy stored in the device is as weak as that of the single photon, and is very easily disturbed. This disruption of the quantum state, also known as decoherence, scrambles the information stored within. The timescale on which we can keep the quantum state coherent decides how many logical operations we can manage to execute, and therefore the complexity of the algorithms we can execute.

In this thesis, we study the physical mechanisms behind quantum state decoherence in superconducting devices, and explore strategies to mitigate them. By identifying and addressing the dominant sources of decoherence, we improve the coherence lifetimes of our devices, which brings us closer to solving complex problems.

### Infrastruktur

Infrastruktur för kemisk avbildning

Chalmers materialanalyslaboratorium

Nanotekniklaboratoriet

### Ämneskategorier

Den kondenserade materiens fysik

### ISBN

978-91-8103-096-9

Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 5554

### Utgivare

Chalmers

Kollektorn, MC2, Kemivägen 9, Chalmers

Opponent: Dr. Mollie Schwartz, MIT Lincoln Laboratory, USA