Superconducting Qubit Fabrication
Superconducting qubits are built from thin-film metals and Josephson junctions on silicon or sapphire, then operated at millikelvin temperatures.
What a superconducting qubit is
A superconducting qubit is a nonlinear microwave resonant circuit in which the two lowest energy levels serve as the computational states. The nonlinearity comes from one or more Josephson junctions, thin insulating barriers between two superconductors through which Cooper pairs tunnel. This nonlinearity makes the level spacing uneven so a single transition frequency can be addressed without accidentally driving higher states.
Materials and patterning
Devices are fabricated on high-purity silicon or sapphire substrates. Superconducting films, commonly aluminum, niobium, or tantalum, are deposited and patterned by lithography and etching to form capacitor pads and resonators. Josephson junctions are typically made by double-angle shadow evaporation of aluminum with a controlled oxidation step to grow the aluminum-oxide tunnel barrier only a nanometer or two thick. Junction area and barrier thickness set the qubit frequency, so control of these steps is exacting.
Coherence and its enemies
Qubit quality is measured by coherence time, how long a superposition survives. The main enemies are two-level-system defects in surface oxides and interfaces, stray infrared and radio-frequency photons, quasiparticles, and magnetic-flux noise. Fabrication improvements target these directly: cleaner interfaces, better substrate surface treatment, tantalum films with a well-ordered native oxide, and careful materials choices have pushed coherence times from nanoseconds decades ago to hundreds of microseconds and beyond.
Why millikelvin
The qubit transition frequency is a few gigahertz, corresponding to a temperature near 0.1 to 0.2 kelvin. To keep the qubit in its ground state and free of thermally excited photons, the device must be far colder than this, which is why superconducting processors live at the mixing chamber of a dilution refrigerator.
- Josephson junctions provide the required nonlinearity
- Films of aluminum, niobium, or tantalum on silicon or sapphire
- Shadow evaporation forms nanometer-thick tunnel barriers
- Coherence limited by defects, stray photons, and quasiparticles