Materials for 2D and 3D quantum devices
Our goals: Understand and mitigate the key limiting mechanism of coherence in superconducting qubits, such as losses in two-level systems in oxides, non-thermal quasiparticles, and the bulk substrate.
- Demonstrate a jump of at least an order of magnitude in coherence times with superconducting 2D transmons with coherence up to miliseconds, along with a decrease in performance spread to enable the high coherence multiqubit processors.
- Demonstrate SRF cavities with coherence times up to tens of seconds.
The SQMS collaboration has launched the largest systematic investigation into the origin of decoherence of materials. By studying performance differences of state-of-the-art qubits with the world’s most advanced characterization techniques, together with superconducting and materials modeling efforts, the Center is building a hierarchy of loss mechanisms that informs how to fabricate the next generation of high-coherence qubits and processors.
As an example, through a three-dimensional analysis of the qubit at the atomic level, SQMS scientists used TOF-SIMS for the first time to reveal impurities such as oxygen, hydrogen, carbon, chlorine, fluorine, sodium, magnesium, and calcium. TOF-SIMS, or time-of-flight secondary ion mass spectrometer, fires ions at a qubit and chips away at it. The ions are analyzed with part-per-million accuracies. With the employment of this technique and of electron microscopy, for the first time applied at cryogenic temperatures, we have identified new features such as nano-hydride formation as one of the factors that contribute to short coherence times. Various other advanced characterization techniques applied to state of the art transmon qubits have shed light on several important sources of decoherence in oxides, interfaces and substrates.
The Nanofabrication Taskforce
The knowledge advances in sources of decoherence from the materials’ efforts guide the development of new processes for quantum device fabrication. SQMS has established the first national nanofabrication Taskforce, bringing together qubits, fabrication experts from Fermilab, NIST, Northwestern, and Rigetti Computing, making devices with new standard processes, and verifying the reproducibility of results across four different state-of-the-art foundries. The first results include newly developed innovative fabrication techniques, such as niobium surface encapsulation and silicon surface passivation, which have systematically improved transmon qubits’ performance to world-leading values.
The Round Robin Experiment
SQMS has launched the Round Robin experiment, the first international qubit chips exchange to study and compare the performance of identical qubits at different test sites worldwide. The collaboration has worked in the first two years to streamline test setups, control systems, qubit fixtures, and packaging and measurement protocols. The underground INFN facilities allow for comparison to above-ground for possible effects of environmental radiation, including possible radioactivity coming from the chip package and fridge environment. Combining measurements with advanced diagnostics, we are shedding new light on the origin of decoherence and performance variations, both intrinsic and extrinsic.
