MetSuperQ
MetSuperQ is a project focused on the metrological investigation of quasiparticles in superconducting quantum processors. Superconducting qubits are among the most promising platforms for practical, error-corrected quantum computing, but their performance is fundamentally limited by quasiparticle poisoning — the presence of unbound electrons (Bogoliubov quasiparticles) in the superconducting films from which the circuits are made.
Quasiparticle Poisoning
In a superconductor at millikelvin temperatures, electrons form Cooper pairs with a binding energy of about one millielectronvolt. Ambient radiation — infrared photons, stray microwave fields, or high-energy particles such as cosmic rays and natural radioactivity — can break these pairs and generate quasiparticles. When a quasiparticle tunnels across the Josephson junction of a qubit, it interacts with the trapped quantum state: it can absorb energy from the electric field, causing the qubit to relax (T1 error), or shift the qubit's transition frequency, causing dephasing (T2 error).
Particularly problematic are high-energy events, such as cosmic ray impacts on the chip substrate, which generate phonon showers that break Cooper pairs across large areas. This can create correlated errors affecting multiple qubits simultaneously — a critical challenge for error correction codes that assume independent errors.
Qubits as Quasiparticle Sensors
While quasiparticles are detrimental to quantum computing, the sensitivity of superconducting qubits to single quasiparticles also makes them uniquely powerful sensors. In MetSuperQ, we exploit charge-sensitive qubit designs to detect and characterize individual quasiparticle tunneling events. This allows us to study the sources, dynamics, and spatial distribution of quasiparticle backgrounds in quantum computing setups with unprecedented precision.
Our measurements provide insights into the infrared photon environment inside dilution refrigerators, the effectiveness of shielding and filtering strategies, and the fundamental physics of quasiparticle generation and recombination in superconducting thin films. These results directly inform the design of next-generation quantum processors with improved coherence times.
Context
This work is carried out at the Quantum Device Lab of ETH Zurich as part of the ETHZ-PSI Quantum Computing Hub. The project combines techniques from superconducting circuit design, nanofabrication, and cryogenic microwave measurements with concepts from particle and detector physics.