Periodic Reporting for period 1 - ConceptQ (New superconducting quantum-electric device concept utilizing increased anharmonicity, simple structure, and insensitivity to charge and flux noise)
Reporting period: 2022-11-01 to 2025-04-30
Quantum technology is an exciting field where new scientific discoveries have great potential to be used in practical applications such as in quantum computing. Although quantum supremacy has been recently demonstrated in fully superconducting qubits, there is a major challenge in promoting these many-qubit processors feasible for technological applications and advanced science experiments: higher fidelity in a power-efficient control and readout architecture is required. This action ConceptQ aims to demonstrate a new superconducting-qubit concept that has a surprisingly simple structure consisting only of standard materials and a single Josephson junction while providing insensitivity to charge and flux noise, and most importantly large anharmonicity. We combine these properties with new ideas in the pursuit of record-breaking fidelities in quantum-logic gates, in initialization, and in readout. Importantly, we introduce cryogenic active components to implement elementary qubit operations at millikelvin temperatures thus paving the way for a power-efficient integrated quantum-classical control system. Finally, we aim to combine the best new methods and designs for a multi-qubit processors and for a demonstration of a set of algorithms at unprecedent fidelity. With these breakthroughs, we aim to contest the transmon, the standard high-fidelity superconducting qubit, thus boosting quantum-technology research and methodology not only in computing but also in sensing and simulation. This potentially opens horizons for novel scientific discoveries in classical cryoelectronics, quantum calorimetry, open quantum systems, and quantum thermodynamics. ConceptQ is a science project, but thanks the quantum industry, it holds great potential for advancement of global wellbeing, e.g. through envisioned long-term applications in security, quantum chemistry, and artificial intelligence.
In the action ConceptQ, we have worked on all of the work packages: WP1 Design and modeling, WP2 Control, WP3 Readout, WP4 Initialization, and WP5 Many-qubit experiments. We have modeled the unimon qubit and made analysis on ways to improve its coherence. One of our main achievements was clearly the demonstration of the unimon qubit and its single-qubit gates up to 99.9% fidelity, which manifests our first key intermediate result. In qubit control, we also demonstrated subharmonic driving of a transmon that was strongly coupled to its drive line but simultaneously protected against decay to the drive line thanks to an on-chip filter. In qubit readout, we managed to demonstrate single-shot readout of a transmon qubit using a thermal sensor at millikelvin temperature. For qubit initialization, we have utilized a single-junction quantum-circuit refrigerator to induce dissipation to a transmon qubit on demand. We also carried out many-qubit experiments using trimon qubits and wrote a script to implement an arbitrary three-qubit gate.
The unimon qubit is a new qubit type well-received in the community and hence take the field forwards. It is beyond the previous state of the art since it combined simple design and larger anharmonicity than that of transmons. As the unimon is further improved in ConceptQ, it has great potential in making its way to commercial quantum processors, improving their fidelities and hence paving the way for commercially relevant applications of quantum computing. The key needs for such market uptake is the demonstration of high-fidelity two-qubit gates, which we are pursuing during the second half of ConceptQ.
Perhaps the greatest achievement thus far in ConceptQ is the demonstration of single-shot readout of a transmon qubit using a thermal detector at millikelvin temperature. We simply used an ultrasensitive calorimeter to measure the energy of the qubit readout pulse. The single-shot fidelity was only roughly 60% but we have carried out modeling work that suggest that much higher fidelities are possible. Thanks to its small form factor, low pump power, and no need for microwave isolators, the calorimetric readout of qubits is of great interest for the quantum hardware industry. The key needs for the market uptake of this result is the demonstration of readout fidelities of 99.9%, surpassing those of existing techniques.
Perhaps the greatest achievement thus far in ConceptQ is the demonstration of single-shot readout of a transmon qubit using a thermal detector at millikelvin temperature. We simply used an ultrasensitive calorimeter to measure the energy of the qubit readout pulse. The single-shot fidelity was only roughly 60% but we have carried out modeling work that suggest that much higher fidelities are possible. Thanks to its small form factor, low pump power, and no need for microwave isolators, the calorimetric readout of qubits is of great interest for the quantum hardware industry. The key needs for the market uptake of this result is the demonstration of readout fidelities of 99.9%, surpassing those of existing techniques.