Cryogenic capacity is a key bottleneck in the scaling of superconducting quantum processors, critically impacted by control wiring. Currently, wiring architectures are often designedfollowing a small set of „best-practice“ heuristics, little changed since they emerged for early, small-scale systems, and not rigorously evaluated for wider contexts. Here, we adopt a whole-system numerical modelling approach and present a systematic optimisation for the coaxial wiring design of a superconducting quantum processor. We identify optimised configurations that substantively outperform conventional designs in device temperature, noise burden, qubit capacity and other practical metrics. Importantly, our analysis delivers varied outcomes for different system contexts and operating regimes, illustrating the benefits of a systematic approach when there is no universal solution. Our results evaluate and sometimes challenge conventional wisdom in relation to design factors such as the utility of 0dB attenuators. We also use a wide exploration of cable configurations—directly enabled by our holistic and flexible numerical modelling approach—to develop more rigorously supported general principles for future cryogenic system design, based on a new conceptual framework to describe attenuator cascades that draws on analogies to low-noise amplifier chains. Our approach, design outcomes, and generalised analyses should also be readily applicable to other cryogenic quantum computing platforms. Finally, we support ready adoption of this approach through a web-based graphical tool which streamlines the analysis of the proposed framework under different contexts. Our results illustrate how our approach can help maximise the available computational resources of quantum processors at all system scales.
Quantum physics cannot be reconciled with the classical philosophy of noncontextual realism. Realism demands that system properties exist independently of whether they are measured,while noncontextuality demands that the results of measurements do not depend on what other measurements are performed in conjunction with them. The Bell-Kochen-Specker theorem states that noncontextual realism cannot reproduce the measurement statistics of a single three-level quantum system (qutrit). Noncontextual realistic models may thus be tested using a single qutrit without relying on the notion of quantum entanglement in contrast to Bell inequality tests. It is challenging to refute such models experimentally, since imperfections may introduce loopholes that enable a realist interpretation. Using a superconducting qutrit with deterministic, binary-outcome readouts, we violate a noncontextuality inequality while addressing the detection, individual-existence and compatibility loopholes. Noncontextuality tests have been carried out in a range of different physical systems and dimensionalities, including neutrons, trapped ions and single photons, but no experiment addressing all three loopholes has been performed in the qutrit scenario where entanglement cannot play a role. Demonstrating state-dependent contextuality of a solid-state system is also an important conceptual ingredient for universal quantum computation in surface-code architectures, currently the most promising route to scalable quantum computing.
The new and rapidly growing field of circuit QED offers extremely exciting prospects for learning about and exercising intimate control over quantum systems, providing flexible, engineerabledesign and strong nonlinearities and interactions in systems consisting of microwave radiation fields and fixed artificial „atoms“. These notes aim to provide a non-expert introduction to the field of circuit QED, to give a basic appreciation of the promise and challenges of the field, along with a number of key concepts that will hopefully be useful for the reader who is new to the field and beginning to explore the research literature. They were written as a pedagogical text designed to complement a course delivered to third-year undergraduate students.
After a introductory section which discusses why studying circuit QED might be worthwhile and interesting, I introduce the basic theory tools from quantum optics and quantum information which are needed to understand the key elements of circuit QED. I also provide a brief overview of superconductivity, focussing on the concepts which are most relevant to operation in the regimes of interest in circuit QED. I then describe the three main types of superconducting qubits, and finally give a basic introduction to decoherence and mixture and how they relate to quantum behaviour in electronic circuits.