A leading platform for quantum information processing are superconducting circuits, which can now comprise tenths of qubits. One of the key challenges in the design of these systems is balancing the tradeoff between access to the qubits for readout and control versus environment induced decoherence. In other words, the same lines used to access the information on the system, unavoidable induce losses and destroy the quantum information store on it. To mitigate this issue, nonreciprocal devices are included between the superconducting qubits and the control electronics. Commercially available microwave nonreciprocal devices are bulky, and impractical for scaling these systems to the scales necessary for the implementation of useful quantum computing algorithms. For this reason, efforts are being directed to the development of on-chip microwave nonreciprocal devices. However, due to the large scale of these systems, their design and modelling is an ongoing challenge. In this thesis, we show how effective dynamical models of superconducting qubits can be obtained when these are coupled by nonreciprocal linear responses, extending the previous work to include on-chip nonreciprocal devices, and more formally all kinds of effectively linear lossless microwave devices. Notably, our formalism quantitatively captures the tradeoff between control and dissipation induced by coupling to the external electronics. In addition, we provide a modern presentation of the theoretical background underpinning circuit synthesis and modelling. Additionally, while scaling to thousands of qubits is necessary for the implementation of useful quantum computing algorithms, this is not the only route where quantum information processing systems can find applications in other fields. In particular, quantum metrology offers a venue where only a few well-tuned qubits and oscillators are needed for a quantum sensing advantage. Examples where these advantages have been realize include dark-matter searches, satellite-free navigational systems, imaging of biological tissues, among others. Despite their promise, quantum sensors must balance the tradeoff between sensitivity to external perturbations and robustness against noise sources. A pathway to limit the detrimental effects of noise and maximize the achievable sensitivity is given by reservoir-engineering and quantum error-correction. Thus, the development of quantum sensors robust against noise is a promising venue with broad applications. In our work, we show how recent protocols developed for the stabilization of bosonic codes can be adapted for quantum sensing. In particular, we show how stabilizing protocols for the Gottesman-Kitaev-Preskill code can be adapted for the quantum sensing of displacements. Quantum sensing of displacement may find applications in for example electric field sensing, force sensing and gravitational waves sensing. Remarkably, the protocol we study nearly saturates the sensitivity allowed by quantum mechanics.
Quantum metrology exploits quantum states to achieve an estimation sensitivity beyond classical limits. In the continuous-variable (CV) regime, the squeezed state has been used to implement deterministic quantum sensing, but the quantum metrology sensitivity of this state is significantly affected by losses or detection inefficiencies, which restrict its applications. In this work, quantum distributed sensing is proposed using optical parametric amplified multimode entanglement generated from squeezed states. It is found that the sensitivity is robust to loss or detection inefficiency when large-gain optical parametric amplification (OPA) is introduced, where a two-mode Einstein--Podolsky--Rosen-entangled state and a four-mode cluster state are exploited for analysis. The quantum sensitivity is greatly improved compared to that without OPA in almost all loss scenarios. The quantum Fisher matrix is calculated for both states to obtain the optimal bound in comparison with our scheme, and it is found that even with a small or moderate OPA gain, quantum sensing can be improved compared with the traditional scheme. The states are also compared with corresponding single-mode squeezed states, finding the parameter ranges where entangled states perform better. This study provides a method for realizing large-scale quantum metrology in real-world applications despite losses or detection inefficiencies.
Quantum sensing promises measurement precision beyond classical limits, but its practical realization is often hindered by decoherence and the challenges of generating and stabilizing entanglement in large-scale systems. Here, we experimentally demonstrate a scalable, scrambling-enhanced quantum sensing protocol, referred to as butterfly metrology, implemented on a cross-shaped superconducting quantum processor. By harnessing quantum information scrambling, the protocol converts local interactions into delocalized metrologically useful correlations, enabling robust signal amplification through interference of the scrambled and polarized quantum states. We validate the time-reversal ability via Loschmidt echo measurements and quantify the information scrambling through out-of-time-ordered correlators, establishing the essential quantum resources of our protocol. Our measurements reveal that the sensing sensitivity surpasses the standard quantum limit (SQL) with increasing qubit number, reaching 3.78 in a 9-qubit configuration, compared to the SQL of 3.0. The scheme further exhibits inherent robustness to coherent control errors and probed signal noise. This work demonstrates a readily scalable path toward practical quantum sensing advantages with prevalent experimental platforms.
Quantum sensors and metrology harness quantum phenomena—such as superposition, entanglement, and squeezing—to achieve precision measurements beyond classical limits. This review presents an overview of the fundamental principles underlying quantum sensing, including quantum coherence, entanglement-assisted measurement, and quantum-enhanced interferometry. Various quantum sensor platforms are discussed, including atomic clocks, nitrogen-vacancy (NV) center magnetometers, superconducting quantum interference devices (SQUIDs), optomechanical sensors, and trapped-ion systems. We explore their operational mechanisms, performance metrics, and technological implementations. Comparative analyses highlight sensitivity, spatial resolution, bandwidth, and environmental robustness. Advances in materials, control techniques, and hybrid architectures are examined, and emerging applications in navigation, geophysics, medical imaging, and fundamental physics are reviewed. Finally, future prospects for scalable and robust quantum metrology are discussed.