Superconducting Quantum Circuits, Qubits and Computing

This paper gives an introduction to the physics and principles of operation of quantized superconducting electrical circuits for quantum information processing.Comment: 59 pages 68 figures. Prepared for Handbook of Theoretical and Computational Nanotechnolog

Physics of the Josephson effect in junctions with ferromagnetic barriers towards quantum circuits and RF applications

Since its first discovering, several key superconducting applications directly use the Josephson effect. The improvement in material science and nanotechnologies allowed to build novel types of hybrid Josephson junctions. A traditional research path first aims at a complete understanding of the processes occurring in hybrid and unconventional Josephson devices, to be integrated in a second stage into real applications, and hopefully in frontier quantum circuits. In my work, I have addressed some key aspects of the physics in Josephson junctions with ferromagnetic barriers (SFS JJs), which fully falls in this category of unconventional junctions. In particular, I discuss the possibility to identify novel self-consistent and complementary protocols for the study of the fundamental physics in a special class of SFS JJs: the tunnel-SFS JJs, which use insulating ferromagnetic or multi-layered insulator-ferromagnet barriers. A special focus is given on the study of the dissipation mechanisms and the unconventional spin-triplet pairing that arises in these novel devices. I here show that the coexistence between tunnel conduction mechanisms and the ferromagnetic ordering in the barrier can be also exploited in quantum coherent devices, such as qubits

Electro-optomechanical equivalent circuits for quantum transduction

Using the techniques of optomechanics, a high-$Q$ mechanical oscillator may serve as a link between electromagnetic modes of vastly different frequencies. This approach has successfully been exploited for the frequency conversion of classical signals and has the potential of performing quantum state transfer between superconducting circuitry and a traveling optical signal. Such transducers are often operated in a linear regime, where the hybrid system can be described using linear response theory based on the Heisenberg-Langevin equations. While mathematically straightforward to solve, this approach yields little intuition about the dynamics of the hybrid system to aid the optimization of the transducer. As an analysis and design tool for such electro-optomechanical transducers, we introduce an equivalent circuit formalism, where the entire transducer is represented by an electrical circuit. Thereby we integrate the transduction functionality of optomechanical systems into the toolbox of electrical engineering allowing the use of its well-established design techniques. This unifying impedance description can be applied both for static (DC) and harmonically varying (AC) drive fields, accommodates arbitrary linear circuits, and is not restricted to the resolved-sideband regime. Furthermore, by establishing the quantized input-output formalism for the equivalent circuit, we obtain the scattering matrix for linear transducers using circuit analysis, and thereby have a complete quantum mechanical characterization of the transducer. Hence, this mapping of the entire transducer to the language of electrical engineering both sheds light on how the transducer performs and can at the same time be used to optimize its performance by aiding the design of a suitable electrical circuit.Comment: 30 pages, 9 figure

Cavity Magnonics

Cavity magnonics deals with the interaction of magnons - elementary excitations in magnetic materials - and confined electromagnetic fields. We introduce the basic physics and review the experimental and theoretical progress of this young field that is gearing up for integration in future quantum technologies. Much of its appeal is derived from the strong magnon-photon coupling and the easily-reached nonlinear regime in microwave cavities. The interaction of magnons with light as detected by Brillouin light scattering is enhanced in magnetic optical resonators, which can be employed to manipulate magnon distributions. The cavity photon-mediated coupling of a magnon mode to a superconducting qubit enables measurements in the single magnon limit.Comment: review article, 54 page

Microwave Quantum Memristors

We propose a design of a superconducting quantum memristive device in the microwave regime, that is, a microwave quantum memristor. It comprises two linked resonators, where the primary one is coupled to a superconducting quantum interference device (SQUID), allowing the adjustment of the resonator properties with an external magnetic flux. The auxiliary resonator is operated through weak measurements, providing feedback to the primary resonator via the SQUID and establishing stable memristive behavior via the external magnetic flux. The device operates with a classical input signal in one cavity while reading the response in the other, serving as a fundamental building block for arrays of microwave quantum memristors. In this sense, we observe that a bipartite setup can retain its memristive behavior while gaining entanglement and quantum correlations. Our findings open the door to the experimental implementation of memristive superconducting quantum devices and arrays of microwave quantum memristors on the path to neuromorphic quantum computing.Comment: 9+6 pages, 10 figure

QUANTUM KEY DISTRIBUTION LABORATORY DEMONSTRATION

Quantum key distribution (QKD) is a method of secure key distribution which provides protection against the tampering and interception of information. Following the Bennet-Brassard 1984 (BB84) protocol of QKD, we select randomly from a set of bases in which to produce polarized photons and send the photons to a receiver, who measures them in a basis randomly selected from the same set. The fact that quantum mechanics prohibits the exact copying of a photon ensures that any eavesdropper who intercepts, measures, and attempts to pass the photons on to the receiver will be unable to faithfully reproduce that signal. The presence of the eavesdropper can then be detected, prior to any exchange of information, by an examination of the error rate between portions of the keys generated by the sender and receiver. Using a biphoton source, we have constructed a QKD system for use in research towards naval applications.Lieutenant, United States NavyApproved for public release. Distribution is unlimited