That superconductivity is a macroscopic quantum phenomenon is undoubtedly the main message to be drawn from the previous introductory chapter. There we saw that flux quantisation is a remarkable demonstration of the strange behaviour exhibited by macroscopic systems which are governed by fundamental quantum laws. In this chapter we introduce the Josephson effects, the second piece of fundamental physics which has been exploited in the development of superconducting electronics. If anything they provide an even clearer view of the quantum nature of superconductivity, through a decidedly esoteric mix of properties.
That superconductivity is a macroscopic quantum phenomenon is undoubtedly the main message to be drawn from the previous introductory chapter. There we saw that flux quantisation is a remarkable demonstration of the strange behaviour exhibited by macroscopic systems which are governed by fundamental quantum laws. In this chapter we introduce the Josephson effects, the second piece of fundamental physics which has been exploited in the development of superconducting electronics. If anything they provide an even clearer view of the quantum nature of superconductivity, through a decidedly esoteric mix of properties.
Good order is the foundation of all good things. E. Burke, Reflections on the Revolution in France , 1790 Josephson effect Superconductivity: A crash course The transition from the theoretical description of hypothetical building blocks of a quantum coherent device to something which can be actually fabricated and controlled is made much easier by the existence of superconductivity . This phenomenon, roughly speaking, allows a macroscopic quantum coherent flow of electrons in a sufficiently cold piece of an appropriate material by establishing a specific long-range order among them. Due to this one can, for example, use macroscopically different states of a superconductor as quantum states of a qubit, with the obvious advantage over “microscopically quantum” systems (like actual atoms) from the point of view of control, measurement and, last but not least, fabrication of structures with the desired parameters and on the desired scale. The phenomenon of superconductivity – the history of its discovery, experimental manifestations, theoretical explanation, open questions, relevance to other branches of physics, and technological applications – requires a thorough treatment, which can be found in any number of books. For our purposes Tinkham (2004) will provide more than sufficient background. We will not wander into the field of exotic/high-temperature superconductors for two simple reasons: the quantum coherent behaviour of the kind we need to realize qubits or other quantum coherent devices has not yet been properly and/or routinely achieved in these systems; and the fabrication of such devices is not yet reliable enough or even feasible.
In the realm of physics, a pivotal moment occurred six decades ago when Brian Josephson made a groundbreaking prediction, setting in motion a series of events that would eventually earn him the prestigious Nobel Prize eleven years later. This prediction centered around what is now known as the Josephson effect, a phenomenon with far-reaching implications. At the heart of this effect lies the Josephson junction (JJ), a device that has become a linchpin in various scientific applications. This chapter delves into the foundational principles of the Josephson effect and the remarkable properties of JJs. From their role in metrology to their application in radiation detectors, these junctions have ushered in a new era of electronics. Exploiting the unique features of superconductive devices, such as high speed, low dissipation, and dispersion, JJs find today practical implementation in the development of superconductive qubits and nanotechnology applications.
Abstract ‘Quantum matter’ describes how superconductors and superfluids can have properties like those found in the strange quantum world of atoms, electrons, and photons. In quantum theory, properties such as energy cannot have any possible value, but are quantized. Energy comes in discrete lumps. Particles can act like waves, be in more than one place at the same time, and interfere with themselves. The weirdness of quantum theory is captured in the paradox of Schrödinger’s cat, where quantum effects occur on the macroscopic scale. This is realized in a superconductor, where magnetic flux is quantized. Brian Josephson proposed an electrical circuit that is the basis for a Superconducting Quantum Interference Device (SQUID). These devices are of technological significance as they are used in quantum computing, and to make precise measurements of fundamental physical constants and of magnetic field strengths. SQUIDs are now used in metrology, the science of precise measurements, being the basis for the international standard for the volt, the unit of electrical voltage, and are used in quantum computing.
In Chapter 2 we made use of a hypothetical arrangement of a strongly coupled superconducting ring, interrupted by a region of weak superconductivity, to derive the essential features of the Josephson effects. In practice this same arrangement is actually employed as the basis of one of the most useful superconducting devices developed so far, the single-junction, or rf, squid . (As mentioned in the Introduction, squid is an acronym for Superconducting QUantum Interference Device.) Here the essential physics behind the operation of these important devices will be described, and in the following section the practical realisation of an rf squid with its associated readout electronics will be explained. Chapter 4 will treat in a similar way dc squid s, which are related devices possessing two weak links in a single ring, whose mode of operation is rather different.
This chapter explores the evolution of superconducting qubits from traditional superconductors, tracing the scientific and technological milestones that have led to their central role in quantum computing. Beginning with an overview of superconductivity, the Josephson effect, qubits, and their types, the chapter highlights superconducting qubits with their theoretical implementation. It then transitions to the development of modern qubit architectures – charge, flux, and transmon qubits – emphasizing advances in material purity, fabrication, and coherence enhancement. Special focus is given to a short description of fabricating granular aluminum qubits for quantum engineering applications. The chapter concludes with current challenges, including decoherence mitigation, material scalability, and integration with hybrid quantum systems. This work aims to provide readers with a comprehensive understanding of the journey from conventional superconductors to quantum information devices.