Controlling Superconducting Circuits with High-Energy Electrons
31-03-2021 | By Liam Critchley
As society and the high-tech industries strive towards making quantum technologies a reality, several materials are being trialled (individually or together) to make the architecture more efficient and commercially viable. These range from semiconductor materials to superconducting materials, both of which are seen as potential materials for the quantum bits (qubits) that can store and transmit data in quantum computers (and other quantum technologies).
Because superconducting circuits have an ultra-low power consumption coupled with a high speed, they offer a lot of promise as a basic building block for quantum technologies (as well as other types of electronics). While the superconducting circuits can be used much in the same way as classical circuits, there is a drive to create switches that function as a classical switch does in a classical circuit, but because of the nature and behaviour of superconducting circuits, they require a slightly different approach.
Superconducting Circuits
Superconducting circuits are seen as an ideal option for quantum technologies as the superconductivity phenomenon is a macro-scale property brought about because of small-scale quantum effects. Superconductivity at its simplest level is the passing of a charge with zero electrical resistance. At the fundamental level, the charge carriers pair up and form a single quantum state (known as a quantum well), and from here, these quantum wells can link to each other via electron tunnelling. Because this phenomenon is an extension and overlapping of the electrons’ wavefunctions, there is zero electrical resistance between the quantum states, enabling superconductivity to take place.
There are only a few materials that are currently capable of producing such circuits. Metal nanowires are seen as one of the front runners. Metal nanowires are one-dimensional materials, so the electrons only travel in one dimension (e.g., in the x-direction), so you don’t get any loss of electrons in the other 2 dimensions as everything is quantumly confined at this level. So, the ability to tunnel through the nanowires using these quantum properties, alongside the movement of electrons in a single dimension, has made them a promising choice for superconducting circuits. Now, these circuits do need to be controlled, so this is where ‘superconducting switches’ are coming in.
The Current State of Superconducting Circuit Switches
Given the superconducting nature of the circuits, there is a drive to create ‘switches’ that can be electrically tuned between a superconducting and a resistive state (i.e., an on and an off state). Research into metallic nanowires has shown that their superconductivity can be controlled using an applied electrical field, so there has been interest in trying to develop these kinds of switches because only a small voltage is needed.
In terms of what has been developed to date, there are several devices created using an electric field to manipulate the superconductivity, but there have also been Josephson junctions created where quasi-particles are injected into the junction to control the superconductivity (but these have had limited source-drain critical currents). There have also been non-Josephson junction architectures created that use electrical currents, magnetic fields, or heat to try and control the channels in superconducting circuit. A lot of the switches being trialled are 3 or 4 junction components that provide a novel functionality to control the different superconducting channels.
High-Energy Electron Switches for Superconducting Circuits
Recent research from IBM has shown a different approach. Rather than utilising an electric field, the introduction of high energy electrons into the nanowires—in this case, a titanium nitride nanowire—alongside the ability for the electrons to tunnel, and the field emission from the gate electrode, could offer an alternative switch mechanism for superconducting circuits.
This approach aimed to quench the level of superconductivity across the wire, which in turn could control the circuit like a switch. To quench the superconducting circuit, the researchers at IBM injected electrons that were at an energy level several orders of magnitude higher than the electrons in the superconducting circuit.
It was found that the ‘switch’ only needed to utilise relatively few electrons to trigger the generation of a large number of quasiparticles at the switch location. This quasiparticle generation quenched the circuit and weakened the superconductivity. This approach is vastly different to when lower energy electrons were trialled (which have energies closer to the circuit energy), as those approaches didn’t really provide a large suppression of the superconductivity in the circuit.
Moreover, the switching time of the switches was recorded to be less than 100 nanoseconds (ns), however, it’s thought that the switching time could be quicker but will be limited by the recombination time of the quasiparticles—which is just over 100 picoseconds.
The creation of this type of switch shows that you don’t need an applied electrical field to suppress a superconducting current—which is an area that has been subjected to a lot of investigation. There are other options out there where you can use other means (such as electrons) to control a superconducting circuit and shows how wide open the field still is at this moment in time.
While good progress has been made here, it doesn’t necessarily mean that this specific type of switch will make it into quantum technologies (when they manifest on a more commercial level), but it offers a different way of thinking and approaching the challenges of controlling these circuits—which could help in the creation of the final technologies. The ability to quench superconductivity using electrons also offers insights into quasi-particle dynamics and could be a way of investigating other quasiparticle mechanisms in the future.
