Bench Talk

(Source: ZinetroN - stock.adobe.com)

As quantum technology matures, the drive to make the physical architecture more efficient and commercially feasible accelerates. As it stands, there has been a lot of progress on the software side, but the physical architecture is taking a long time to reach commercial readiness, so we won't see it's use in our computers anytime soon.

A number of methods are being trialed for the physical infrastructure. The entanglement of photons gets the most attention, and there’s interest in using semiconductor materials to entangle and transport quantum bit (qubits) in quantum computing technologies. However, photons are not the only way of creating and entangling qubits, electrons and polarized atoms can also be used. Beyond semiconducting photonic infrastructure, designers are creating electron-based qubits and transporting the data held within them via superconducting circuits.

Superconducting circuits are a special type of electrical circuit that has zero electrical resistance. Superconductor materials have zero electrical resistance—due to the overlap of the valence and conduction bands in the electron orbital of superconductor materials—and this frictionless electronic conduction can be harnessed into circuits.

Superconducting Materials

Superconducting materials are one of the leading architectures—alongside semiconductors—for quantum computers. The presence of superconductivity in these circuits is a key, macroscale property that is brought about because of the individual material constituents in the circuit. Superconducting materials exhibit specific nanoscale quantum effects that build together to create a circuit where an electrical current can be passed without any electrical resistance at all.

At the fundamental level, superconducting materials function the way they do because of the sub-atomic interactions in the material. At this level, the charge carriers form a single quantum state, known as a quantum, well (also can be referred to as a potential well). Inside these quantum wells, the electrons become physically confined. However, even though the electrons are physical confined, they are not necessarily electronically confined.

If each quantum well isolated away from any other quantum well, then it will be electronically confined. However, electrons can tunnel, so even if they are physically in one place, their wavefunction can extend beyond their physical reach and outside of the quantum well. So, if you have quantum wells located near each other, then the wavefunctions of each confined electron can overlap and link up with each other. When the electrons inside the quantum wells link up, you can pass an electronic current between them without any resistance because there is zero electrical resistance between quantum states. This is a superconductive current and can be used as a fundamental, mechanistic building block for superconducting circuits.

While the properties of these circuits are beneficial, there are only certain materials that exhibit these features, and there are fewer materials still that can direct the superconducting current in a manner that is more controlled. Because of this, nanowires have become one of the more promising options for superconducting circuits.

Because nanowires are a one-dimensional (1D) material, the electrons are quantumly confined in two spatial dimensions. This means there is one spatial dimension where the electrons can tunnel, which allows the current to flow in one direction, much like a classical circuit. To be effective and functional like classical circuits, this superconductivity needs to be harnessed and controlled—and this is where different quantum components come in and functional superconducting circuits can be created.

Building Superconducting Circuits

Superconducting circuits are seen as a low-power consumption option for transporting superconducting qubits around quantum channels and are seen as one of the potential building blocks of technologies. In terms of the basic architecture and setup, semiconductor circuits are similar to classical circuits and still require many components that classical setups do, including power sources, switches, gates, quantum memory, readouts, etc., that can be built as integrated circuits and form superconducting chips that could power quantum operations. The main difference is that these components need to be able to handle, transmit, and communicate qubits rather than classical bits.

Beyond the different components, a number of different types of superconducting qubit which have been realized can be used to build superconducting circuits. These are phase, charge and flux qubits and systems are being built that include both one and multiple qubit systems. These circuits are still in their infancy, but new ways are being developed to control and manipulate these circuits so that they can store and transfer data, and a range of magnetic fields, electrical fields, and high-energy electron injections are all trying to achieve this aim so that superconducting chips can function in a manner similar to classical setups.

Many of the different components in superconducting circuits resemble, and try to mirror, the effects of their classical counterparts, but with the ability to run quantum algorithms, e.g., switches. One distinguishing feature of superconducting quantum circuits is the use of Josephson junctions, as these are not present in normal, conducting circuits. Josephson junctions are a weak connection (made of an insulator) between two superconducting wires, where the electrons can tunnel from one wire to the next. The Josephson junction allows the wavefunctions on either side to become continuous and prevents any current from exceeding the critical current. Because of this, Josephson junctions are important in many superconducting chip designs as well in the different quantum components as well.

Each type of architecture that has been theorized for quantum technologies has their own inherent advantages and disadvantages. This includes the well-talked about photonic systems, as well as superconducting circuits. As each technology develops, the different advantages and disadvantages will change as new developments are made and as new challenges arrive, so this not likely to be a static talking point.

As it stands, there are number of advantages of using semiconducting circuits as the fundamental building block of quantum technologies. One advantage is on the qubit side because this method of entanglement can produce strongly coupled qubits, and both single and multi-qubit systems can be generated. Moreover, the qubit potential of superconducting qubits can be controlled, and the nature of the single qubits can be used to build a ‘universal’ set of quantum gates.

As far as disadvantages go, the current through a Josephson junction tends to be very small and they can be privy to noise, so more work needs to be done to reduce the noise levels as it is important for semiconducting circuits. Beyond this, the other disadvantage of superconducting circuits is that their coherence is limited by defects in the Josephson junction tunnel barriers and the fabrication of the device can influence the qubit parameters, so care needs to be taken with the fabrication to ensure that the qubit parameters do not vary too much.

One of the main advantages of these systems as a complete circuit, is that scientists and engineers are fabricating them onto chips like classical computers. So, there is potential here for these chips to be scalable. However, there is still work to be done with these superconducting chips to control and interconnect the components without needing to add additional layers to the chip (making them bulkier). So, while it can be seen as advantage, it’s a potential advantage for the future once some of the chip integration issues are solved.

It’s no secret that superconducting circuits, and other quantum architectures for that matter, are still some ways from being commercially feasible for quantum computing. However, if you look at the developments of classical computing systems over the last two or three decades, then it’s not unreasonable to think that basic quantum computers are just around the corner—especially given the amount of work going into them—and in two or three more decades, that we could have high performing quantum computers.

Superconducting circuits offer the potential to facilitate superconducting qubits (from electrons) in quantum channels and offers different advantages and challenges over photon-based architectures. It’ll be interesting to see in the coming years which of these two front-running architectures will come to the fore, whether we will see quantum systems where both architectures are used, or if something else materializes along the way.