Superconducting qubits

Electrical current flows through electrical circuits like water through a pipe. However, this “pipe” isn’t actually hollow. It is, in fact, a conductive material, usually metallic, that consists of atoms. The electrons in the electrical current pass from atom to atom, but they also collide with some atoms along the way. These collisions with atoms result in the transfer of kinetic energy away from the electrons, thus slowing the electrons down. This is known as “resistance.”

The amount of electrical resistance that the electrons encounter depends on the length, cross-sectional area, and temperature of the conductor. We often add resistance to electrical circuits intentionally in order to direct the flow of current, protect against surges in voltage, allocate correct voltages, and more.  

But something interesting happens when a conductor is cooled below what is called its transition temperature. The material now conducts electricity with zero resistance, a phenomenon known as superconducting.

What is Superconductor

Without resistance, the electrical current in a loop can continue indefinitely without a power source and without loss to heat. This usually happens at temperatures close to absolute zero, however materials have been discovered that transition at relatively high temperatures. And although superconduction does not generate heat, it does, however, generate exterior magnetic fields. These magnetic fields allow the emergence of superconducting qubits and superconducting quantum computing.

The Emergence of Superconducting Qubits

A qubit is the fundamental unit of quantum information, and quite a few modalities are currently being researched and developed. A superconducting computer uses these superconducting loops as its qubits. They exhibit the behavior of atoms, and are thus often referred to as “artificial atoms.” A superconducting qubit can be in a ground state, an excited state, and up until measurement, a superposition of both states. Some are capable of being in higher excited states, although they become known as qutrits or qudits at those levels.

For a deeper technical understanding of superconducting qubits, feel free to download and read:

• “A Quantum Engineer’s Guide to Superconducting Qubits” by a team from the Massachusetts Institute of Technology, Chalmers University of Technology, and MIT Lincoln Laboratory
• “Introduction to superconducting qubits” by Yuriy Makhlin from the Landau Institute for Theoretical Physics
• “Superconducting Circuit Companion—an Introduction with Worked Examples” published in PRX Quantum by a team from Aarhus University

Advantages and Challenges in the Application of Superconducting Qubits

Although superconducting qubits offer a number of advantages over other modalities, they also face some significant challenges.  

Some of the advantages are:

• The first electrical circuits date back more than two centuries.  
• The dilution refrigeration necessary to sufficiently cool them dates back many decades.
• The theory also has several decades of maturity.
• Because they are fabricated, they can be configured to exhibit specific properties.
• A quantum superconductor has useful applications beyond quantum computing.

And some of the challenges are:

• Because they are fabricated, they can exhibit high error rates and even outright fail.
• Although they may look identical, no two superconducting qubits are actually identical.
• Because of the error rates, considerable quantum error correction (QEC) is required.
• Coherence times are relatively short, limiting the volume of computation possible.
• Considerable shielding is required to protect against environmental noise.

Because they use existing technologies, superconducting quantum computers are probably the most abundant. And with size measured in qubit counts, they are the second largest quantum computers, behind only neutral atom quantum computers. However, two of the above challenges are particularly significant. First, superconducting qubits or their wiring can be so defective that they outright fail. And, second, the shielding they require is physical, which makes superconducting quantum computers the second largest physically, behind ion trap quantum computers.