Physics of Qubits

In this section, we’ll describe the types of physical qubits being deployed in today’s quantum computers. The discussion will be primarily qualitative because understanding the details requires substantial background in physics, materials science, and engineering. With the rapidly developing technology in QC, what we write here will most likely be out of date within a year or two. Moreover, as we mentioned before, most QC users will not need to know those details, any more than the typical classical computer user needs to understand the details of the semiconductor devices used in their laptops and smart phones. Of course, the details are important if you want to build a QC. In fact, there are many fascinating and challenging issues to be addressed to build fully functioning quantum computers and, as we mentioned before, lots of great jobs in the labs and businesses that are designing and building those machines.

Before looking at specific systems, let’s talk about what is needed in a QC qubit. To me, it is amazing that we can controllably manipulate individual atoms or other atomic-like particles. Transistors on silicon wafers in classical computers are small, but the atomic world is even smaller. Of course, a qubit must be a quantum object whose state can be manipulated into superposition states and changed via gates by means of external, controllable stimuli. These stimuli can be laser light or other electromagnetic fields applied at different places and times in the circuit. These quantum objects must be available in large enough quantities and have nearly identical properties to make a useful system. As mentioned earlier, the largest general-purpose QC systems at this time have about 50 qubits, but that number is likely to increase dramatically over the next few years.

The qubits must be able to be measured. In most practical QCs, the measurement bases stay fixed, and we apply various gates to transform the states of the qubits. Using different basis states is a pathway to extracting the maximum information from a quantum system. Importantly, the objects must be robustly immune to unwanted outside influences to limit changes in those states. While we can use error-correction protocols as we discussed earlier, there are limits to the fixes, and it is more efficient, when possible, to reduce the effects due to undesirable outside influences.

The two types of qubits currently used in general-purpose quantum computers are superconducting circuits and trapped atomic ions. In both these systems, the qubits are acted upon and measured through interactions with electromagnetic fields, making use of the tremendous advances in lasers and microwave devices developed over the past 50 years or so.