Types of Qubits

Most people hear about superconducting qubits from IBM’s system, however, those are not the only type of qubits. Three common types of qubits are superconducting qubits, photonic qubits, and Rydberg atom qubits.

Superconducting Qubits

These qubits are created by reducing the temperature of a material to make it become a better conductor, in which for some materials the resistance sharply drops to near 0 at a critical temperature. This makes them known as superconductors through this superconductivity property. Electronic circuits made with these superconductors show quantum properties at really low temperatures, which allows them to be used for quantum computing. To maintain these low temperatures, a dilution refrigerator is needed. Furthermore, to interact with the qubits, microwave equipment is used. The combination of these two devices allows the qubits to be kept cold as well as operated with. The main benefit of this type of qubit is that it comes on a chip, just like classical CPUs. Additionally, superconducting qubits are compatible with existing fabrication infrastructure and have high qubits gates and readout (measured by fidelity). However, these qubits do have their drawbacks, including the need to cool the material at microkelvin, which will require bigger dilution fridges with more qubits.

Photonic Qubits

First, photons are quanta of light, meaning that they can be controlled by prisms and silicon photonic chips. For this type of system, beamsplitters and mirrors serve as the quantum gates. However, due to their nature, they are hard to entangle. Compared to superconducting qubits, can be left at room temperature. Additionally, they are compatible with existing fabrication infrastructure as well and have low single-photon error rates. One of the other downsides to using this type of system is the unreliability of single-photon generation.

Rydberg Atom Qubits

Like they sound, Rydberg atom qubits are made with atoms that have only one valence electron (an electron in the outermost shell of the atom). The state of these qubits are changed by moving the valence electron between its initial shell and a shell beyond that. This would mean a superposition would be when the valence electron is between its ground shell and its excited shell. To control these qubits, optical tweezers are used, which focus onto a point and push the atoms. The benefit of this type of system is that they can manage a large number of qubits, have high decoherence time, and are highly customizable with respect to qubit location. However, there are physical crowding limits of single-qubit gates, and they have high error rates for two-qubit gates.

All of these different systems just go to show how much development and research is still needed. It may turn out that future quantum computers may use multiple types of qubits at the same time.