The theoretical aspects of qubit preparation, readout, single and double qubit gates in both optical and ion-trapped QCs
The talk consisted of two physical models for a qubit : Trapped Ion qubit and Photonic qubit.
Trapped ion qubits are generated by holding ions in a place with electromagnetic traps and using 2 energy levels in an ion from qubit states.
This is done using an oscillating electric field (Paul traps) or static electric and magnetic field, as Earnshaw’s theorem prevents us from doing so, using only static electric fields.
Paul traps consist of strong confinement in 2 directions and a relatively weaker one in the 3rd direction. To quantize the motion in the 3rd direction and to increase the stability of the trapped ion in general, they are cooled. State preparation and readout is very easily done in trapped ion qubit with high fidelity.
Single qubit gates are executed by using a laser for transition. For two-qubit gates, interaction between ions is used; as an example the execution of Controlled-Z was discussed in the talk.
This section was concluded by stating that trapped ion qubits having advantages like high coherence time and fidelity while the disadvantages were that the gates were quite slow and scalability is difficult in this model.
Photonic qubit has two different modes: Polarisation qubit and Path encoded qubit. State preparation in this is a problem as creating a single photon and having it in pure state is difficult. This problem is solved by Spontaneous parametric down conversion (SPDC). Readout seems easy theoretically while it poses a great challenge practically as single photon detection is quite hard. An efficient way is to use superconducting nanowire single-photon detector.
(SNSPD). Although, single qubit gates are performed very easily using linear optical devices such as phase shifters and beam splitters in the case of Path encoded qubits. One of the greatest challenges is to execute a two-qubit gate as photons do not interact easily. KLM scheme uses the fact that photon detection is a non linear phenomenon and makes the implementation of two qubit gates easier.
This section was concluded by discussing the pros like it works on room temperature because of no coherence, easier scaling and its usefulness in the areas of quantum communication, and cons like photon loss at generation and detection and infeasibility of two qubit gates.
Finally, the talk and the mini series was concluded with a brief review of all types of qubits discussed.
Kartik Patekar is a fourth year undergraduate student in the Department of Physics at IIT Bombay