What are the different types of qubits used in quantum computers and their advantages/disadvantages?

The different types of qubits used in quantum computers include superconducting qubits, trapped ion qubits, topological qubits, and photon qubits. Superconducting qubits are the most widely used and offer relatively simple fabrication techniques, but suffer from susceptibility to noise and decoherence. Trapped ion qubits have long coherence times and high fidelity operations, but are challenging to scale up due to intricate control requirements. Topological qubits harness anyons for fault-tolerant computing but are currently still under development. Photon qubits offer long communication distances and low error rates but face challenges with regard to Gaussian states generation and detection.

Long answer

Quantum computers rely on quantum bits or qubits as the fundamental units of information processing. Several different types of physical systems have been explored to implement qubits.

1. Superconducting qubits: These are tiny circuits made from superconducting materials that can carry electrical currents without resistance when cooled at extremely low temperatures. Superconducting qubits offer relatively simple fabrication techniques compatible with existing semiconductor technology, enabling potential large-scale integration of many-qubit systems. However, they are highly susceptible to noise from various environmental sources such as thermal fluctuations and electromagnetic fields, which limits their coherence times and introduces errors in computations.

2. Trapped ion qubits: In this approach, individual ions (charged atoms) are confined using electromagnetic fields in a vacuum chamber. The internal electronic states of the ions serve as the information carriers or qubits. Trapped ion qubits exhibit very long coherence times due to exceptional isolation from the environment and can achieve high-fidelity gate operations required for fault-tolerant quantum computing. However, scaling up trapped ion systems is challenging due to complex control requirements involved in individually manipulating each ion.

3. Topological qubits: These quantum bits rely on exotic particles called anyons that arise in certain types of two-dimensional systems known as topological materials. Topological qubits possess properties that make them immune to certain types of errors, making them robust against decoherence. However, implementing topological qubits is still a nascent field, and considerable effort is required to fabricate and stabilize the material structures needed for their realization.

4. Photon qubits: These qubits are encoded in the states of individual photons or light particles. Photon qubits offer several advantages such as potential for long-distance communication, low inherent error rates, and compatibility with existing fiber-optic technology. However, generating and detecting single-photon states with high fidelity remains challenging, impeding the development of large-scale photonic quantum computing systems.

In conclusion, different types of qubits have their own strengths and limitations. Superconducting qubits are widely used but suffer from noise susceptibility; trapped ion qubits exhibit long coherence times and high fidelity operations but face scaling issues; topological qubits offer promising error-correcting properties but are still under development; photon qubits have advantages in communication and low error rates but face technological challenges in high-fidelity generation and detection of single photons. The choice of qubit type depends on the specific requirements of the quantum computing application at hand.