Qubits, or quantum bits, are the fundamental units of quantum information. Unlike classical bits, which can be either 0 or 1, qubits can exist in a superposition of both states simultaneously. This property, along with quantum entanglement (where qubits become interconnected and the state of one instantly affects the state of another, regardless of distance). The principles of quantum mechanics govern the behavior of qubits, and harnessing their unique properties gives quantum computers their potential power.
Qubits (Quantum Bits) are programmable, just as their classical bit is programmable. Quantum Programming languages enable users to program and manipulate the state of a qubit or multiple qubits. Quantum programming and software development has become a nascent but growing industry with various quantum programming languages on offer. Some of the most popular today are Qiskit, Q#, and Cirq, and which one best suits the needs of the programmer. Many use a Bloch-Sphere to better under the Qubit.
There isn’t just one qubit type, there isn’t just one technology, there are multiple. Below, we introduce the different types of qubits that researchers are working on. We have also looked at qubit growth, which charts the evolution of the different types of qubit and the qubit numbers that are now achievable.
Superconducting Qubits
Superconducting qubits are among the most widely researched and used in quantum computing today. They are tiny circuits made from superconducting materials, which can carry an electric current without resistance. When cooled to extremely low temperatures, these circuits exhibit quantum mechanical effects. Companies like IBM, Google, and Rigetti Computing have been at the forefront of developing quantum computers based on superconducting qubits. The advantage of this approach is the relatively advanced state of superconducting technology and the ability to scale up by adding more qubits. However, they require extremely cold environments (close to absolute zero) to function, which presents challenges in terms of practicality and error rates.
Trapped Ion Qubits
Trapped ion qubits are based on individual ions trapped and isolated in an electromagnetic field. Lasers are then used to perform quantum operations on these ions. The advantage of trapped ion qubits is their long coherence times, meaning they can maintain their quantum state for a relatively long duration without being disturbed. This can potentially lead to more accurate quantum computations. Companies like IonQ are pioneering the development of trapped ion quantum computers. One of the challenges with this approach is scaling up the system, as adding more ions to the trap can increase complexity.
Topological Qubits
Topological qubits are a more recent and less developed approach to quantum computing but hold promise due to their inherent error resistance. They rely on anyons, particles that exist only in two-dimensional spaces. When these anyons are braided around each other, they form stable quantum states that can be used for computation. The advantage of topological qubits is that they are less susceptible to local errors, as the information is stored non-locally in the braiding of anyons. Microsoft has been a significant proponent of this approach. However, the practical realization of topological qubits remains a significant challenge.
Photonic Qubits
Photonic qubits utilize the quantum properties of photons, the elementary particles of light, for quantum computation. In this approach, quantum information is encoded in properties of photons such as polarization. Photonic quantum computers have the advantage of being inherently fault-tolerant and can operate at room temperature. Companies like Xanadu Quantum Technologies are exploring this approach. The challenge with photonic qubits lies in effectively manipulating and entangling photons for complex computations.
Semiconducting Qubits
Semiconducting qubits are made using semiconductor materials, typically silicon or gallium arsenide. Unlike classical bits that can be either a 0 or a 1, qubits can be in a superposition of both states simultaneously. This superposition allows quantum computers to process a vast amount of information at once. Intel is using its prowess in semiconductors to explore quantum computing using its vast experience.
Single Spin Qubits rely on the spin of a single electron in a quantum dot (a small, confined region in a semiconductor). The quantum state of the qubit is represented by the electron’s spin direction, either up or down. Single-spin qubits are advantageous because they can be controlled using electric fields and can be integrated into existing semiconductor fabrication techniques.
Hybrid Qubits combine the properties of both charge qubits and spin qubits. They utilize the charge of electrons and their spin to encode quantum information. This hybrid nature allows for faster gate operations and potentially longer coherence times.
Summary
There is no clear winner in the race for the best qubit technology. Just like the early days of classical computing, there were a variety of technologies that eventually gave way to the silicon revolution. But the fundamentals didn’t change: bits were at the epicenter as the universal computation unit.