Creating a low voltage to represent a logical “0” and a high voltage to represent a logical “1” is straightforward.

🤷‍♂️ Creating a superposition of low and high voltages however is not.

A quantum computer is commonly envisioned to be a machine that exploits the full complexity of a many-particle quantum wavefunction to solve computational problems [LJL+10].

• The current state of quantum computing technologies is summarised by the keywords Noisy Intermediate-Scale Quantum (NISQ).
• NISQ computers are subject to substantial error rates and has a limited number of qubits [LLSK22, Sec. 1].

For the construction of quantum computers, laser serves as an inspiration because it is quantum mechanics that enables laser waves to be generated in phase [LJL+10].

Just as there are many possible materials for lasers (e.g., crystals, organic dye molecules, semiconductors, free electrons), there are many materials under consideration for quantum computers; see [LJL+10] and [MM12, Ch. 6].

Quantum bits are often imagined to be constructed from the smallest form of matter, e.g., an isolated atom, through ion traps and optical lattices, but they can also be made in components far larger than consumer electronics, e.g., a superconducting system [LJL+10].

Below we discuss three main technologies [LLSK22, Sec. 6]: 1️⃣ trapped ions, 2️⃣ photonics, and 3️⃣ superconducting qubits.

Photonics: Photonics has always been a prominent candidate for realising qubits [LLSK22].

For generating qubits, photonics offers the following advantages [SP19, PAB+20, LLSK22]:

• Photons are clean and decoherence-free quantum systems for which single-qubit operations can be easily performed with high fidelity, making photons a flagship system for studying quantum mechanics and developing quantum technologies.
• Quantum entanglement, teleportation, QKD, and early quantum computing demonstrations were pioneered in photonics because photons represent a naturally mobile and low-noise system with quantum-limited detection readily available.
• The quantum states of individual photons can be manipulated with high precision using interferometry, an experimental staple that has been under continuous development since the 19th century.
• The ability to generate large numbers of photons and the development of integrated platforms, improved sources and detectors, novel noise-tolerant theoretical approaches render photonics a leading contender for both quantum information processing and quantum networking.
• Nowadays, photonic quantum computing represents a promising path to medium- and large-scale processing.
• Photonics is the primary technology for realising quantum communications.

Superconducting qubits: They are currently the leading contenders in the race for large-scale quantum computing [LLSK22]. Superconducting qubits are the technology big-tech companies like Google and IBM have been focusing on.

In 2019, a large research team consisting of Google and multiple American and European universities demonstrated “quantum supremacy” on a programmable superconducting quantum processor called “Sycamore”, which consists of a two-dimensional array of 54 transmon qubits [AAB+19]:

• In the superconducting circuit of Sycamore, conduction electrons condense into a macroscopic quantum state, such that currents and voltages behave quantum-mechanically.
• Transmon is short for “transmission-line shunted plasma oscillation”.
• The employed transmon qubits can be thought of as nonlinear superconducting resonators at 5-7 GHz, and each qubit encodes the two lowest quantum eigenstates of the resonant circuit.
• Each qubit is connected to its four neighbouring qubits using an adjustable coupler for tuning inter-qubit coupling. 💡 Coupling qubits is essential for implementing two-qubit gates.
• Each qubit is also connected to a linear resonator used to read out the qubit state.
• Sycamore’s record might have been broken by China’s Zuchongzi in 2021 [Cho21].
• In 2023, Google demonstrated quantum supremacy again with an increased qubit count of 70 [MVM+23].
• IBM is slated to launch its 1121-bit NISQ computer called Condor.