Quantum Bit Modalities/Architectures

This is an advance summary of a forthcoming article in the Oxford Research Encyclopedia of Physics. Please check back later for the full article. The theory of quantum mechanics provides an accurate description of nature at the fundamental level of elementary particles, such as photons, electrons, and larger objects like atoms, molecules, and more macroscopic systems. Any such physical system with two distinct energy levels can be used to represent a quantum bit, or qubit, which provides the equivalent to a classical bit within the context of quantum mechanics. As such, a qubit can be in a well-defined physical state representing one “classical bit” of information. Yet, it also allows for fundamental quantum phenomena such as superposition and mutual entanglement, making these effects available as a resource. Quantum information processing aims to use qubits and quantum effects to attain an advantage in computation and simulation, communication, or the measurement of physical parameters. Much like the classical bits realized by transistors in silicon are at the foundation of many modern devices, quantum bits form the building blocks out of which quantum devices can be constructed that allow for the use of qubits as a resource. Since the 1990s, many physical systems have been investigated and prototyped as quantum bits, leading to implementations that range from photonics, to atoms and , as well as solid state devices in the form of tailored impurities in a material or superconducting electrical circuits. Each physical approach differs in how the quantum bits are stored, how they are being manipulated, and how quantum states are read out. Research in this area is often cross-cutting between different areas of physics, often covering atomic, optical, and solid state physics and combining fundamental with applied science and engineering. Tying these efforts together is a joint set of metrics that describes the qubits’ ability to retain a quantum mechanical state and the ability to manipulate and read out this state. Examples are phase coherence and fidelity of measurement and operations. Further aspects include the scalability with respect to current technological capabilities, speed, and amenability to error correction.