Physical Qubit

A qubit is the fundamental unit of quantum information. It is the quantum analogue of the classical bit and can be implemented either as a physical qubit or as a logical qubit. Whereas the physical implementation of a bit is a transistor, the qubit physical implementation is the physical qubit. While there is more than one classification of transistor, there is also more than one qubit modality. And as the states of transistors change with the application of different voltages, the states of qubits change with laser pulses, microwave pulses, or by other analog means. One key difference that is often overlooked is that although qubits are, by definition, two-state systems, like bits, some modalities are not limited to two states.

Physical qubits are highly susceptible to environmental noise and are prone to errors to an extent that makes their usage impractical. This has led to considerable research into  

Quantum Error Correction (QEC) and the concept of logical qubits. For more information, ScienceDirect’s “Physical Qubits” page suggests a few books that may be of interest.

What is Physical Qubit?

In the Noisy Intermediate-Scale Quantum (NISQ) era, the term “qubit” is largely synonymous with a physical qubit. A 256-qubit neutral atom quantum computer, for example, has 256 atoms. The ratio is 1:1.

However, digital mode quantum computers can be agnostic to physical qubits. A quantum circuit may be designed inefficiently, and therefore encoding imperfect physical qubits may involve mapping circuit qubits to different physical qubits during transpilation. One reason for mapping is to select physical qubits with lower error rates. A second reason is to select physical qubits that are not only connected, but have connections with low error rates.

It is worth noting that modalities matter. Atoms, for example, are not manufactured, therefore they are identical and perfect and don’t have different calibrations to select from. Atoms also enjoy all-to-all connectivity. Therefore, mapping is not necessary; the atom in the graph is the atom in the array.

Physical Qubit Implementations

Physical qubits can take a number of different forms. The variety highlights that each modality has its strengths and weaknesses, which encourages research into novel modalities. The best-known modalities are:

• Atoms, aka “neutral atoms” or “cold atoms,” which encode information into the physical geometric arrangements of the atoms
• Electron spins, which encode information within the intrinsic spins of individual electrons trapped either within quantum dots, vacuum chambers, or carbon lattice vacancies
• Ions, which encode information within the stable energy levels of the ions, as well as in the collective motions of the ions within the vacuum chamber
• Nitrogen vacancy, aka “NV Center,” which encode information either within the nuclear spins of nitrogen atoms or within the intrinsic spins of electrons in the vacancies
• Photons, which encode information within the paths that photons might travel, as well as in the number of photons that might follow each path
• Quantum dots, which encode information in superpositions of whether or not individual electrons are physically present

• Superconducting qubits, which cryogenically cool electronic circuits to temperatures so low that they have the behaviors of artificially-created atoms
• Topological qubits, which encode information within the patterns created by the physical movements of the qubits

It’s worth noting that not all of these have been realized, and those that have been realized have been implemented at different scales. The largest publicly-available quantum computer, for example, is the 256-atom Aquila, while the largest known NV Center device has two qubits. Topological qubits remain theoretical.

The Difference Between Logical Qubit and Physical Qubit

The primary distinction between logical qubit vs physical qubit is that the former is composed of the latter. Logical qubits are high-level abstractions consisting of physical qubits that implement quantum error correction codes (QECC). As a result, logical qubits have lower error rates, longer coherence times, and make useful quantum computing possible.

For more information, Aanshsavla’s Medium article titled “Physical and Logical Qubits” provides high-level definitions. Caution is advised in selecting sources of information, though, as the answer to a Stack Overflow question “What is the difference between a physical and a logical qubit?” incorrectly describes logical qubits as simulated ideal qubits.