# Qubit

## Summary


In classical computing and digital communications, a bit is the most basic unit of information. Similarly, a qubit or quantum bit is the basic unit of quantum information in quantum computing. A qubit is the quantum version of the classic binary bit physically perceived by a two-state device. 

A quantum computer can utilise a variety of basic particles such as electrons or photons. In fact, ions accomplish success through their charging or separation, which serves as a symbol of 0 and/or 1. A qubit is a unit of measurement for each of these particles. The character and conduct of these particles, as demonstrated in quantum theory form the basis of quantum computing. The two most important aspects of quantum physics are the principles of Superposition and Entanglement.

## Discussion

### What is the physics of Qubit?

**Qubit is a two-dimensional quantum-mechanical system (or two-dimensional) system**, one of the simplest quantum systems that reflects the rarity of quantum mechanics. Examples include electron rotation where two levels can be considered as spin up and spin down; or polarization of a single photon in which two regions can be considered as direct polarization and horizontal polarization. 

In the classical system, the bit will have to be in 0 or 1. However, quantum mechanics allows qubit to occupy a high degree of coherence for both regions simultaneously, a basic position in quantum mechanics and quantum computing. 

To create a qubit, an object capable of achieving quantum superposition between two states is required. One type of qubit is an atomic nucleus. The orientation of its magnetic moment, that is, its "spin", can point in different directions in relation to a magnetic field, such as up or down. The difficulty is in locating and then dealing with that solitary atom. 


### How is Qubit represented in the form of an equation?

The two basis states (or vectors) are commonly expressed as 0 and 1 (pronounced 'ket 0' and 'ket 1'), as this corresponds to the standard bra-ket nomenclature for quantum states. As a result, a qubit can be viewed as a quantum mechanical counterpart of a traditional data bit. A linear quantum superposition of those two states is a pure qubit state. **Each qubit can be represented as a linear combination of 0 and 1 in this way:** 

**where α and are β the amplitudes of complex probability.** 

**α and β are constrained by the equation:** 

**\(∣α∣^2 + ∣β∣^2 = 1\).** 

The probability amplitudes, α and β, encode more than just the probabilities of the outcomes of a measurement; the relative phase between α and β is for example responsible for quantum interference, as seen in the two-slit experiment. 


### How is a Qubit different from a Bit?

Qubits are counted in the same way that bits are, using the binary system of 0 and 1. A bit's state can only be 0 or 1, whereas a qubit's state can be both 0 and 1 at the same time. 

According to quantum mechanics, a qubit's general state can be a coherent superposition of both. The duration of qubit coherence, is used to compare the quality of qubits as it indicates how long a qubit keeps its information and, as a result, determines its lifetime. 

A measurement of a qubit, would disrupt its coherence and irreversibly damage the superposition state, although a measurement of a classical bit would not. One bit can be entirely encoded in a single qubit. A qubit can store more data, such as up to two bits utilizing superdense coding. Superdense coding, often known as dense coding, is a quantum communication protocol in quantum information theory. It allows a sender and receiver to communicate a number of classical bits of information using only a few qubits, given that the sender and receiver had previously shared an entangled resource. 


### What technologies help in keeping Qubits in their coherent state?

**Superconducting Qubits:** 

The most sophisticated qubit technology is currently superconducting qubits. Most existing quantum computers, including the one that "surpassed" the world's fastest supercomputer, use superconducting qubits. They use Josephson junctions, which are metal-insulator-metal sandwiches. Scientists chill these materials to incredibly low temperatures to make them into superconductors - materials that transmit electricity without loss. Pairs of electrons, for example, pass through the material as if they were single particles. The quantum states are more long-lived as a result of this movement than in conventional materials. 

**Defects and Qubits:** 

The movements of electrons in materials are altered by the gaps formed by deficiencies in a material's structure. These gaps confine electrons in particular quantum materials, allowing researchers to access and regulate their spins. Unlike superconductors, these qubits don't have to be kept at extremely low temperatures always. They have the potential for lengthy coherence times and could be mass-produced. 

Quantum computing can also use three-state or multilayer states, which can be decoupled to conduct operations in qubits, while two-state systems are the most common. The storage is always consistent, which helps with data storage and the development of complex systems. 


### What does it mean for two Qubits to be entangled?

**Entanglement** is another counter-intuitive phenomenon in quantum physics. When each particle's quantum state cannot be characterised independently of the quantum state of the other, a pair or group of particles is said to be entangled. Although the parts of the system are not in a distinct state, the quantum state of the system as a whole can be stated. 

There is a particular link between two qubits when they are entangled. The results of the measurements will reveal the entanglement. The measurements on the various qubits could result in a 0 or a 1. However, the result of one qubit's measurement will always be correlated with the result of the other qubit's measurement. Even when the particles are separated by a considerable distance, this is always the case. The Bell states are an example of such states. 


### What is Qudit?

**The superpositions that qubits can allow them to assist in the execution of two calculations at the same time.** When two qubits are quantum-mechanically coupled, or entangled, they can aid in the simultaneous execution of four calculations; three qubits, eight calculations; and so on. As a result, a quantum computer with 300 qubits might execute more computations in an instant than the known universe's atoms, solving certain problems significantly faster than classical computers. However, superpositions are extremely fragile, making working with several qubits problematic. 

**Qudit is a multi-level computational unit that surpasses the two-level qubit.** Qudit, as compared to qubit, has a broader state space in which to store and process information, allowing for a reduction in circuit complexity, simplification of the experimental setup, and improved algorithm performance. 

## Milestones

1959

Richard Feynman states the possibility of using quantum effects for computation. 

1973

Alexander Holevo publishes a paper showing that n qubits can carry more than n classical bits of information, but at most n classical bits are accessible (a result known as "Holevo's theorem" or "Holevo's bound"). Charles H. Bennett shows that computation can be done reversibly. 

1976

Polish mathematical physicist Roman Stanisław Ingarden publishes a seminal paper entitled "Quantum Information Theory" in Reports on Mathematical Physics, vol. 10, 43–72. It is one of the first attempts at creating a quantum information theory, showing that Shannon information theory cannot directly be generalized to the quantum case, but rather that it is possible to construct a quantum information theory, which is a generalization of Shannon's theory, within the formalism of a generalized quantum mechanics of open systems and a generalized concept of observables (the so-called semi-observables). 

May  
2016

IBM Research announces that for the first time ever it is making quantum computing available to members of the public via the cloud. 

Oct  
2017

IBM Research scientists successfully "break the 49-qubit simulation barrier". 

2017

IBM, Intel, and Google each report testing quantum processors containing 50, 49, and 72 qubits. 

Jun  
2018

Intel begins testing a silicon-based spin-qubit processor. 

Mar  
2012

Caltech physicist John Preskill describes the moment when "well-controlled quantum systems can perform tasks surpassing what can be done in the classical world" as the arrival of "quantum supremacy." 

Oct  
2019

Google announces it has achieved quantum supremacy - marking a huge milestone in the advancement of practical quantum computing.