# Quantum Computing for Healthcare: A Review
## 2 Quantum Computing: History, Background, and Enabling Technologies
### Quantum Computing vs. Classical Computing  
We refer the reader to Figure 1 for a differentiation of quantum computing paradigms with classical computing approaches in terms of their strengths, weaknesses, and applicability. Unlike conventional computers that operate in terms of bits, the basic units of operation in a quantum computer are referred to as quantum bits, or _\"qubits\"_, that possess two states or levels, i.e., they can represent a single bit in both '1' and '0' simultaneously. Quantum physical systems, which leverage, for example, the orientation of a photon or spin of an electron, are used to create qubits. We note that quantum computers can come in various varieties, including one-qubit computers [37], two-qubit computers [38], and higher-qubit quantum computers. Key advancements in quantum computing were made earlier in the year 2000 when the very first 5-qubit quantum computer was invented [39]. Since then, many important advancements have been made so far, and the best-known quantum computer of the current era is IBM's newest quantum-computing chip that contains 433 qubits [40]. However, the literature suggests that the minimum number of qubits to realize quantum supremacy is 50 [41]. Quantum supremacy is defined as the ability of a programmable quantum device, which is capable of solving a problem that cannot be solved by classical computers in a feasible amount of time [42]. The behavior of qubits relates directly to the behavior of a spinning electron orbiting an atom's nucleus, which can demonstrate three key quantum properties: quantum superposition, quantum entanglement, and quantum interference [43]. * Quantum superposition refers to the fact that a spinning electron's position cannot be pinpointed to any specific location at any time. On the contrary, it is calculated as a probability distribution in which the electron can exist at all locations at all times with varying probabilities. Quantum computers rely on quantum superposition, in that they use a group of qubits for calculations and, while classical computer bits may take on only states 0 and 1, qubits can be either a 0 or 1, or a linear combination of both. These linear combinations are termed superposition states. Since a qubit can exist in two states, the computing capacity of a qubit quantum computer grows exponentially in the form of \(2^{q}\). Figure 1: Comparison of _Classical Computing vs. Quantum Computing_ in terms of four key attributes: (1) computing units; (2) computing capacity; (3) suitability; and (4) and error rates. * _Quantum entanglement_ takes place in a highly intertwined pair of systems, such that knowledge of anyone immediately provides information about the other, regardless of the distance between them. This nonintuitive fact was described by [PERSON] as \"spooky action at a distance\", because it went against the rule that information could never be communicated beyond light speed. Quantum entanglement in physics is when two systems such as photons or electrons are so highly interlinked that obtaining information about one's state (for example, the direction of one electron's upward spin) would provide instantaneous information about the other's state, such as, for example, the direction of the second electron's downward spin, no matter how far apart they are. Modifying one entangled qubit's state therefore immediately perturbs the paired qubit's state in quantum computers. Thereby, entanglement leads to the increased computational efficiency of quantum computers. Since processing one qubit provides knowledge about many qubits, doubling the number of qubits does not necessarily increase the number of entangled qubits. Quantum entanglement is therefore necessary for the exponentially faster performance of a quantum algorithm as compared with its classical counterpart. * _Quantum interference_ occurs because, at the subatomic scale, particles have wavelike properties. These wavelike properties are often attributed to location, for example, where around a nucleus an electron might be.