The Principles of Quantum Computing Explained
Today, a variety of companies are producing mainstream quantum hardware and making tools available to developers, turning quantum computing technology that was theoretical a few decades ago into a reality.
Introduction
During one of his Messenger Lectures at MIT in 1964, the renowned Nobel prize laureate and theoretical physicist, Richard Feynman, was quoted as saying “I think I can safely say that no one can understand quantum mechanics”. Feynman emphasized the counter intuitiveness of quantum mechanics, and encouraged listeners at his lecture to simply accept how atoms behave at the quantum level, rather than trying to apply a classical understanding onto it [1].
At its core, quantum theory describes how light and matter behave at the subatomic level. Quantum theory explains how particles can appear in two different places at the same time, how light can behave both as a particle and a wave, and how electrical current can flow both clockwise and counter-clockwise in a wire. These ideas can seem strange to us, even bizarre, yet quantum mechanics gave rise to a new world of possibilities science, technology and information processing.
What is a quantum computer?
While classical computers use bits that can be either 0 or 1, quantum computers use quantum bits (qubits) that can be 0, 1 or both at the same time, suspended in superposition. Qubits are created by manipulating and measuring systems that exhibit quantum mechanical behaviour. Because qubits can hold superposition and exhibit interference, they can solve problems differently than classical computers.
Quantum computers perform quantum computations by manipulating the quantum states of qubits in a controlled way to perform algorithms [2]. Quantum computers can adopt an arbitrary quantum state from an arbitrary input quantum state. This enables quantum computers to accurately compute the behaviour of small particles that follow the laws of quantum mechanics, such as the behaviour of an electron in a hydrogen molecule. Quantum computers can also be used to efficiently run optimization and machine learning algorithms.
For example, a classical computer might apply a brute force method to solve a maze by trying every possible path and remembering the paths that don’t work. A quantum computer, on the other hand, may not need to test all paths in the maze to arrive at the solution. Instead, given a snapshot of the maze, a quantum computer relies on measuring the probability amplitudes of qubits to determine the outcome. Since the amplitudes behave like waves, the solution is found when the waves overlap.
Principles of quantum computing
Quantum computing relies on four key principles:
Superposition – represents all possible combinations of a qubit through a complex multi-dimensional computational space. Superposition allows the representation of complex problems in new ways using these computational spaces. The quantum state is measured by collapsing it from the superposition of possibilities into a binary state that can be registered as binary code using 0 and 1[3].
Entanglement – the ability of qubits to correlate their state with other qubits. Entanglement implies close connections among qubits in a quantum system, such that each qubit can immediately determine information about other qubits in the system.
Interference – qubits placed in a state of collective superposition structure information in a way that looks like waves, with amplitudes associated with each wave. These waves can either peak at a particular level or cancel each other out, thus amplifying the probability or canceling it out for a specific outcome. Amplifying or canceling out a probability are both forms of interference.
Decoherence – occurs when a system collapses from a quantum state to a non-quantum state. This can be triggered intentionally through measurement of the quantum system or other unintentional factors. Quantum computers require avoiding or minimizing decoherence.
Combining these principles can help explain how quantum computers work. By preparing a superposition of quantum states, a quantum circuit written by the user uses operations to entangle qubits and generate interference patterns, as governed by a quantum algorithm. Outcomes are either canceled out or amplified through interference, and the amplified outcomes serve as the solution to the computation.
Conclusion
Today, a variety of companies are producing mainstream quantum hardware and making tools available to developers, turning quantum computing technology that was theoretical a few decades ago into a reality. Superconducting quantum processors are being delivered at regular intervals, increasing quantum computing speed and capacity. Researchers are continuing to make quantum computers even more useful, while overcoming challenges related to scaling quantum hardware and software, quantum error correction and quantum algorithms.
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