Quantum Tunnelling and NISQ

Quantum Tunnelling

In classical physics, if the gravitational potential of a barrier is too high - i.e. greater than the initial kinetic energy of a free particle such as an electron trying to pass over the barrier - the object simply cannot pass over the barrier. In this scenario, the particle simply bounces against the barrier and eventually stops. In quantum mechanics, the particle behaves as a wave. This electron wave diminishes as it encounters the barrier, but despite the existence of a higher gravitational potential, there is always a probability that part of the wave will pass the barrier onto the other side. This is due to a phenomenon known as quantum tunnelling. Through quantum tunnelling, the particle can move through the barrier as a wave and appear on the other side.

The phenomenon of quantum tunnelling can be explained through the macroscopic analogy of a rock rolling up a steep hill. In this scenario, we know that the ball can only go over the hill if its kinetic energy is higher than the gravitational potential of the hill itself. Otherwise, the ball will simply roll back down the hill. In order to pass over the hill, the ball will either need to surpass the gravitational potential of the hill through additional energy (e.g. an individual rolling the ball up the hill), or use quantum tunnelling to simply pass through the barrier as a wave.

Quantum Tunneling is what gives some quantum computers the potential to not only complete tasks faster but to potentially complete tasks a classical computer simply could not do within the confines of classical physics.

Noisy Intermediate-Scale Quantum

What is NISQ?

NISQ, “Noisy Intermediate-Scale Quantum,” refers to a current stage in the development of quantum computing technology.

N: Noisy

The “N” in NISQ stands for “Noisy,” highlighting a core challenge in current quantum computing technology. Quantum bits, or qubits, in NISQ computers are prone to errors due to environmental interference and other factors, leading to quantum information loss, known as decoherence. This noise affects the accuracy and reliability of quantum computations, necessitating the development of sophisticated error mitigation techniques to manage and reduce the impact of these errors. Despite these challenges, the noisy nature of these qubits does not entirely prevent them from performing certain computations that could potentially outperform classical computers.

I in NISQ: Intermediate-Scale

The “I” refers to “Intermediate-Scale,” indicating the size of quantum computers during this phase. NISQ devices typically possess a modest number of qubits, ranging from several dozen to a few hundred. This scale is substantial enough to potentially perform tasks beyond the capabilities of classical computers but not large enough for implementing full-scale quantum error correction. This intermediate scale is a significant step in quantum computing, bridging the gap between early experimental quantum devices and the eventual goal of large-scale, fault-tolerant quantum computers.

S in NISQ: Scale

The “S” in NISQ, which is part of the term “Intermediate-Scale,” emphasizes the current capacity of quantum computers in terms of qubit count. The scale of NISQ devices is a critical factor, as it determines the complexity of problems these computers can tackle. While these quantum computers do not yet have the thousands or millions of qubits required for widespread practical use and robust error correction, the intermediate scale provides a testing ground for various quantum algorithms and helps understand the practical limitations and capabilities of quantum computing at this stage.

Q in NISQ: Quantum

The “Q” stands for “Quantum,” the fundamental aspect of this technology. NISQ devices operate on quantum mechanical principles, leveraging the unique properties of quantum states like superposition and entanglement to perform calculations. This quantum nature allows NISQ computers to explore computational pathways and algorithms in a manner fundamentally different from classical computers. Despite the current limitations in scale and noise, the quantum aspect of NISQ machines opens up possibilities for solving certain complex problems more efficiently than classical systems, exploring new realms of computational theory and practice.

What next?

What we have now: NISQ is valuable for scientific exploration. But there is no proposed application of NISQ computing with commercial value for which quantum advantage has been demonstrated when compared to the best classical hardware running the best algorithms for solving the same problems.

What we can reasonably foresee. Nor are there persuasive theoretical arguments indicating that commercially viable applications will be found that do not use quantum error-correcting codes and fault-tolerant quantum computing.