We can imagine a world where quantum computers will be able to design powerful new drugs by simulating the behaviour of individual molecules, and optimize complex supply chains to help companies source the parts they need and assemble products in the most efficient way possible.
Introduction
Quantum computing is an entirely new dimension of computing leveraging the laws of quantum mechanics. Quantum computers apply superposition and entanglement at the universe’s smallest scales and coldest temperatures. They also adopt a multidisciplinary approach comprising of computer science, physics and mathematics to enable scientist to solve complex problems.
While today’s quantum computers remain rudimentary and error-prone, they have the potential to provide significant performance gains, and dramatically increased computation speeds to perform complex computational tasks that can take classical computers years to complete. Numerous governments, universities and vendors around the world are investing heavily in harnessing quantum computing technology to achieve fault-tolerant and reliable systems.
In this article, I provide a detailed examination of the key concepts underlying quantum computers, and how they promise to open the potential for massive advancements in a variety of scientific and industrial applications.
Superposition and entanglement
Qubits are the most basic units of processing in quantum computers. Qubits rely on the use of particles such electrons and photons, that can be suspended in states of 0, 1 or any states in between. This ability of qubits to be in more than one state at a time is what gives quantum computers their processing power. However, it is the application of superposition and entanglement through interference to qubits that allow quantum computers to produce reliable outcomes.
To better understand superposition, we refer to the famous thought experiment involving a cat as imagined by the physicist, Erwin Shrödinger. Shrödinger’s experiment imagined a cat sealed in a box with a poison trap that can be triggered by a decaying radioactive atom. Since the decay of the radioactive atom is uncertain, at any given moment the cat could be in a superposition of states such that it is either dead or alive [1]. It is only when someone opens the box and observes the cat does its state become definite or its state “collapses” to being either dead or alive.
Superposition is difficult to explain through analogies; however it is also possible to imagine a coin tossed and spinning fast in the air. As long as the coin continues spinning then its state can be considered both heads and tails. It is only when the coin is stopped does one observe its state as either heads or tails.
Quantum theory also implies that particles can be linked with each other, such that when the state of one particle changes it will instantly impact the state of the other, regardless of the distance between them. This is what is referred to as entanglement, and it is what allows qubits to correlate their states with each other and thus scale their processing power exponentially.
In Shrödinger’s cat experiment, entanglement can be described as having several cats in the box that are entangled in a superposition of states such all cats in the box are either dead or alive. When someone opens the box, their state then collapses such that the cats in the box are all observed to be either dead or alive. Entanglement means that two particles are always connected, and they are never independent of each other. This is how nature works at the atomic level.
Interference
Quantum interference refers to a phenomenon where the probability amplitudes of quantum states combine, either constructively or destructively, to influence the likelihood of an outcome. In classical interference, physical waves such as sound or water can overlap such that they amplify or cancel each other out. Quantum interference is different in that is it based on the wave-like behaviour of particles such as electrons, photons and atoms [2].
In quantum theory, particles are described via wavefunctions, which contain complex-value probability amplitudes. We can think of a particle going through two indistinguishable paths such as two slits in a barrier as the two-slit experiment describes. In this experiment, particles such as photons or electrons are fired one at a time at a wall with two narrow slits and a screen placed behind it. Each particle must pass through slit A, slit B or a combination of both. The expectation would be that particles would pass through one slit or the other, and that the screen would show two bright spots as the particles pass through.
Instead of observing two spots on the screen, a series of bright and dark fringes are observed – an interference pattern. The fringe pattern is characteristic of the behaviour of waves rather than particles, where the bright areas indicate wave amplitudes that amplified each other, while the dark ones are waves that canceled out. This behaviour can be described as [3]:
- Constructive interference, where the wave amplitudes add up, thus increasing the probability of a particular outcome.
- Destructive interference, where the amplitudes cancel each other out, thus reducing or eliminating the chance of an outcome.
What is fascinating about the fringe pattern observed is that it can appear even when particles are sent one at a time. Therefore, instead of interfering with each other particles are interfering with themselves, thus taking both paths simultaneously in superposition.
Interference is what gives quantum computers their superiority over classical computers. It allows quantum systems to guide computations by enhancing the probability of correct answers while supressing wrong ones. Once qubits are transformed and entangled, their probability amplitudes evolve through interference. All possible computations are performed simultaneously and are allowed to interact through entanglement.
A critical condition of interference is that the paths followed by qubits are indistinguishable, such that it is not possible to determine which path a qubit takes, even in principle. Any form of measurement collapses the wavefunction, thus destroying the superposition and possibility of interference. Interference underlies the power of quantum computing, and it remains a key component in unlocking the full potential of quantum technology.
Measurement
In the final stage of quantum computing, states collapse into classical outcomes upon measurement. These outcomes are not random and are fundamentally determined by whether computational paths leading to them have interfered constructively or destructively.
A state where computational paths leading to it have interfered destructively will have a probability close to 0. Similarly, a state where computational paths leading to it have interfered constructively will have a significantly amplified likelihood.
Instead of measuring outcomes sequentially, quantum computers exploit the wave-like nature of qubits to allow all possible computational paths to co-exist and interfere. This creates a probabilistic landscape where the correct answers become the most likely outcomes.
Quantum bits (Qubits)
Computers process information using bits that store information using 0’s and 1’s. Bits can be represented using physical objects such as bar magnets or switches placed in either a state of up or down. Bits can maintain their state for a long time, thus allowing them to represent stored information in a stable and long-lasting fashion. However, bits are limited in their ability to store information when compared to qubits. While bits can exist in either a stare of 0 or 1, qubits can exist in a superposition of multiple states of 0, 1 or any state in between.
The superposition of qubits is what makes them superior to classical bits. It is possible to think of a qubit as an electron spinning in a magnetic field. The electron could be spinning with the field, known as spin-up state, or against the field, knows as spin-down state. Suppose it is possible to change the direction of the electron’s spin using a pulse of energy such as a laser. If only half a pulse of laser energy is used and all external influences are isolated, then we can imagine the electron in superposition where it is in all possible states at once [4].
Superposition increases the computational power of qubits exponentially depending on the number of qubits in a quantum computer. Whereas two classical bits can contain only two pieces of information (01 and 10), two qubits can store a superposition of four combinations of 0 and 1 simultaneously, three qubits can store eight combinations, and so on. Therefore, a quantum computer can perform 2N computations, where N is the number of qubits.
Conclusion
Through exponential scaling, unique algorithms and the continued evolution of quantum hardware, quantum computing has the potential to revolutionize industries like cryptography, material science, pharmaceuticals and logistics. We can imagine a world where quantum computers will be able to design powerful new drugs by simulating the behaviour of individual molecules, and optimize complex supply chains to help companies source the parts they need and assemble products in the most efficient way possible. Other more impactful applications could be computers that could break the encryption that safeguards our private information on the internet.
Governments, companies and research labs are working tirelessly to harness the potential of this emerging technology. Quantum computing, combined with the capabilities and advancements in AI, has the potential to achieve artificial general intelligence (AGI). By enabling rapid data processing and computation, improved learning capabilities and parallel processing, quantum computers can process extensive datasets, enabling the improved learning capabilities needed for AGI. As quantum computers continue to rapidly evolve, it is essential for us to harness their potential in ways that further advance humanity’s future and well-being.
References
[1] Quantum Computing Explained
[2] What is quantum interference and how does it work?
[3] Quantum interference in Quantum Computing: 2025 Full Guide
