Quantum computing explained

Quantum computing is a hot topic among scientists, journalists and investors. With exciting recent developments in the field, quantum computing is fast becoming mainstream. Google Trends shows that searches on the term have significantly increased over the past five years. With its mind-bending properties, and claims of world changing potential, who wouldn’t be curious?

To make quantum computing accessible to all, we will answer some key questions on the topic over the next few weeks. What is quantum computing? How can it change the world? What are the key challenges? Who is working on this, and how? Our series of three blogposts takes a step back from the technical detail to help make sense of quantum computing, for everyone.

quantum computing explained

What is quantum computing?

The best place to start is to compare quantum computing to conventional, or ‘classical’ computing. Classical computers handle digital information in binary code, where each ‘bit’ is represented as a 0 or a 1. Quantum computers are made up of qubits – quantum bits. The amazing thing about qubits is that they can exist as both 0 and 1 at the same time. This is due to a phenomenon known as superposition. Imagine a spinning coin – the coin is simultaneously heads and tails until it stops spinning and lands with either heads or tails facing up. A qubit is similar – it remains somewhere between 0 and 1 until it is measured and collapses into one state or the other.

Another feature of a quantum computer is entanglement. When qubits interact, they can become connected, or ‘entangled’, so that measuring one qubit affects the state on the other qubit, no matter how far apart they are. In fact, they could be at either ends of the universe and the principle will remain true! Entanglement is a fundamental ingredient of quantum computation and is necessary to gain an advantage over classical computation.

Maria Violaris, PhD student and previous intern at Riverlane, explains that, “Being able to control and measure qubits is the ‘bread-and-butter’ of quantum computing, as it allows us to test out ideas to find information of interest to solve problems. Keeping qubits in the in-between state between a 0 or a 1 is the main challenge of building qubits for quantum computers.”

Superposition and entanglement help to give a quantum computer its power. A classical computer typically approaches a problem by trying out one route, then trying another, and another, until it finds the solution. However, thanks to superposition and entanglement, a quantum computer can simultaneously try all possible routes, meaning it has the ability to process multiple complex calculations. And it can do this at speed. The computational power increases exponentially the more qubits you have. Additionally, quantum computers have the ability to perform certain calculations that classical computers are simply not capable of. Put together, this can lead to ‘quantum advantage’, where a quantum computer significantly outperforms a classical computer.

The problem with qubits

Qubits are tiny. They might be a single atom, a particle of light, or a miniscule superconducting circuit, and they are manipulated by micropulses. Controlling qubits is extremely difficult: they are highly sensitive to environmental factors, known as noise, which can cause ‘decoherence’ – a loss of information. Many technologies require their qubits to be kept exceptionally cold, at temperatures of minus 273.3˚C: a fraction above absolute zero. This is the lowest temperature that is theoretically possible!

In order for a quantum computer to become useful, more qubits are needed, but due to their fragile nature, more qubits = more problems. For context, IBM’s current largest quantum computer consists of 65 qubits, and in 2019 Google claimed to demonstrate quantum advantage with 53 qubits. This is pretty amazing, but to really put this technology to use we need more – thousands, maybe millions of qubits!

Building quantum software

But it is not just the physical hardware that’s important. As with conventional computers, quantum computers need software to run algorithms, which act as a set of instructions. Quantum algorithms work by applying ‘gates’ – a gate is an operation which changes the state of a qubit. Several gates can be applied, then qubits are measured to give useful information about a particular problem. For example, a gate can cause a qubit to be ‘flipped’ from a 0 state to a 1, while a different gate can cause the qubit to go into superposition. Quantum scientists use gates to tell the quantum hardware how to manipulate the qubit into different states, to find out information of interest (based on probability distributions) and to help solve problems.

An example would be factoring. Shor’s algorithm was published in 1994 and confirmed that a quantum computer is capable of solving the following command: ‘find the prime factors of ‘n’’. Shor’s algorithm demonstrates an exponential speedup over the fastest classical algorithm and was the first of its kind to show evidence of a quantum computer’s power. As the security of the internet is based on how hard it is to factor a number, this example demonstrates how disruptive this technology can be.

Quantum scientists today are looking to improve existing algorithms and develop new ones. However, as previously mentioned, qubits are fragile and highly susceptible to noise. This noise can lead to errors in any computation we perform, and so for a quantum computer to be useful, we need to deal with these errors. This is challenging, not only because we don’t fully understand how qubits work (even Einstein was baffled by quantum theory!), but also because these errors occur so frequently. Specialist software is necessary to help correct these errors at a quick rate. Developing this software is an active area of research at Riverlane.

Building a quantum operating system

Much like a conventional computer, quantum computers need an operating system to make them useful and useable. Riverlane is building a quantum operating system, which can be used on different types of quantum hardware and that squeezes the most out of every precious qubit.


The quantum computing stack is made up of layers, consisting of qubits, FPGAs (field programmable gate arrays), and a CPU (computer processing unit).


Riverlane has a team of physicists, chemists, mathematicians, computer scientists, and FPGA experts that work on the challenges outlined in this blogpost. We also collaborate with quantum hardware companies such as Oxford Ionics and Universal Quantum to find ways of overcoming the difficulties more quickly. Building Deltaflow is a step towards our ultimate goal – to make quantum computers useful, sooner.


Quantum computers will have the ability to solve problems that are currently impossible, crunching through numbers with speed and accuracy. This could enable the accurate simulation of the molecules in drugs, which would allow for faster development of new medicines. It could also improve our ability to design more efficient batteries and better fertilisers to grow crops. The potential of quantum computing is staggering.

In upcoming blogposts, we will explore real world applications for quantum computers in more detail. We’ll also look into what steps the industry is taking to get closer to unleashing the power of quantum computers.


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