Since the first claims of so-called “quantum supremacy” in 2019, the transition from university laboratory to commercial application has been steadily accelerating for quantum computers. While the timeline for the development of useful quantum computers remains unclear, the associated technical and commercial infrastructures are increasingly advanced.
There are now several established quantum programming languages (e.g., Cirq, Qiskit, and Q#), service offerings from big companies for remote access to quantum computing including Google Quantum AI, IBM Quantum and Microsoft Azure Quantum, and several quantum computer companies listed on stock exchanges (IonQ, Rigetti etc).
But what are quantum computers? How do they work? And are they going to replace our conventional computing devices?
What makes a quantum computer special?
Over the last 70 years, advances in information processing have been made by connecting an increasingly large number of transistors together in very specific arrangements. While the complexity of these transistor networks can be overwhelming, they all work on the decidedly simple principle that a signal in a computer’s circuitry will either be 0 or 1, represented by a low or high voltage. This 0 or 1 is called a “bit”, and when enough bits are combined, they can represent pretty much any piece of information you want.
A quantum computer instead uses quantum bits, known as “qubits”. Like conventional bits, when qubits are measured their value will also be either 0 or 1. Unlike bits, however, a qubit need not necessarily be the same every time you measure it. For example, a single qubit could be set up so that its value would be 0 half the times you measure it and 1 the other half. Technically, until measured, the qubit is said to be in a superposition of both states.
The extra flexibility they provide means that much more information can be encoded and processed per qubit than per bit. Depending on the type of quantum computer, the relationship between the number of variables considered and the number of qubits can be exponential. This means that a relatively small number of qubits can be used to process an astonishingly large number of values simultaneously and this can be exploited for certain problems which can be solved very efficiently, such as cracking encryption or route finding.
For that reason, quantum computers represent a potential leap in processing capability, but they come with a caveat. They are not all-purpose like personal computers; they are extremely sophisticated, demanding and expensive machines.
Qubits are not represented by voltage levels, nor can you think of them moving around a circuit and passing through a series of logic gates as occurs in a conventional computer. Depending on the implementation, the qubits could be photons (light particles) or single atoms, for example. The “logic gates” can be complex pulses of microwave radiation which modify the respective probability of you ending up with a 0 or 1, without the qubits moving physically at all.
The challenge with all approaches is that they involve controlling and measuring the properties of single “quanta” such as particles. This presents extraordinary technical challenges.
The Challenge(s)
Single quanta are very susceptible to “noise” from the external environment so quantum computers need to operate at extremely low temperatures, often just a few hundredths of a degree above absolute zero, to mitigate this interference. To put that in context, liquid nitrogen will cool something to 77°C above absolute zero. And getting colder than that requires more innovative (and expensive) approaches.
In most cases, a dilution refrigerator is used, which can typically reduce temperatures to 0.01 Kelvin. You may be familiar with pictures of quantum computers appearing as a series of large gold-coloured discs with wires and pipes passing through them. But this is not actually the quantum computer; it’s the inside of a dilution refrigerator. The quantum computer will be a relatively small chip (a few cm across) right at the bottom of the fridge.
All of this means you are not likely to have a quantum computer in your home anytime soon. In the short to medium term at least, we are likely to be using quantum computers in a similar way to how we used conventional computers in the 1950’s and 1960’s; large institutions such as major computer chip manufacturers and universities will host quantum computers, and users will request time to access the compute resources via terminals. This is happening with an increasing number of organizations offering Quantum Computing as a Service (QCaaS).
What to expect from the future
So how will quantum computers change things? The unsatisfying answer to this question is, we don’t yet know. If you search for the potential applications of quantum computers, the results will include things like financial modelling, materials discovery, and pharmaceutical development. While undoubtedly important, these fields tend not to generate the same level of excitement in the general public compared with something new and immediately engaging like interactive A.I. models. Part of this may be due to physicists’ infamous inability to adequately promote and explain their work. However, history has shown us that if you create something with incredible capability (and quantum computers may have huge capability), then humans will find impressive ways to exploit it.
There is a good chance that an as yet unconsidered application of quantum computers could transform our world beyond recognition. As with all great technological disruptions, very few will see it coming, and we may have to be patient. When Bardeen, Brattain and Shockley demonstrated the first transistor based on a lump of germanium in 1947, there is no way they could have imagined a world where 10 billion could be contained in an area 1 cm × 1 cm, at a cost $0.00000001 per transistor.
The practical challenges associated with quantum computers should not be ignored but most now agree that there are no impassable barriers left and the remaining issues are solvable. Once these hurdles are overcome, quantum computers will change the world. This could be via the facilitation of encryption cracking and the significant consequences this would have for data security, banking, and communication. In the short to medium term at least, these machines will remain the pinnacle of computer engineering.
This is a guest post by Dr. John Labram, Associate Professor in the Electronic & Electrical Engineering Department of University College London