UK: Quantum Computing

Last Updated: 17 April 2019
Article by Justin Hill

Quantum computers are no longer in the realms of science fiction. But how are they different from classical computers?

For something to exhibit a quantum state it needs to be either extremely small, extremely cold or extremely isolated. If you have ever seen IBM Q (advertised as the world's most advanced civilian quantum computer), you will know that it looks a little like a steampunk chandelier. The reason for this is that the computer is trying to keep itself extremely cold. It manages to reduce the temperature of the system down to as low as -273.10°C, which is 0.01°C above absolute zero – the coldest temperature possible.  

There are a number of strange principles in quantum physics that underpin this technology.

First, quantum superposition. A good way to explain superposition is to imagine a coin. If I were to flip the coin and let it land and then ask you whether it was heads or tails, no doubt you would be able to provide me with an answer. But what about if I were to spin the coin? When the coin is spinning, is it heads or tails? It's not really either, it's kind of both. When spinning, the coin is in a state of superposition.

In modern computers, like the one you are using to read the electronic version of this article, information is represented in 1s and 0s. The smallest unit of data in a computer is called a bit. Just like a coin, a bit has two states – instead or heads of tails, though, we have 1 and 0. In quantum computing we do not use bits, instead we use "qubits" – short for quantum bit. The qubit has two states, but the difference between a bit and a qubit is essentially that a qubit, like the spinning coin, can also behave as if it is both states simultaneously – it is in "superposition".

The second key principle in quantum physics we need to understand is quantum entanglement. Entanglement is essentially where we now have two coins spinning and the behaviour of one is linked to the other. The state of one of the entangled coins (head or tails) correlates with the state of the other entangled coin. Entangled particles are connected, but the nature of the relationship between them can vary. If an action is performed on one, the other is affected, regardless of the distance between them. What this means, sticking with our coin example, is that when the two entangled coins stop spinning, if they were entangled in a way that meant that the coins would always land on the same side, they will both land on heads or on tails at the exact same instant, regardless of the distance between them. Einstein called this phenomenon "spooky action at distance" and it remained unexplained by conventional theory.

But what does this mean for computing?

This all means that a qubit is simultaneously in both states before being measured, and when measured it assumes a state with its entangled qubit counterpart instantaneously. As a result a two-qubit register in a quantum computer can store four binary pairs simultaneously, whereas a two-bit register in a classical computer could store only one binary pair. This increased data density and configurability makes quantum computers far more powerful and much faster than our classical computers.

This will not be universally true for all types of computing task, though. They will not necessarily be faster (and may actually be slower) for browsing the web or streaming your favourite shows. It would be a mistake to think of them as a replacement for classical computers for all tasks. Quantum computers approach problems in a different way. Using algorithms designed specifically for them, they will vastly reduce the amount of operation needed to solve a particular calculation, for example. To really understand how much more powerful a quantum computer can be in these kinds of instances, we must look briefly at the maths.

Let's say you have a quantum computer with 300 qubits: how many bits would you need to produce this much power?

N qubits = 2N classical bits.

Because qubits can be either a 1 or a 0 at any given time, as you increase the number by one the capacity greatly increases under the power of exponentials. Therefore, if we had 300 qubits, we would need 2300 classical bits to recreate this much computational power. To put this 2300 number in perspective, that would be as many classical bits as there are known particles in the universe.

The graphic way to understand how this increase in power can be utilised is to consider the eight queens puzzle in chess.

The eight queens puzzle is the problem of trying to find all the ways in which you can place eight queens on an 8x8 chessboard without any of the queens being able to take another. This means that a solution requires that no two queens are occupying the same row, column or diagonal. To solve this with bits in a classical computer, you would need to pass every possible iteration of the chessboard through the computer, which would be programmed to tell you whether you had found a solution or not. You would not be able to stop trying iterations until you had tried them all, because you would never know if you had found them all until you had tested every outcome. There are around 4 billion possible iterations of the eight queens on the board and there are 96 correct solutions. The problem with classical computing is that, even if you were to find the 96 solutions in your first 96 guesses, you couldn't be sure you had found them all until testing all of the 4 billion iterations. Using a quantum computer and qubits, you could input every possible iteration at once and then be given all of the possible solutions at once too.

I know what you are thinking – if it's just that simple, why don't we all have one already?

Another strange principle in physics that we need to be aware of is the measurement problem. The measurement problem is essentially the fact that you cannot ever measure something without affecting it. If you want to know how cold the screen you are reading this on is, you might touch it. But, by touching it, you are changing its temperature and thus unable to take the true measurement. We are presented with this same problem when trying to view the results a quantum computer gives us. We feed the information into the computer in a state of superposition, but it also comes out in a state of superposition. When we want to look at the results, we fire microwaves at those results to measure them. These microwaves interfere with the result. The microwaves interact with the particles they are measuring and in doing so change the information we are looking at, and consequentially affect the result. These changes produce errors in our findings, and these errors are the biggest difficulty facing quantum computing. How can we minimise these errors to improve our results?

Quantum computer developers are using the same technology found in sound cancelling headphones to resolve this problem – called interference. If you have ever worn sound cancelling headphones you will know that when you put them on you can hear much less of your surroundings. The headphones are receiving background sounds from your environment and then instantaneously emitting a counteracting frequency to cancel them out so you are left with a zeroed effect – no background sound. There are two kinds of interference: constructive and destructive. Constructive interference is when two waves are exactly in phase and the peaks of the waves align leading to an adding effect (think two flames meeting to increase the size of the overall flame). Deconstructive interference is when two waves are exactly out of phase and the peaks of one wave align with the troughs of the other wave, leading to a subtracting/cancelling effect. By using interference we are able to control quantum states and amplify the kinds of signals tending toward the right answer and cancel the ones tending toward the wrong answer.

This is a massive leap forward for computing and is set to result in a number of problems and opportunities for many industries.

One issue is that the usual way in which we mask/conceal the majority of our data online (encryption) is at risk. Currently, the safety of our encrypted data relies on the difficulty of finding the prime factors of certain large numbers. The ability to find these numbers is only limited by the amount of computational power we have at our disposal dedicated to doing this. With quantum computing, this limiting factor is removed. On the flip side, the technology itself can also be utilised to improve cloud security, which will actually enhance private data safety. Reassuringly, certain alternative encryption techniques are expected to remain quantum resistant because the maths involved in decrypting is not well suited to being executed on quantum computers.

In general, the increase in our capacity for solving complex problems is set to skyrocket. This is great news not only for the research and development communities but also for business. The development of machine learning is currently restricted by how long it takes to train and improve the machine's responses. Put simply, the machine is fed data and then produces a result; that result is then compared with the desired result and, where they are not the same, the machine's programming is altered/improved. This is a very time-consuming exercise. By using quantum computing this whole process could be sped up exponentially.

While the possibilities are endless, the technology is also expected to be utilised in the financial services sector to make better investments by finding new ways to model financial data and identify risk factors. Quantum computers could also be harnessed in the supply chain and in logistics applications. For example, when a delivery vehicle is mapping the "best route" it does not only look at the fastest and most direct roads, as any of us would, but it also seeks to minimise the amount of right turns the vehicle makes (left turns in the US). This is because turning across traffic does not only slow you down but also increases risk. Though this was not a product of quantum computing, it is an interesting example of how there are often strange ways to optimise a system, which could be much more easily tested and proven by using quantum computers.

Quantum computing may not improve your experience of streaming Netflix, but it will soon lead to breakthroughs in many industries. Watch this space.

Dentons is the world's first polycentric global law firm. A top 20 firm on the Acritas 2015 Global Elite Brand Index, the Firm is committed to challenging the status quo in delivering consistent and uncompromising quality and value in new and inventive ways. Driven to provide clients a competitive edge, and connected to the communities where its clients want to do business, Dentons knows that understanding local cultures is crucial to successfully completing a deal, resolving a dispute or solving a business challenge. Now the world's largest law firm, Dentons' global team builds agile, tailored solutions to meet the local, national and global needs of private and public clients of any size in more than 125 locations serving 50-plus countries. www.dentons.com.

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