Quantum Computing: The Transformative Technology of the Qubit Revolution

QUANTUM

COMPUTING

The Transformative Technology of the Qubit Revolution

BRIAN CLEGG

For Gillian, Chelsea and Rebecca

ACKNOWLEDGEMENTS

My thanks to the team at Icon Books who have helped shape this series, notably Duncan Heath, Robert Sharman and Andrew Furlow.

My interest in computing was shaped by my first exposure at the Manchester Grammar School, where a young teacher encouraged us to punch holes in cards (by hand), send them off to a computer facility in London by post and wait a week for the result to come back in the post. After mixed experiences at university, I fell in love with computing again at British Airways, under the guidance of two mentors, John Carney and Keith Rapley, sadly both no longer with us. It was there that I learned that computer programming is an amalgam of fun and frustration, combining as it does the challenges of puzzle solving and of writing.

Although my coding experience is long in the past, it helps me appreciate the ingenuity of those who attempt to solve the many problems facing anyone who wishes to harness quantum capabilities to bring in a new computer revolution.

CONTENTS

Title Page

Dedication

Acknowledgements

1 Instructions for a ghost engine

2 Making a world bit by bit

3 The soft touch

4 Quantum strangeness

5 Quantum algorithms

6 Quantum hardware

7 To infinity and beyond

Further Reading

Index

About the Author

Copyright

quantum (ˈkwɒntəm)

A minimum amount of a physical quantity that can exist due to the physical limits in nature, meaning that the item can only be varied in such units. Describes the properties of the particles that make up light and matter, which behave entirely differently from familiar objects, with probability at the heart of their behaviour. From the post-classical Latin ‘quantum’ meaning amount, quantity or determination of quantity.

computing (kəmˈpjuːtɪŋ)

The action or an example of calculation or counting. Since the 20th century, the use of computers, particularly electronic computers, to perform computations mechanically. From the Latin ‘computare’: to calculate, account, reckon or count up.

quantum computing (ˈkwɒntəm kəmˈpjuːtɪŋ)

Performing calculations with a device that makes use of the special properties of quantum particles, such as photons of light or electrons, in order to perform certain operations exponentially faster than is possible with a conventional computer.

1

INSTRUCTIONS FOR A GHOST ENGINE

Program one: 1843

In 1840, the British inventor and polymath Charles Babbage gave a number of lectures in Turin, taking as his subject an as-yet-unconstructed device, the Analytical Engine. Born in 1791, Babbage had a sufficiently large inheritance from his father – a goldsmith and banker – never to need to take gainful employment. He enjoyed the social life of the salon as much as his work. It is said that his inspiration to explore mechanical means of calculation was helping out his friend, the astronomer John Herschel, to check astronomical tables. The experience was tedious beyond measure and Babbage is said to have cried out, ‘I wish to God these calculations had been executed by steam!’

Had the Analytical Engine ever been built, it would arguably have been the world’s first computer in the modern sense of the word. Babbage had been playing the role of a computer for Herschel – that is, a person who undertook calculations. The terminology dates back at least to the seventeenth century – it was only in the mid-twentieth century that the term was shifted from human beings to machines. Although it was entirely mechanical, the Analytical Engine was intended to hold both its data and its programs* on punched cards, based on the cards that had been devised to produce intricate patterns on the Jacquard silk-weaving loom. Unlike its semi-constructed (but never finished by Babbage) predecessor, the Difference Engine, where the instructions on what to do with the data were built into the machinery, the Analytical Engine’s instructions could be varied to taste.

Two years later, in an impressive piece of internationalism, a minor Italian military engineer Luigi Federico Menabrea (later to unexpectedly become Prime Minister of Italy) published a write-up of Babbage’s Turin talks, written in French for a Swiss publication, the Bibliothèque Universelle de Genève. Left in that periodical, this memoir would no doubt have rapidly disappeared into obscurity. However, in 1843 it was translated into English by Ada King, the Countess of Lovelace. In truth, ‘translation’ is a distinctly weak term for the resultant document, as King added copious notes that tripled the length of the piece, speculating on the future use of the unbuilt Analytical Engine and describing how it could be programmed for a number of tasks.

It is thanks to this single document that Ada Lovelace, as King is usually known, has gained the reputation of being the world’s first computer programmer. There is no doubt that Lovelace succeeded in bringing the potential of the Engine to a wider audience, though the degree to which she was indeed the first programmer has been disputed. One certainty is that the machine these instructions were intended for was never built – realistically, it could not have been constructed with the mechanical tolerances of the time. And so, strictly speaking, we should say that the document contained algorithms, in the form of tables that reflected the structure of the Engine, rather than computer programs in the modern sense.

Algorithms are structured instructions that could be anything from the sequence of actions required to brew a cup of tea to complex manipulations of data to solve a mathematical problem. They don’t require any computer – they can be worked by hand – but can, as was the case here, be structured in such a way that they fit well with a computer’s architecture.

Unfortunately, in the entirely desirable urge to provide good female role models from the past, Lovelace’s contribution has been exaggerated. Lovelace was the daughter of the poet Lord Byron and Annabella Milbanke. As a child, Lovelace was encouraged by her mother to study mathematics. She is often described as a mathematician, but it would be accurate to describe her as an undergraduate-level maths student. From letters between Babbage and Lovelace, it seems well-established that the notes that Lovelace added to Menabrea’s work were strongly influenced by Babbage. And even taking algorithms as programs, we know that Lovelace was not the first. This is because, as historian of science Thony Christie points out:

The Menabrea Memoir that Ada had translated already contained examples of programs for the Analytical Engine that Babbage had used to illustrate his Turin lectures and had actually developed several years before. The notes contain further examples from the same source that Babbage supplied to the authoress. The only new program example developed for the notes was the one to determine the so-called Bernoulli numbers.

We do know that Babbage, in his lectures, described algorithms that could have become programs for the Analytical Engine, had it ever been built. Usually with a totally new piece of technology like this we have to wait for some kind of prototype to be constructed before we can be certain of the device’s capabilities. But, remarkably, the Analytical Engine algorithms clearly showed the remarkable power that the machine would be capable of, had it ever been built. Rather than wait for programs to be developed, the Engine could instantly leap into action.

Having algorithms that were ready to go on a technology that was impossible to construct at the time was remarkable. That such a thing should happen twice seems even more surprising. Yet 153 years after the publication of Lovelace’s translation and notes, a very similar occurrence would play out. This time, the imagined engines in question would invoke the power of the quantum.

Program two: 1996

By 1996, electronic computers had been part of government and business establishments for decades, and had become relatively commonplace in homes, since personal computers moved from the realm of enthusiasts’ toys to commercial products in the 1980s. Unlike the Analytical Engine, electronic computers were too complex to be designed by a single person – and although some early programs were the work of an individual, many were constructed by teams.

Although the underlying concepts behind the electronic computer would continue to be developed, and these devices would continue to become increasingly powerful for decades to come, by the 1990s scientists were already aware of limitations in the way that such computers worked. Fifteen years before our key date, the physicist Richard Feynman had speculated about constructing computers where the basic unit of operation was not the traditional bit, which was limited to holding values of 0 or 1, but rather a ‘quantum bit’, based on a quantum particle such as a photon or electron, which could be in an intermediate state with a potentially infinite set of possibilities.

By the 1990s, teams were thinking about or attempting to construct such quantum computers. The challenges they faced were huge. In 1996, ability to manipulate individual quantum particles was in its infancy. The technology required to build a quantum computer was entirely impractical. Yet, just like the algorithms for the Analytical Engine, it proved possible to devise an algorithm for quantum computers that, should the machines ever work, could revolutionise the business of searching for data – a task that lies at the very heart of the computer business.

This particular quantum algorithm was devised by Lov Grover, then working at Bell Labs in America. Grover was born in 1961 in the North Indian city of Roorkee. His original intention was to become an electrical engineer – perhaps not a surprising ambition, as he lived in the city that was home to Asia’s first specialist technology establishment, founded in the 1840s to give training in engineering. However, Grover did not attend the University of Roorkee (now the Indian Institute of Technology Roorkee), but rather the Indian Institute of Technology in Delhi.

A career in electrical engineering was still Grover’s intention when he emigrated to the USA, but by 1985, when he had achieved a PhD in the subject at Stanford University, his interest had grown in the quantum applications of physics – his doctorate concerned a device that typifies the oddities of the quantum world, the laser, and it was quantum physics that drove his thinking when he joined Bell Labs.

Then the research arm of the communication company AT&T, Bell Labs was not unlike the research department of a modern technology giant such as Google or Apple today. Researchers at the laboratories were given an impressive degree of freedom to explore new ideas, as a result of which the company was rewarded with dramatic developments – often in and around computing. It was at Bell Labs, for example, that John Bardeen and Walter Brattain, under the direction of William Shockley, had come up with the transistor. In the 70s, Bell had been responsible for developing UNIX, the operating system that still is central to much computing, and the C programming language which, with its derivates, still dominates the world of conventional computer programming.

When Grover joined Bell, the idea of developing algorithms that could run on quantum computers was already in the air, with the first devised in 1994. The freedom to do what Grover described as ‘forward-looking research’ was still available at Bell and he almost immediately devised his quantum computing search algorithm. He would later write up his new idea in the journal Physics Review Letters as ‘Quantum Mechanics Helps in Searching for a Needle in a Haystack’. His idea could, in theory, transform the business of searching.

At the time, search engines† as we now know them were in their infancy. Prior to this point, if you were an early adopter of the internet and wanted to find your way around the World Wide Web, you would use a curated list of links – a manual index. The leading search engine in 1996, AltaVista, only started operation the year before. Google would begin life as a research project in 1996. But though search engines per se were a novelty, databases had been at the heart of much computing for decades – the need to quickly find and retrieve data was the driving force behind much commercial computer use, and anything that could speed up that process would clearly be attractive. What Grover realised is that with a quantum computer, he could not only speed up searching, he could supercharge it.

We can think of a database as an electronic version of a card index. Each database consists of a set of records, with a record being the equivalent of a single card. To find our way around the records, the database is indexed. In the card index, that is done simply by putting the cards in alphabetical order of the heading – but an electronic database can have multiple indices, enabling it to effectively re-order