Light is the ultimate information carrier. It can be generated with high efficiency and sent over huge distances using optical fibres to transmit internet, TV, and telephone signals. Our research aims to harness the special quantum properties of light to make quantum computers a reality and provide perfectly secure communications.

The most detailed description of light uses quantum theory, which tells us that light comes in small packets of energy called photons. These are neither particles nor waves, and are best referred to as quanta. However, they do display some properties of both particles and waves: photons can be detected at very precise positions and times like particles and they exhibit interference like waves. Because photons have these special quantum properties, they can be used to process information in a fundamentally different way than ordinary light. We can use them to create a perfectly secure communication channel via quantum key distribution, and, in the future, we can use photons as the information carriers in a quantum computer.

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How long does it take a classical computer (running a clock speed of 2 GHz) to factor a 100 digit number? And what

about a quantum computer?

QUANTUM SPEED TESTPhotons can also be used as the fundamental building blocks of a quantum computer. Quantum computers are often said to be much faster than normal, or “classical” computers, but what does this mean?

To answer this, we need to look at how computers actually operate. Some problems are hard for a computer to solve, while others are easy. An example of an easy problem is multiplication: in just a few steps you can multiply two relatively large numbers. Going in the opposite direction —finding the prime factors of a number—is a much harder problem. In practical computing terms this means that it takes many more steps in the factoring computation to get to the final answer than are needed for the multiplication computation. For example, it is relatively easy to work out by long-multiplication that 19 times 23 is 437. However, it takes much longer to factor 437 into 19 and 23. Try it with the number 3953.

On a classical computer, the fastest method—or algorithm—for factoring large numbers is the “number sieve algorithm”. If N is the number you wish to factor, the classical computer needs about exp[1.9 (log N)⅓] steps. For large N this is a very big number! On the other hand, a quantum computer can do this computation in much fewer steps. Much, much fewer: (log N)3. We call this an exponential speed-up. If both computers run at the same clock cycle, the quantum computer will be much faster than the classical computer. You can see how much faster in this app: choose a large number to factor, and see how much time a quantum computer requires, compared to a classical computer.

You may wonder what all the fuss is about. We care about factoring a great deal! The asymmetry in hardness of multiplication and factoring can be exploited to create cryptographic codes in which encryption (via multiplication) is fast, but decryption without a key is slow (factoring).

The cryptography used today is pretty much all based on this mathematical property. If a quantum computer can quickly factor very large numbers, all these cryptographic schemes will be insecure. However, photons come again to the rescue: as we saw above, in quantum cryptography we can use photons to detect the presence of an eavesdropper.

Speed of

Quantum

Computing

COMMUNICATING WITH LIGHTLight is the ultimate data carrier. Optical fibres form the backbone of the internet providing high capacity date links between countries, large cities and more recently to cabinets at the end of your road.

Using total internal reflection optical fibres guide ultra-short pulses of laser light, with a modern system able to carry data at the rate of 1.6 million bits per second along a single fibre. This is equivalent to 43 DVDs every second or 25 million simultaneous telephone calls. This is more than 10,000 times faster than current 4G rates. In research laboratories data rates as high as 26 terabits/s have been achieved.

One reason why fibres can carry such large amounts of data is that multiple wavelengths or colours can be injected into the optical fibre. There is no interaction between these different colours so at the far end of the fibre they can be extracted and separated. A modern long distance system might use 160 different wavelengths with each wavelength carrying 10 gigabits per second to give a total capacity of 1.6 terabits per second. This is known as wavelength-division multiplexing.

The demonstration is a single channel low data rate optical fibre system. The amplitude of the audio signal is used to vary (modulate) the frequency of the laser pulses. This is referred to as frequency modulation (FM) which is much less sensitive to interference than modulating the amplitude (AM) and so results in a higher quality system. For a similar reason FM analogue radio has a higher quality than AM radio.

Light from the laser travels across the breadboard via a chopper wheel, which can be used to block the light, confirming that it is carrying the data. The light is picked up by an optical fibre and fed to a photo-detector and amplifier. In a real fibre system the laser would inject light directly into the fibre.

Each laser pulse is approximately 10 millionths of a second (10 μs) long and contains one hundred thousand million photons. With so many photons it is easy to ‘steal’ some and use them to eaves-drop into the data. A beam splitter can be moved into the laser beam, which splits off half the light and directs this via the mirror onto a second optical fibre and eavesdropper box.

It is now possible to build a communication system where each pulse contains only one photon. Data is encoded via the polarisation state of the photons. Although it is possible for an eavesdropper to intercept the photons, these have to be re-transmitted to try to conceal this interception. However, single photons form a quantum system so the act of measurement changes their properties. This change can be detected by the two legitimate people using the system. Hence, it is possible to construct a system secured by the laws of physics, where any attempt to eavesdrop can be detected.

This is a quantum cryptography system.

Photonic systems combine both light and electronics and can be shrunk down to sizes below one millionth of a metre (1 μm). Nano-scale semiconductor objects known as quantum dots can be used to produce a regular sequence of single photons for use in quantum cryptography systems and the quantum mechanical properties of the photons can be used as the building blocks of an optical quantum computer. The application of photons to quantum computing is explored in the other areas of our exhibition.

SINGLE PHOTON CHALLENGEA photon is the smallest element of light, and just a single photon can be used as a carrier of information. Devices that are able to count single photons one by one are called single photon detectors and are one of the key technologies required for reliable quantum computing and quantum communication

How difficult is it to detect a single photon? An average produces approximately 100 billion billions photons per second. Digital cameras collect around half a million photons for a single photograph. Human eyes are also very sensitive organs, and we have made a demonstration setup in which you can test your eyes against the single-photon detector. Our source of photons is a light emitting diode (LED) that generates short light pulses of various intensities. Light from the LED is coupled straight to the optical fibre and is split equally into three fibre outputs. Two are connected to player’s oculars and one is connected to a detector. You should be able to see dim flashes as the photons hit your eye’s retina.

• What is the ultimate limit

of human eye sensitivity?

• What is the minimum number of

photons that we can notice?

• Read chapter 60 ‘The Telegraph’ of the Count of

Monte Cristo by Alexandre Duma’s for an early example (1820s) of the use of light to carry data.

This is also an example of how the communication

can be compromised.• Research the operation of a quantum cryptography

system, particularly how the data is encoded using

the polarisation of the single photons and why an

attempt to eavesdrop this data can be detected

by the users of the system. What techniques can

be used to create the required regular stream of

single photons?• Why can light carry data at much higher rates

than radio? • Investigate the components of a modern long

distance fibre optic system. These include the lasers, detectors and amplifiers. How many different wavelength channels do the systems use?

• Can you think of an experiment to show that light of two different wavelengths or colours does not interact?

QUANTUM INTERFERENCE OF PHOTONSWhile photons behave as particles when they trigger detectors and our eyes, they behave as waves when they encounter each other. An important quantum interference effect occurs when two photons enter opposite sides of a beam splitter.

Beam splitters are often made out of cubes of glass, and can be found in SLR cameras to divert half of the incoming light to camera’s light sensors and the other half to the eyepiece.

On a chip, these beam splitters are not made of glass cubes, but are instead made of tiny semiconductor waveguides routing photons closely to each other.

When a photon enters a beam splitter, it will randomly choose the output, either crossing over or staying in the same waveguide. When two photons enter from both sides, you may expect that the two photons end up all over the place, because they each independently and randomly choose whether to cross over or not. However, this is not the case! If the photons are identical in frequency (colour) they will always emerge together on either exit of the beam splitter. This purely quantum property of light is the key feature for quantum computation.

For our exhibition we have made a large beam splitter model in which you can explore this quantum behaviour of the photons.

Scan here to watch our YouTube video explaining quantum interference

• Why does light reflect from flat surfaces, like glass in windows?

Photons carry

informationThe Core of Computation

Readout

of the

computation