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The Scicurious Brain

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Scicurious Guest Writer! Quantum Machines Become Reality


Please welcome this month's Scicurious Guest Writer, Rory Fenton!

As a child, I would close my eyes and think that everyone else was plunged into darkness too. It doesn't take long to realize that, actually, the rest of the world remains the same whether you're looking at it or not. But twenty years and a physics degree later I've started to ask that same question I thought I solved at age three; do things really stay the same when we aren't looking at them? According to quantum mechanics, the answer is no. In fact it is precisely when we aren't looking that the universe is at its most unusual. This theory could be finding its way into everyday life, from medical devices to computers. The child in all of us might just have been onto something.

(Quantum Lit: A Tale of Two Cities, at 1/25,000 of its normal size. Source)

Quantum mechanics could soon be running your computer thanks to recent leaps in nanotechnology. As technology gets smaller, computer components could soon be built on an atomic level. This means smaller, faster computers. It could also lead to machines behaving very quirkily indeed.

Quantum mechanics is the theory of the very smallest particles of physics; such as electrons, protons and neutrinos. Its discovery at the start of the last century was nothing short of a revolution in the way physicists viewed the world, even more so than Einstein's relativity. The impact of its conclusions on our understanding of the universe were so radical that many scientists strongly resisted the idea at first. What was under attack was the cornerstone of science; precise measurement. Precise measurement was fundamental to science. The only thing that should affect the accuracy of your measurements is how good your microscope or ruler or clock or whatever is. In theory, it should be possible to measure anything with perfect accuracy. But at the start of the last century, quantum mechanics came along and turned this view on its head.

The strangeness of quantum mechanics comes from the math it uses. The mathematics of the old, precise science are just like the mathematics of everyday life where the order of our numbers doesn't matter. So 6+2 is the same as 2+6 and 4x3 is the same as 3x4. The mathematics of quantum mechanics are more like words; the order of your letters matters. Think of the difference between having a pet dog and having a pet god. The fact that these are two different things means that when we take them away, unlike with 3x4-4x3, we don't get zero. What's more, the part we're left with (admittedly, thinking of “dog minus god” won't get us very far here) is an "imaginary number", which means the square root of a negative number, and isn't exactly the kind of thing you can measure with a ruler. This is what physicists call “the uncertainty principle”; the fact that because quantum mechanics uses the mathematics of letters rather than the mathematics of numbers, we will always be left with something in our experiment we can't properly measure. What's more, the fact that we have made a measurement will affect how our experiment develops. The path taken by an electron will depend on whether you are measuring its position or not. Imagine throwing a ball in the air only to find where it lands depends on whether you looked at it or not. This is science confirming the childhood superstition; the world is affected by you looking at it.

When this uncertainty principle was first discovered it was a slap in the face of science. It no longer mattered how precise our measuring devices were, there would always be a tiny part of our experiments we couldn't measure. The one consolation of this new theory was the size of this unknown quantity; it was too small to matter to our everyday world of tables, cars, trees and wrist watches so the scientific world could breath a sigh of relief. That is, until we started to make the quantum world our own.

The computer revolution holds that smaller is better and the nano revolution is the most extreme expression of this. Computers and mechanical machines are being built on increasingly small scales following a trend that started with a challenge from Nobel Prize winner Richard Feynman. In a 1959 lecture, Feynman set out his vision for the technology of the future. He imagined building a mechanical set of hands at 1/4 the scale of our own, which could then be used to construct a further set of hands at 1/16th scale, which could then be used to construct even smaller hands and so on, allowing the construction and manipulation of tiny devices by the smallest pair of hands. He also announced two prizes of $1,000, small in value but, from so important a physicist, huge in prestige. The first prize announced was for the first person to create a motor just 1/64th of an inch across. Feynman was surprised to be giving out the prize just a year later, not to a scientist but to a highly skilled craftsman, William McLellan, who built such a working motor using the conventional tools of a microscope and tweezers and weighing just one four-thousandth of a gram. Feynman's second prize took longer to be claimed. It was to be given to the first person to shrink text to a scale small enough that the entire Encyclopaedia Britannica could be written on a pin head, making the text 25,000 times smaller. This was eventually achieved by Stanford graduate student Tom Newman in 1985, who shrunk the first paragraph of A Tale of Two Cities to 1/25,000 of it's normal size, using a beam of electrons to scratch the surface of a thin plastic membrane.

Feyman's challenge ignited the field of microelectromechanical systems: machines with both electrical and mechanical parts, built on the scale of one thousandth of a millimetre, a micrometer. Although initially more about researchers showing off, these devices soon found their way into everyday life from medical devices to earthquake detectors. Their small scales made them very sensitive to small changes; perfect for making very sensitive detectors. Your car airbag will likely contain a microelectromechanical system, just waiting to be head-butted.

The field of tiny machines has come far since 1959 but they were never quite small enough to have to worry about quantum mechanics. This is now changing with the past decade seeing the first nanoelectromechanical systems built; machines now on the scale of one millionth of a millimetre. At this scale, scientists can no longer rely on classical mechanics to govern their creations as quantum uncertainty is big enough to matter. This means the quantum realm holds and a whole new world of possibilities opens up.

The most successful nanoelectromechanical systems (NEMS) so far built have been tiny levers. Levers are ideal NEMS because of the way they vibrate at tiny scales. The resonant frequency of a lever, the frequency at which it most strongly vibrates, is inversely proportional to its length. This means that if you make a lever half as small, it will vibrate twice as quickly. If you can make a lever one billion times smaller than your finger, it will vibrate very quickly indeed. Current technology allows for levers just 10 millionths of a millimetre in length which will vibrate 10 billion times every second. For comparison with an everyday machine, a car engine won't exceed one hundred vibrations per second.

Levers will interact with neighboring electrical and magnetic fields, allowing them to detect nearby particles. The quick movement of a NEM lever means that any sudden changes in its motion can be spotted very quickly. If I tap you on the arm every ten seconds and ask you to tell me when you notice a delay, you'll have to wait at least ten seconds to tell I'm taking longer. If I tap you every second, you'll detect changes a lot quicker. This makes NEM levers perfect for ultra-sensitive detectors, able to quickly detect anything that disturbs their regular vibrations. In theory, a NEM lever detector can have a sensitivity all the way down to the quantum limit, allowing it to detect individual protons, which could lead to ultra-sensitive MRI scanning. Modern medical MRI scanners require about one million billion protons to get a signal; NEM devices have already displayed accuracies of ten times that and are set to get even more accurate. Where an MRI scanner could scan a brain, a NEM scanner could show us individual neurons.

Other than potential medical uses, this level of precision offers exciting glimpses into the quantum universe. Physicists know the laws of quantum mechanics by inferring them from the relatively large scale of atoms but what they have yet to achieve is direct measurement of quantum activity. With a NEM device, as detectors get smaller and more accurate, this could soon be possible.

When an atom gets excited it doesn't gradually build up energy, rather it “jumps” to the next energy level without ever passing between. It's like counting 1,2,3,4 without mentioning 1.01, 1.02 etc. We can tell this because excited atoms release light at very specific frequencies, corresponding to exactly "1" or exactly "2" and never 1.546 or whatever. Now imagine a NEM device measuring the energy of an atom. As these tiny measuring levers get smaller, physicists are hoping soon to be able to directly measure one of these energy jumps, a so called “quantum leap”, providing the most direct evidence yet for quantum mechanics.

Another curious quirk of the quantum world is entanglement; the tendency of two separate objects to be in some way “connected”, with actions on one affecting the other instantly and at any distance. Einstein rightly termed this “spooky”. It is possible for one of these NEM levers to become entangled with nearby electrons. Eventually, environmental factors (such as heat vibrations) will cancel out this entanglement. By measuring the time taken for this cancelling out it will be possible to show that the lever was indeed entangled, giving again a direct measurement of a quantum phenomenon, undeniable proof that nature really can be "spooky".

The most exciting and practical future application of one of these tiny machines is in computing. A tiny computer component, such as a transistor, could control the movements of individual electrons with great accuracy. Transistors are tiny switches, their invention in the 40's paved the way for smaller, lighter and more affordable computers. The computer you're reading this on will contain millions of them. Physicists have proposed a "quantum shuttle” to carry electrons individually across a gap. This involves a “quantum dot” which is a device that traps electrons. The dot oscillates between two wires, allowing electrons to hop on and off. By controlling the energies of the two wires it is possible to ensure the electrons travel in only one direction. A nearby charged nano lever could be used to subtly adjust the oscillation of the dot, controlling the current. The electrons will be traveling using a phenomenon called “quantum tunneling” which enables them to pass through electric fields that would otherwise be too strong for them, saving energy. Not only is this more efficient, it is also much faster than a conventional transistor; about one thousand times faster.

It's an incredible thought that computers that started as mechanical calculating machines and soon became much faster electronic devices, could once again be made of mechanical, moving parts. When you then consider that billions of these would fit in your hand, you can see how exciting this technology is for the future of computing.

NEMS, of course, are not without their challenges. The biggest hurdle to clear is temperature. Heat causes objects to vibrate. On a human scale this isn't at all noticeable but for atoms even the lowest temperatures can rock them like a small dinghy in an almighty storm, which makes attempts to control them futile. Countering this requires temperatures of just a few miliKelvin; just a few thousandths of a degree about absolute zero, the temperature at which all motion stops. These temperatures are only available in a few labs. What's more, the incredible speeds at which these devices are make to vibrate requires very high quality and precise engineering to avoid damage.

These challenges are significant but not necessarily any greater than those faced in creating the original computer; the first transistors, at first only for use in labs soon became the subject of Moore's Law- halving in cost every two years. Nanoelectromechanical systems represent one of the most exciting and promising fields in current technological research. From ultra-sensitive detectors to super-fast computing and insights into the quantum universe, these tiny machines offer big promises. How long it will take before you have one in your smart phone is not yet clear but, in the words of Richard Feynman, there is plenty of room at the bottom.

Rory Fenton is an MSc Epidemiology student at Imperial College London, where he received an MSci in Theoretical Physics. He worked on the theory of quantum machines but is now into global health data analysis. He tweets as @roryfenton.

The views expressed are those of the author and are not necessarily those of Scientific American.

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