bit A contraction of BInary digiT. The basic unit of storage in computing, a bit can have a value of 0 or 1.
conservation of momentum The momentum of an object is its mass times its velocity. Momentum is conserved, so if for instance a stationary particle (with zero momentum) splits into two moving particles, those particles must have equal and opposite momentum.
encryption The use of codes and cyphers to conceal the meaning of information.
hidden variables Einstein and others doubted the probability-based nature of quantum theory. They believed that there was an underlying reality that provided the actual value observed, rather than its being dependent on probability. The concealed values are called hidden variables.
local reality The idea that a quantum particle can only influence another particle if it is nearby (locality) and if its properties have real values (reality). Einstein’s EPR thought experiment (see here) established that either quantum theory was flawed or local reality did not exist for quantum particles.
microwave cavity A metal chamber holding an electromagnetic wave in the microwave part of the spectrum. The chamber acts as a resonator, just as a string can vibrate with a particular frequency, but rather than a physical wave fitting between two fixed points on a string, in a microwave cavity there is an electromagnetic wave fitting between two ‘fixed’ points created by the metal walls of the chamber.
MRI scanner A medical device, formerly known as a NMR (nuclear magnetic resonance) scanner, that uses powerful, often superconducting magnets to manipulate the quantum spin of protons in the nucleus of the hydrogen in water molecules, usually in living things. When the molecules flip they act as tiny transmitters, whose output is then detected. See here.
one-time pad An unbreakable means of encryption devised in 1918. Each character in the message to be encrypted has a randomly selected value added to it. The final message is itself random and not susceptible to cracking. Despite being unbreakable, it was not widely used because the list of values (the ‘pad’) must be provided to both ends of the communication and this pad can be intercepted.
quantum dots Nanoparticles of a semiconductor that act as an artificial atom. They are used in quantum technology, notably in electronics and solar cells, and as qubits in quantum computers. See here.
quantum entanglement A fundamental aspect of quantum theory: two (or more) quantum particles can be linked together in such a way that a change made to the state of one particle is reflected instantly in the other, however far apart they are separated. Einstein believed that this was impossible, as the particles ‘communicate’ faster than the speed of light, but it has been repeatedly demonstrated in experiment.
qubit The quantum computing equivalent of a bit. Where a bit can only have a value of 0 or 1, a qubit can be in a superposition of states where it represents the probabilities of being 0 or 1. The qubits can also be entangled, multiplying up the combinations of values, so that a much larger computation can be done with qubits than could be done with the same number of bits.
superposition When a quantum particle has a state with, say, two possible values it will not have an actual value but rather a superposition – a collection of probabilities of being in the states – until it is measured, when it collapses to an actual value. A tossed coin has two states but no superposition. Before we look, the coin already has one of these values. But a quantum particle has no value, just probabilities, while in superposition.
In 1935, Albert Einstein joined with younger colleagues Boris Podolsky and Nathan Rosen to write an academic paper he hoped would prove his long-held belief that quantum theory was incorrect. Formally entitled Can Quantum-Mechanical Description of Physical Reality Be Considered Complete? it is universally known by its authors’ initials, ‘EPR’. The paper imagines a particle splitting into two, with the new particles flying off in opposite directions. According to quantum theory, after a while the momentum of the particles does not have an absolute value, merely a range of probabilities. Measure the momentum of one particle and it takes a specific value. Instantly, however far apart the particles are, the other particle’s momentum must become equal and opposite to fulfil conservation of momentum. A similar effect can be obtained on measurements of position. The EPR paper concludes that either quantum theory is wrong, and there are hidden values that specify the momentum (and so on) before measurement, or locality – the idea that one thing cannot influence another at a distance without something passing between them – has to be thrown out. Einstein and friends conclude ‘No reasonable definition of reality could be expected to permit this.’ Here, Einstein thought, was the clincher. Experiments would prove him wrong.
3-SECOND FLASH
Einstein’s EPR thought experiment, designed to destroy quantum theory, demonstrated when put into practice that Einstein was mistaken.
3-MINUTE THOUGHT
EPR can be a little confusing as it refers to both the momentum and position of the particle, which is reminiscent of the uncertainty principle. But EPR would have worked just as well with a single property. When Schrödinger pointed out how using two properties hid the meaning of the paper, Einstein replied that using two properties ‘ist mir Wurst’, literally ‘is sausage to me’, colloquial German for: ‘I couldn’t care less.’
RELATED THEORIES
HEISENBERG’S UNCERTAINTY PRINCIPLE
3-SECOND BIOGRAPHIES
ALBERT EINSTEIN
1879–1955
German-born physicist who, though sceptical, contributed to quantum theory
BORIS PODOLSKY
1896–1966
American physicist who may have worded EPR
NATHAN ROSEN
1909–95
American-Israeli scientist who worked on EPR and later devised the idea of the wormhole
30-SECOND TEXT
Brian Clegg
The spooky linkage of entanglement means that observing a value in one particle instantly influences the other.
The basic idea of Schrödinger’s Cat is the superposition of quantum states: both the atomic nucleus and the cat in the box are in two states simultaneously. If you open the box, you find the cat either dead or alive, and the nucleus decayed or intact. In quantum-speak, the cat and the atomic nucleus are ‘entangled’. Typically, two identical particles created in one process are entangled, and remain so even if they become separated by long distances. Both these particles are in a superposition of two quantum states, but if you check on one of them, your measurement immediately affects the quantum state of the other particle. Einstein and colleagues Podolsky and Rosen argued that if two particles remain entangled over long distances, a physical influence between them has to travel faster than light, which contradicts the theory of relativity. In 1964, John Bell produced a measurement that enabled experimenters to distinguish between a link that took place at the moment of measurement and one in which there were ‘hidden variables’ that set up the values that would be measured before the particles separated. This distinguishing factor was Bell’s inequality. In 1984, Alain Aspect performed such an experiment on photons, supplying experimental proof of their entanglement.
3-SECOND FLASH
Einstein disagreed with Schrödinger’s idea of quantum entanglement, which he called ‘spooky action at a distance’, but did not live to see the experiments that vindicated Schrödinger.
3-MINUTE THOUGHT
Two entangled particles can be viewed as a single physical object, even if they are light years apart. Quantum entanglement will be a powerful tool in future computing and in data encryption. While bits in current computers are switched with electric pulses, qubits will be linked by entangling them with subatomic particles.
RELATED THEORIES
3-SECOND BIOGRAPHY
ALAIN ASPECT
1947–
French experimental physicist who demonstrated quantum entanglement
30-SECOND TEXT
Alexander Hellemans
The endangered cat in Schrödinger’s thought experiment is entangled with the nucleus of a decaying atom.
John Bell’s brothers and sister left school at 14, so it was a surprise when young Stewart (the family referred to him by his middle name to distinguish him from his father) announced he intended to go to university and become a scientist. Although this was something new for the Bell family, his mother encouraged it, wanting ‘the Prof’, as they sometimes called him, to have a life ‘where he could wear his Sunday suit all week!’
Bell attended Belfast Technical High School and Queen’s University, Belfast. Afterwards, rather than stay in academia, the financially conscious Bell got a job with the UK Atomic Research Establishment at Harwell in England. While at Harwell he met his wife-to-be, Mary Ross, a Scottish physicist, and the pair secured posts at CERN, the European Centre for Nuclear Research near Geneva.
Although particle physics was Bell’s bread and butter job, a sabbatical in 1963 gave him a chance really to think about quantum theory, a subject that had always fascinated him. Bell had some sympathy with Einstein’s view that there was something uncomfortable about quantum theory, that there ought to be reality underlying the apparent randomness. He once said of quantum physics: ‘I hesitated to think it might be wrong, but I knew that it was rotten.’
Einstein’s EPR thought experiment (see here) had shown that either there was a big hole in quantum theory or that local reality was untrue. Local reality meant a world that didn’t depend on probability and that did not allow distant particles somehow to communicate instantly, Bell came up with his own thought experiment, providing a measurement that would distinguish between the two possibilities. He was a theorist and did not know how this measurement could be put into practice, but he had set a benchmark with what became known as ‘Bell’s theorem’ that made it possible to check the validity of the remarkable claims made by quantum theory. If experimental results fell statistically outside a certain range – known as ‘Bell’s inequality’ – then Bell’s theorem was true and local reality was doomed.
Later experiments addressed Bell’s theorem and showed that Einstein and, in his heart, Bell were wrong. Quantum theory did appear to be correct and did violate local reality. Bell’s untimely and unexpected death at the age of 62 brought to an end the career of a thoughtful and inspired scientist.
Brian Clegg
28 July 1928
Born in Belfast to John Bell and Annie née Brownlee
1948
Receives experimental physics degree from Queen’s University, Belfast
1949
Receives mathematical physics degree from Queen’s and goes to work at the UK Atomic Research Establishment, Harwell, Oxfordshire
1954
Marries Mary Ross
1956
Completes PhD in physics at University of Birmingham
1960
The Bells move to work at CERN near Geneva
1964
Publishes breakthrough paper, ‘On the Einstein-Podolsky-Rosen Paradox’, specifying Bell’s inequality
1972
American group of John Clauser, Abner Shimony, Michael Horne and Richard Holt provides first experimental verification of Bell’s theorem, supporting quantum theory, but their approach has a possible loophole
1982
French physicist Alain Aspect closes the loophole, vindicating quantum theory using Bell’s theorem
1987
Elected Foreign Honorary Member of the American Academy of Arts and Sciences
1 October 1990
Dies in Geneva, Switzerland
2008
The John Stewart Bell Prize for research in fundamental issues in quantum mechanics is created
As long as we have had written communication, we have tried to keep some messages secret. Many codes and cyphers are easily broken, but there is a totally secure method: the one-time pad. Here a random value is added to each character to be encrypted. The result is a truly random piece of text – but it can be decoded with the key. Such pads are rarely used because it is too easy for the key to be discovered by conventional spying. But quantum physics overcomes this problem. Quantum cryptography originated with Charles Bennett and Gilles Brassard, who used the polarization of individual photons as the key. This did provide a one-time pad, but had technical issues that could lay it open to interception. Quantum entanglement, however, provides a one-time pad key that does not exist before the message is sent. Usually the randomness of the value communicated instantly by quantum entanglement is a disadvantage. But if that random value is used as the key, it will be available to decode the message as soon as it is encrypted. What’s more, it is possible to test whether particles are still entangled – so the system automatically detects any interception of the quantum key.
3-SECOND FLASH
Quantum particles, and particularly entangled particles, make great carriers of secret data, providing their own random one-time pad.
3-MINUTE THOUGHT
In 2004, Anton Zeilinger, one of the foremost experimental physicists working on quantum entanglement, demonstrated the use of an entangled one-time pad in a way that beat any lab experiment for its dramatic scope. He set up a 600ft (500-metre) link through the sewers between City Hall and the Bank of Austria in Vienna and (with permission) used an entanglement-encrypted message to transfer 3,000 euros from the Mayor of Vienna’s funds to the University’s account.
RELATED THEORIES
3-SECOND BIOGRAPHIES
CHARLES H. BENNETT
1943–
American physicist and information theorist mostly working at IBM
ANTON ZEILINGER
1945–
Austrian quantum physicist specializing in entanglement – with a flair for dramatic demonstrations
GILLES BRASSARD
1955–
French-Canadian computer scientist and cryptographer
30-SECOND TEXT
Brian Clegg
Quantum encryption was used to safe-guard an electronic money transfer in Vienna in 2004.
Electrons have a quantum property called ‘spin’. They can spin clockwise or anticlockwise, and it is the electron spin that is responsible for the magnetic properties of certain materials. By hitting electrons with a laser pulse, you can coax them into a superposition state – that is, they assume both quantum states at the same time: rather than having a specific direction of spin they have only probabilities of spinning in each direction simultaneously. The two directions of spin can be assigned values of 0 and 1, corresponding to the 0 or 1 setting of a conventional computer bit, but the superposition and the associated probabilities means that a qubit holds more information. Such a system is called a quantum bit or ‘qubit’. Photons, which can be polarized both horizontally and vertically, and atomic nuclei, by assuming two nuclear spin states simultaneously, are other examples of qubits. The superposition state is very delicate: the smallest disturbance, such as an attempt to detect the quantum state of the subatomic particle, causes it to revert back to a non-superposition state, a phenomenon known as decoherence. Quantum entanglement is important in the use of qubits to link data without causing decoherence.
3-SECOND FLASH
Qubits behave like bits. They can be on or off, but in effect they can be on and off at the same time.
3-MINUTE THOUGHT
Qubits will be at the heart of future quantum computers. Any particle or system that can assume two or more quantum states can function as a qubit. Researchers create qubits by several experimental means. For example, they can lock electrons in quantum dots and manipulate their spins with laser beams. The spin of atomic nuclei can be manipulated with radio waves, as in MRI scanners. Serge Haroche pioneered the storing of quantum data by trapped photons in microwave cavities.
RELATED THEORIES
3-SECOND BIOGRAPHY
1944–
French physicist who shared (with David J. Wineland) the 2012 Nobel Prize in Physics for his experimental research in quantum physics
30-SECOND TEXT
Alexander Hellemans
Measuring spin will always produce ‘up’ or ‘down’; which one depends on the qubit’s underlying probabilistic state.
Today’s computers contain millions of tiny transistors that use electric charges to store data as bits. The presence of a charge corresponds to a 1, and its absence to a 0 – this information is called a binary digit, or ‘bit’. Computers process numbers by representing them with series of bits that can be switched on or off individually. For example, with four bits you can represent the numbers 0 to 7 as 0000, 0001, 0011, 0111, 1111, 1110, 1100 and 1000. A conventional computer processes these data one at a time. Four cubits, however, with each cubit being a superposition of 0 and 1, will represent these 8 numbers simultaneously, allowing them to be processed in parallel. However, the enormous power of quantum computers becomes apparent when the number of qubits increases. Ten cubits allow the simultaneous processing of 1,023 numbers. The enormous computation power quantum computers are expected to achieve is mindboggling: 20 qubits can process 1 million parallel calculations; with 40 qubits, the number of parallel computations will increase to 1 million million. Although the creation of qubits that remain entangled will require the development of new technologies, researchers are hopeful of using a large number of qubits to achieve enormous computing power.
3-SECOND FLASH
Qubits will play a central role in quantum computers because they will allow parallel processing on a massive scale.
3-MINUTE THOUGHT
Richard Feynman suggested that tiny quantum mechanical computers would be able to simulate quantum systems. Besides the modelling physical process quantum computers will break any records in mathematics. For example, they will be able to factor numbers of 400 digits in a few seconds, enabling the cracking of encryption keys used in banking.
RELATED THEORIES
3-SECOND BIOGRAPHY
RICHARD FEYNMAN
1918–88
American physicist who suggested computers that would obey quantum mechanical laws
30-SECOND TEXT
Alexander Hellemans
In a quantum computer, quantum bits or qubits replace the traditional bit to enable parallel computation.
On a dark, moonless night in 2012 scientists set the current distance record for quantum teleportation: 89 miles (144 kilometres), using a laser to beam photons between different islands of the Canaries. These photons were intimately connected to one another via the quantum property of entanglement, so that an action made on one of the pair immediately affected its entangled partner, however distant. The team, led by Anton Zeilinger at the University of Vienna, sent one of an entangled pair of photons through the air to a detector on the next island. They then used that pair as a quantum communication line to send information about another quantum object, reconstructing it at the other end of the line. Quantum teleportation sounds much like sci-fi, so when computer scientist Charles Bennett of IBM in New York and colleagues first proposed it in 1993 it attracted immediate attention. It is now a serious area of research, with applications in quantum technologies for computing and telecommunications. It has been demonstrated in various systems, including between clouds of caesium atoms and within electric circuits. Scientists now have their eyes on space: teleporting to orbiting satellites may be essential for a global quantum communications network.
3-SECOND FLASH
In quantum teleportation, all the information about a quantum object is scanned and recreated in a new place using entangled particles that form the ends of a quantum communication line.
3-MINUTE THOUGHT
Quantum teleportation does not allow faster- than-light communication, because to reconstruct your quantum object at the end of the line you need instructions from the sender, which are sent via a classical communication line. But it does get round the no-cloning rule, which prevents you making a perfect copy of a quantum object. Instead, teleportation works by shifting where the quantum information is located, and in the process destroys the original.
RELATED THEORIES
3-SECOND BIOGRAPHIES
1943–
Fellow at IBM Research whose work focuses on the relation between physics and information
ANTON ZEILINGER
1945–
Austrian quantum physicist and head of the team that pioneered long-distance quantum teleportation experiments
30-SECOND TEXT
Sophie Hebden
Quantum teleportation between the Canary Islands was a precursor to satellite links.
Ancient Greek philosopher Zeno formulated a number of paradoxes that appear to prove that motion is impossible. In terms of classical physics, these paradoxes are easily explained away as fallacies. But in 1977 George Sudarshan and a colleague at the University of Texas drew a parallel between the observation that an arrow in flight does not appear to be moving if we take a single moment in time and a little known quantum phenomenon now termed the quantum Zeno effect. As real-world examples tend to be complicated, it is easier to illustrate the quantum Zeno effect with a thought experiment. The probability that a radioactive atom will decay in a given interval of time is often said to be constant, but this isn’t strictly true. Immediately after the atom has been observed in an undecayed state, its rate of decay is zero – although it quickly ramps up to its ‘constant’ value. But if another observation is made before this has had a chance to happen, the decay rate is pushed back down to zero … and so on as long as repeated observations are made. It may not be true that a watched kettle never boils – but a watched atom never decays!
3-SECOND FLASH
If a quantum system is observed frequently enough, it will never change state – even if the system is unstable.
3-MINUTE THOUGHT
Ever since the quantum Zeno effect was discovered, physicists have been trying to build practical applications around it. But it’s possible Nature got there first. According to one theory, migratory birds can detect the Earth’s magnetic field using pairs of entangled electrons located inside their eyes. What is not clear, however, is how the birds maintain the necessary entangled state sufficiently long for the mechanism to work. The answer may be that they exploit the quantum Zeno effect.
RELATED THEORIES
3-SECOND BIOGRAPHIES
fl. 5th century BCE
Greek philosopher who suggested that precise observation could freeze an arrow in flight
1931–
Indian physicist and prolific researcher in quantum optics and fundamental physics
30-SECOND TEXT
Andrew May
Zeno’s arrow did not appear to be moving when watched, and a quantum particle does not decay when observed.
