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“NO ONE understands quantum mechanics”. Many who have attempted to get to grips with our most basic theory of the workings of material reality would nod vigorously at this quote from Nobel laureate (and quantum and particle physicist) Richard Feynman. With its dead-and-alive cats, particles in multiple places at once, spooky influences travelling over distances and strange implication that we all, somehow, might as observers be implicated in the fabrication of reality, the assaults of quantum theory on our intuition are legion, and legendary. Still, nothing ventured, nothing gained. This fifth New Scientist Essential Guide aims to be a primer of what we do know: how quantum theory came to be, how it fits together and the problems it presents, as well as giving perspectives on technological advances it is…

SOME middle-aged men have trains in their attics. Niels Bohr had Werner Heisenberg. In the winter of 1926-1927, the brilliant young German was working as Bohr’s chief assistant, billeted in a garret at the top of the great Dane’s Copenhagen institute. After a day’s work, Bohr would come up to Heisenberg’s eyrie to chew the quantum fat. They often sat up late into the night, in intense debate over the meaning of the revolutionary quantum theory, then in its infancy. One puzzle they pondered was that exposed by the trails of droplets left by electrons as they passed through cloud chambers, an apparatus used to track the movements of charged particles. When Heisenberg tried calculating these seemingly precise trajectories using the equations of quantum mechanics, he failed. One evening in…

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NIELS BOHR  BORN COPENHAGEN, DENMARK, 7 OCTOBER 1885 DIED COPENHAGEN, DENMARK, 18 NOVEMBER 1962 In 1913, Bohr used the principle of quantisation to postulate that electrons in atoms can exist only in discrete energy states, and thus explained the spectrum of light emitted by a hydrogen atom. This work ensured he followed Einstein to the Nobel prize for physics in 1922. Bohr went on to develop the principle of complementarity, the bedrock of the dominant “Copenhagen” interpretation of quantum theory. This states that the quantum world is both wave- and particle-like; it is the act of measurement that causes it to show one face or the other. ALBERT EINSTEIN BORN ULM, GERMANY 14 MARCH 1879 DIED PRINCETON, NEW JERSEY, 18 APRIL 1955 In his annus mirabilis of 1905, Einstein not…

“If you think you understand quantum mechanics, you haven’t understood quantum mechanics.” Nowhere does that jibe, supposedly made by physicist Richard Feynman half a century ago, ring truer than with the conundrum known as the quantum measurement problem. This is the question of how our concrete, classical reality emerges from the fuzzy quantum world, and what role, if any, we play in that. Standard quantum theory gives seemingly absurd answers, so many rival interpretations have sprung up – all taking as their starting point science’s most notorious thought experiment.…

JOHN WHEELER at Princeton University once said that we live in a “participatory universe” – one that can’t be meaningfully described without invoking our active involvement. It certainly does seem that wave function collapse from many possibilities to one observed reality hinges on some kind of intervention. “Nothing is more astonishing about quantum mechanics,” Wheeler wrote, “than its allowing one to consider seriously… that the universe would be nothing without observership.” If you take quantum mechanics seriously, some element of observer-dependent subjectivity seems impossible to avoid. Enter Markus Müller, the self-described “madman” and a theorist at the University of Vienna. His answer is to represent each individual’s experience at every instant as a string of bits of information – 1s and 0s, like binary computer code. Each observer’s history then…

The promise of quantum computers rests largely on two algorithms. One, developed in 1994 by Peter Shor, then of Bell Laboratories, provides a way for a quantum computer to speedily find the prime factors of large numbers. Classical computers effectively have to try to divide the given number by all possible prime factors (2, 3, 5, 7, 11 and so on) in turn, whereas quantum computers can do these divisions simultaneously. Conventional encryption methods rely on the fact that classical computers cannot factorise efficiently. If Shor’s algorithm were ever implemented on a large scale, encrypted information such as the PIN for your bank card would be vulnerable to hacking – and quantum cryptography would be the only viable defence. There is no need to worry just yet: demonstrations so far,…

“NATURE abhors a vacuum.” This sentiment, which first popped up in Greek philosophy some 2500 years ago, continues to excite debate among scientists and philosophers. The concept of a true void, apart from inducing a queasy feeling, strikes many people as preposterous or even meaningless. If two bodies are separated by nothing, should they not be in contact? How can “emptiness” keep things apart, or have properties such as size or boundaries? While we continue to struggle with such notions, our idea of the vacuum has moved on. We now know, thanks to quantum theory, that empty space is richer than a mere absence of things – and this richness plays an indispensable part in much of modern physics. Even among the ancient Greeks, the void divided loyalties. One influential…

QUANTUM mechanics is often called a theory of the very small. In reality, it explains phenomena on a vast range of scales – from elementary particles and their interactions, through atoms and molecules, all the way to neutron stars and the supernovae that spawn them. So far, essentially all its predictions have been confirmed by experiments. It is the most successful theory of material reality we have ever had. So why have so many physicists, from Albert Einstein onwards, taken the view that quantum theory is wrong? The reasons lie in its mysterious nature, in the phenomena it doesn’t explain and the answers it doesn’t give. That is reason enough to seek what might lie beyond it. I believe we already have the outline of what this deeper answer looks…

Quantum theory is perhaps the most rigorously tested theory in the history of science. It is the fundamental theory of subatomic particles and their interactions, and the basis of models of how three of the four forces of nature work, gravity excluded. It has never yet delivered a wrong result – and yet quantum theory’s random nature and prediction of weird, probabilistic effects and spooky influences between objects that don’t seem to be in contact challenges our intuitions about how reality should work. Let our journey into this strange realm begin.…

WERNER HEISENBERG BORN WÜRZBURG, GERMANY 5 DECEMBER 1901 DIED MUNICH, GERMANY 1 FEBRUARY 1976 Just 25 when he came up with his eponymous principle, Heisenberg made other seminal contributions to the emerging quantum theory – indeed, he won the 1932 Nobel prize in physics “for the creation of quantum mechanics”. The following year, Adolf Hitler’s Nazi regime came to power in Germany. He was investigated by the SS, who condemned him for supporting the “Jewish” theory of relativity. But the regime decided he was too useful to be silenced, and he later became a central figure in German efforts to build a nuclear bomb. Heisenberg’s own attitude towards these efforts remained equivocal, as shown in transcripts of conversations secretly recorded in 1945 when he and other German scientists were captured…

EVER since its mathematical rules began to crystallise in the 1920s, quantum theory has been an affront to our sense of decency. In the world of things we can see and grasp, objects have robust properties that can be described by long-established, “classical” physical theories. In this realm, a cat has a binary position on the dead-alive scale. It also has a definite weight, fur colour, whisker length, position in space. You can measure these things at any time you like without the expectation of ambiguity or sudden change (except maybe in position: cats do sometimes move in mysterious ways). In a word, classical reality is what we would call real. You wouldn’t labour this point if, as we saw in the previous chapter, the quantum world weren’t so different.…

WHILE a graduate student at Princeton University, Hugh Everett III called the enforced separation of the quantum world from the everyday, classical one a “monstrosity”, and decided to find out what happened if the wave function did not collapse. The mathematics he came up with showed that the universe would split every time a measurement is made – or in human terms, whenever we make a decision with multiple possible outcomes. That, in a nutshell, is the many-worlds interpretation. In this interpretation, every decision you take in this world creates new universes: one for each and every choice you could possibly make. There’s a boundless collection of parallel worlds, full of innumerable near-copies of you and everyone else. The leading physicists of Everett’s day, in particular Niels Bohr, one of…

Up till now our discussions of quantum theory may have seemed rather abstract, and far removed from our everyday world. But quantum theory’s practical applications have the potential to revolutionise our lives. In some sense, they already have – the transistor, the laser and many other technologies already exploit basic quantum principles. But a new wave of quantum computing kit could exploit quantum weirdness to the full to give us technological superpowers. Vlatko Vedral starts with the basics.…

1980 PAUL BENIOFF DESCRIBES THE FIRST QUANTUM MECHANICAL MODEL OF A COMPUTER, SHOWING THAT QUANTUM COMPUTERS ARE THEORETICALLY POSSIBLE. 1985 DAVID DEUTSCH DEVELOPS THE IDEA OF A UNIVERSAL QUANTUM COMPUTER: A WAY TO MATHEMATICALLY UNDERSTAND WHAT IS POSSIBLE ON A QUANTUM COMPUTER. 1994 PETER SHOR DEVELOPS “SHOR’S ALGORITHM”, WHICH WOULD ALLOW A QUANTUM COMPUTER TO FACTOR LARGE NUMBERS MUCH FASTER THAN THE BEST CLASSICAL ALGORITHM. 1995 SEVERAL RESEARCHERS INDEPENDENTLY PROPOSE THE FIRST QUANTUM ERROR-CORRECTION METHODS TO AUTOMATICALLY FIX ERRORS THAT CAN EASILY ARISE IN A QUANTUM COMPUTER. 1997 QUANTUM TELEPORTATION, THE TRANSMISSION OF QUANTUM INFORMATION FROM ONE PLACE TO ANOTHER, IS FIRST DEMONSTRATED FOR SINGLE PHOTONS. 1998 RESEARCHERS AT THE UNIVERSITY OF OXFORD RUN A QUANTUM ALGORITHM FOR THE FIRST TIME, ON A 2-QUBIT QUANTUM COMPUTER. 2001 A TEAM AT…

A CENTURY ago, Einstein’s theories of relativity revolutionised our ideas of time. They gave it a physical identity as part of space-time, a malleable fabric on which reality’s events play out. Einstein’s ideas are mathematically sound and, as with the predictions of quantum theory, have been rigorously confirmed by experiments, but moulding together space and time as if they were the same thing doesn’t square with our experience. True, both space and time separate objects and events. But we are free to move as we please in space, at least in theory. Time, on the other hand, dictates a direction of travel, trapping us in an eternal present as it conveys us from the past to the future. If this flowing time is not contained within our fundamental description, where…

ONE glaring incompatibility between quantum theory and general relativity concerns black holes, entities whose gravitational attraction is so strong that not even light can escape their pull. If you throw a quantum particle into one, information about its properties simply disappears. This is deeply weird: ordinarily, any missing information in quantum mechanics can always be found by expanding your viewpoint. Think of it as a missing set of keys: they are never really missing, only being searched for in the wrong places. Information about the physical properties of a particle lost to a black hole, however, can’t be found anywhere – even if you include the whole universe in your search. This black hole information paradox has bothered many great physicists, including Stephen Hawking. The trouble doesn’t end there. Take…

MAX PLANCK’S light-bulb calculations swiftly encountered a problem. The physics he knew predicted that a heated object should absorb and emit electromagnetic radiation at all frequencies, meaning it should generate infinite amounts of energy. Experimental data, not to mention common sense, suggested otherwise. It took Planck a few years before he had his own light-bulb moment: if energy is absorbed or emitted in discrete packets, then the energy radiated by a heated object would be finite. The theory seemed absurd at the time, but once physicists started looking, these “quanta” popped up all over the place. As quantum theory developed, it became clear that not just energy, but many other properties such as electric charge and spin – a property itself only discovered once quantum theory was applied to the…

IN THE most basic version of the double-slit experiment, first performed in 1801 way before quantum theory came along, a beam of light illuminates a plate with two parallel slits cut into it. The light that passes through the slits hits a screen, creating stripes of light and darkness as a result of the interference between the two light waves that went through the slits. This interference is proof that light is not just a classical particle, as Isaac Newton had argued, but also a wave. The weirdness of the quantum version is that interference happens even if you just send one photon through the double-slit set up. That is, it does if you measure the effects at a screen beyond the slits – then you measure a classical interference…

THROW a ball against a wall. Watch it bounce. Catch it. So far, so good. The ball didn’t suddenly disappear through a wall, or spontaneously turn into something else. This perfectly normal, predictable behaviour you just witnessed is classical physics in action. Up until 1900, it was just called physics. Whether you were discussing particles or planets, the rules that governed the bouncing of that ball could be used to describe anything in the universe. The world made sense. Then quantum mechanics happened. The way it replaced hard certainties with wave functions and probabilities had some startling consequences. It meant, for example, that when faced with a hard barrier, a particle had a non-zero probability of being located on the other side, allowing it to perform the seemingly impossible feat…

ERWIN SCHRÖDINGER BORN VIENNA, AUSTRIA 12 AUGUST 1887 DIED VIENNA, AUSTRIA, 4 JANUARY 1961 Fated to be remembered only for his cat – thought to be inspired by a moggie called Milton he owned when a fellow of Magdalen College at the University of Oxford – Erwin Schrödinger worked out the basic dynamics of quantum theory in the mid-1920s, receiving the Nobel prize in physics in 1933 together with Paul Dirac for “the discovery of new productive forms of atomic theory”. That same year Schrödinger, a noted opponent of Nazism, had been forced to leave Germany. He eventually settled in Ireland, becoming a naturalised Irish citizen in 1948, where he furthered his eclectic research interests, including writing his famous book What is Life?, in which he attempted to apply the…

DAVID MERMIN thinks he has something sensible to say about interpreting quantum theory. An atomic physicist at Cornell University in Ithaca, New York, he has spent most of his half-century-long career rejecting philosophical musings about the nature of quantum theory. A few years back, he had an epiphany. We can solve our quantum conundrums, by abandoning the ingrained idea that we can achieve an objective view of reality. According to this provocative idea, the world is not uncertain – we are. It is not Mermin’s idea: in fact, he spent more than a decade arguing against it with its originators, Carlton Caves of the University of New Mexico in Albuquerque, Christopher Fuchs of the Perimeter Institute for Theoretical Physics in Waterloo, Canada, and Rüdiger Schack of Royal Holloway, University of…

Ever fancied playing the role of Schrödinger’s cat? You’ll need a gun whose firing is controlled by a quantum property, such as an atom’s spin, which has two possible states when measured. If the Copenhagen interpretation is right, you have the familiar 50-50 odds of survival. The more times you “play”, the less likely you are to survive. If the multiverse is real, on the other hand, there always will be a universe in which “you” are alive, no matter how long you play. What’s more, you might always end up in it, thanks to the exalted status of the “observer” in quantum mechanics. You would just hear a series of clicks as the gun failed to fire every time – and realise you’re immortal. But be warned: even if…

WHAT makes quantum computers so different? Conventional, classical computers process information using the presence or absence of electrical charge or current. These classical bits have two positions, on (1) and off (0). Semiconductor switches – transistors – flip these bits, making logic gates such as AND, OR and NOT. Combining these gates, we can compute anything that is in principle computable. In quantum computation, the switching is between quantum states. Quantum objects can generally be in many states at once: an atom may simultaneously be in different locations or energy states, a photon in more than one state of polarisation, and so on. In general, quantum theory allows us to distinguish two of these states at any one time. In essence, a quantum bit, or qubit, is in a “superposition”…

REACHING “quantum supremacy” represented a long-sought stride towards realising the immense promise of quantum computers. It involved demonstrating that a quantum processor consisting of 54 superconducting quantum bits, or qubits, was able to perform a random sampling calculation – essentially verifying that a set of numbers is randomly distributed – exponentially faster than any standard computer. Google’s Sycamore device did it in just 3 minutes and 20 seconds, although one of the qubits had to be turned off as it wasn’t working properly. According to Google, the calculation would have taken IBM’s Summit, the world’s most powerful supercomputer, some 10,000 years. IBM immediately pushed back on that claim, insisting that with some clever classical programming, its machine can solve the problem in 2.5 days. IBM, which has its own 53-qubit…