A WORLD WITHOUT CAUSE AND EFFECT Logic-defying experiments into quantum causality scramble the notion of time itself. BY PHILIP BALL lbert Einstein is heading out for his This finding1 in 2015 made the quantum the constraints of a predefined causal structure daily stroll and has to pass through world seem even stranger than scientists had might solve some problems faster than con- Atwo doorways. First he walks through thought. Walther’s experiments mash up cau- ventional quantum computers,” says quantum the green door, and then through the red one. sality: the idea that one thing leads to another. theorist Giulio Chiribella of the University of Or wait — did he go through the red first and It is as if the physicists have scrambled the con- Hong Kong. then the green? It must have been one or the cept of time itself, so that it seems to run in two What’s more, thinking about the ‘causal struc- other. The events had have to happened in a directions at once. ture’ of quantum mechanics — which events EDGAR BĄK BY ILLUSTRATION sequence, right? In everyday language, that sounds nonsen- precede or succeed others — might prove to be Not if Einstein were riding on one of the sical. But within the mathematical formalism more productive, and ultimately more intuitive, photons ricocheting through Philip Walther’s of quantum theory, ambiguity about causation than couching it in the typical mind-bending lab at the University of Vienna. Walther’s group emerges in a perfectly logical and consistent language that describes photons as being both has shown that it is impossible to say in which way. And by creating systems that lack a clear waves and particles, or events as blurred by a order these photons pass through a pair of flow of cause and effect2, researchers now haze of uncertainty. gates as they zip around the lab. It’s not that this think they can tap into a rich realm of pos- And because causation is really about how information gets lost or jumbled — it simply sibilities. Some suggest that they could boost objects influence one another across time doesn’t exist. In Walther’s experiments, there the already phenomenal potential of quantum and space, this new approach could pro- is no well-defined order of events. computing. “A quantum computer free from vide the first steps towards uniting the two 590 | NATURE | VOL 546 | 29 JUNE© 20172017 Mac millan Publishers Li mited, part of Spri nger Nature. All ri ghts reserved. ©2017 Mac millan Publishers Li mited, part of Spri nger Nature. All ri ghts reserved. FEATURE NEWS cornerstone theories of physics and resolving this logic has unravelled over the past decade, and 1, the second qubit experiences a causal one of the most profound scientific challenges as researchers have realized that it is possible superposition of both sequences — meaning today. “Causality lies at the interface between to imagine quantum scenarios in which one there is no defined order to the particle’s tra- quantum mechanics and general relativity,” simply can’t say which of two related events versal of the gates (see ‘Trippy journeys’). says Walther’s collaborator Časlav Brukner, a happens first. Three years later, Chiribella proposed an theorist at the Institute for Quantum Optics Classically, this situation sounds impossible. explicit experimental procedure for enact- and Quantum Information in Vienna, “and so True, we might not actually know whether A or ing this idea5; Walther, Brukner and their it could help us to think about how one could B happened first — but one of them surely did. colleagues subsequently worked out how to merge the two conceptually.” Quantum indeterminacy, however, isn’t a lack implement it in the lab1. The Vienna team uses of knowledge; it’s a fundamental prohibition a series of ‘waveplates’ (crystals that change a TANGLES IN TIME on pronouncing on any ‘true state of affairs’ photon’s polarization) and partial mirrors that Causation has been a key issue in quantum before a measurement is made. reflect light and also let some pass through. mechanics since the mid-1930s, when Einstein These devices act as the logic gates A and B challenged the apparent randomness that Niels AMBIGUOUS ACTION to manipulate the polarization of a test pho- Bohr and Werner Heisenberg had installed at Brukner’s group in Vienna, Chiribella’s team ton. A control qubit determines whether the heart of the theory. Bohr and Heisenberg’s and others have been pioneering efforts to the photon experiences AB or BA — or Copenhagen interpretation insisted that the explore this ambiguous causality in quantum a causal superposition of both. But any outcome of a quantum measurement — such mechanics3,4. They have devised ways to create attempt to find out whether the pho- as checking the orientation of a photon’s plane related events A and B such that no one can ton goes through gate A or gate B first of polarization — is determined at random, say whether A preceded and led to (in a sense will destroy the superposition of gate and only in the instant that the measurement ‘caused’) B, or vice versa. This arrangement ordering. is made. No reason can be adduced to explain enables information to be shared between A and Having demonstrated causal indetermi- that particular outcome. But in 1935, Einstein B in ways that are ruled out if there is a definite nacy experimentally, the Vienna team wanted and his young colleagues Boris Podolsky and causal order. In other words, an indeterminate to go further. It’s one thing to create a quantum Nathan Rosen (now collectively denoted causal order lets researchers do things with superposition of causal states, in which it is sim- EPR) described a thought experiment that quantum systems that are otherwise impossible. ply not determined what caused what (that is, pushed Bohr’s interpretation to a seemingly The trick they use involves creating a special whether the gate order is AB or BA). But the impossible conclusion. type of quantum ‘superposition’. Super positions researchers wondered whether it is possible to The EPR experiment involves two particles, of quantum states are well known: a spin, for preserve causal ambiguity even if they spy on A and B, that have been prepared with inter- example, can be placed in a superposition of up the photon as it travels through various gates. dependent, or ‘entangled’, properties. For and down states. And the two spins in the EPR At face value, this would seem to violate the example, if A has an upward-pointing ‘spin’ experiment are in a super position — in that idea that sustaining a superposition depends (crudely, a quantum property that can be pic- case involving two particles. It’s often said that on not trying to measure it. But researchers tured a little bit like the orientation of a bar a quantum object in a superposition exists in are now realizing that in quantum mechanics, magnet), then B must be down, and vice versa. two states at once, but more properly it simply it’s not exactly what you do that matters, but Both pairs of orientations are possible. But cannot be said in advance what the outcome of what you know. researchers can discover the actual orientation only when they make a measurement on one of the particles. According to the Copenhagen interpretation, that measurement doesn’t just AN INDETERMINATE CAUSAL ORDER LETS reveal the particle’s state; it actually fixes it in that instant. That means it also instantly fixes RESEARCHERS DO THINGS WITH QUANTUM the state of the particle’s entangled partner — however far away that partner is. But Einstein SYSTEMS THAT ARE OTHERWISE IMPOSSIBLE. considered this apparent instant action at a distance impossible, because it would require faster-than-light interaction across space, a measurement would be. The two observable Last year, Walther and his colleagues which is forbidden by his special theory of rela- states can be used as the binary states (1 and 0) devised a way to measure the photon as it tivity. Einstein was convinced that this invali- of quantum bits, or qubits, which are the basic passes through the two gates without immedi- dated the Copenhagen interpretation, and elements of quantum computers. ately changing what they know about it6. They that particles A and B must already have well- The researchers extend this concept by cre- encode the result of the measurement in the defined spins before anybody looks at them. ating a causal superposition. In this case, the photon itself, but do not read it out at the time. Measurements of entangled particles show, two states represent sequences of events: a par- Because the photon goes through the whole however, that the observed correlation between ticle goes first through gate A and then through circuit before it is detected and the measure- the spins can’t be explained on the basis of pre- gate B (so that A’s output state determines B’s ment is revealed, that information can’t be existing properties. But these correlations don’t input), or vice versa. used to reconstruct the gate order. It’s as if you actually violate relativity because they can’t be In 2009, Chiribella and his co-workers came asked someone to keep a record of how they used to communicate faster than light. Quite up with a theoretical way to do an experiment feel during a trip and then relay the informa- how the relationship arises is hard to explain like this using a single qubit as a switch that tion to you later — so that you can’t deduce in any intuitive cause-and-effect way.