Exotic quantum rifts have been created with charged atoms, and they exist in a superposition of being in two places at once. This is a first step towards better understanding the behaviour of such quantum defects in everything from materials to an entire universe.
Defects are ubiquitous 鈥� think of tears in textiles or cloudy imperfections in shiny crystals 鈥� but in quantum systems, they can have the extra property of being topological. That means the overall structure of the system where they occur conspires to make them impossible to fix or remove. The most extreme examples are hypothetical cosmic strings, which are snags in the fabric of space-time. But less dramatic topological defects have been observed in experiments with liquid crystals, quantum magnets and extremely cold atoms.
Now, at Tsinghua聽University in China and his colleagues put one such defect into a quantum superposition. They used a quantum simulator made from 21 ytterbium ions that they controlled with lasers and electric fields. The researchers arranged these charged atoms into one narrow line while making sure that most of them had the same quantum spin as their neighbours. If an ion had an upwards or downwards spin, the ions to its left and right did too, except in one special place: a topological defect in the middle of the chain. An ion at the edge of this defect had a neighbour with the opposite spin.
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The researchers could control where in the line this defect occurred, and they used this to place the same defect in two places simultaneously. More precisely, they created a quantum superposition where it was impossible to tell whether the defect was between the 10th and 11th ions, or between the 11th and 12th ions. This is similar to Erwin Schr枚dinger鈥檚 famous thought experiment where it is impossible to tell if a hypothetical cat is living or dead until you observe it, and it collapses into one outcome or the other.
Such cat-like defects have been theorised previously but never created in experiment. Wu says doing so required that he and his team calibrate their lasers extremely precisely, as any errors in using them would have destroyed the typically fragile superposition 鈥� even though each individual defect is surprisingly stable for a quantum system.
Although the superposition of defects was difficult to achieve experimentally, such superpositions can occur naturally during quantum phase transitions 鈥� when an object that possesses the property of quantum spin changes to a new spin state, akin to when liquid water changes phase to become ice. In 2011, at Los Alamos National Laboratory in New Mexico and his colleagues calculated that, when a spin system undergoes a quantum phase transition, topological defects will appear in Schr枚dinger鈥檚 cat-like superposition states. Instead of indecision between alive and dead, the situation would be as if a post-phase-change ice crystal 鈥渃annot decide鈥� which arrangement of atoms to adopt and thus where that arrangement鈥檚 defects will be.
Zurek says that all experiments to date have only been able to聽carry out the聽鈥減ostmortem examination鈥�, studying the superpositions after measurements have collapsed them. He notes that, in the new experiment,聽the researchers create superpositions of defects 鈥渂y hand鈥�, which could be a first step towards understanding what happens to聽superpositions聽when they crop up naturally. The experiment is elegant, but its future version could be even more interesting, says Zurek, if the researchers used cat states of defects that are further apart from each other.
at the University of Luxembourg says the new experiment is in an area of research that has been largely overlooked. It could mark a beginning of using ion-based simulators, a nascent quantum technology, to clarify fundamental quantum phenomena. Eventually, he says, these simulations may even relate to complex questions about how quantum properties in the universe, such as superposition states, disappear to produce the non-quantum world that we experience.
Science Advances
