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Some problems are too hard for even quantum computers<\/p>\n
Yaroslav Kushta\/Getty Images<\/p>\n<\/p>\n
Researchers have identified a \u201cnightmare scenario\u201d calculation related to exotic types of quantum matter that would be impossible to solve, even for a very efficient quantum computer.<\/p>\n
Without the complexity of quantum states of matter, determining the phase of a material can be relatively simple. Take water, for example \u2013 it is straightforward to tell whether it is in a solid or liquid phase. The quantum version of this task, however, can be a lot more daunting. Thomas Schuster<\/a> at the California Institute of Technology and his colleagues have now proven identifying quantum phases of matter can get too difficult even for quantum computers<\/a>.<\/p>\n They mathematically analysed a scenario where a quantum computer is presented with a set of measurements about a quantum state of an object and has to identify its phase<\/a>. Schuster says this is not always an impossible problem, but his team proved for a substantial portion of quantum phases of matter \u2013 the more exotic relatives of liquid water and ice<\/a>, such as \u201ctopological\u201d phases that feature odd electric currents \u2013 a quantum computer may need to calculate for an impossibly long time. The situation is like the worst version of a lab experiment where identifying the properties of a sample would require keeping an instrument on for billions or trillions of years.<\/p>\n This doesn\u2019t make quantum computers practically obsolete for this task. Schuster says these phases are unlikely to show up in actual experiments with materials or quantum computers \u2013 they are more of a diagnostic for where our understanding of quantum computation is currently lacking than an imminent practical threat. \u201cThey\u2019re like a nightmare scenario that would be very bad if it appears. It probably doesn\u2019t appear, but we should understand it better,\u201d he says.<\/p>\n