A quantum computer made of charged atoms can catch its own errors when performing any operation – a meaningful step towards more reliable and practical quantum computers.
Conventional computers routinely flag and correct their own errors, so to truly outperform them, quantum computers will have to do the same. However, quantum effects can make errors cascade quickly through the quits, or quantum bits, that make up these devices.
Lukas Poster at the University of Innsbruck in Austria and his colleagues have now built a quantum computer that runs any calculation without errors.
Any operation of a quantum computer can be broken down into simpler computations called quantum gates. These can be applied to a cubit by hitting it with a laser to change its quantum state. The researchers created a set of gates that are necessary building blocks for more complicated computations and that keep even the most complex of them error-free.
They used electric fields to trap 14 calcium ions in place, forming two so-called logical cubits, each made of seven entangled ions. Two more cubits served as flags that let the computer know when some erroneous computation needed to be corrected.
With this universal set of gates, you can approximate every calculation that you can possibly want to do with a quantum computer, says Poster. The researchers demonstrated that each of the logic gates within their system works, reducing the errors.
While this set-up may be useful for certain types of quantum computers, it might not work for more complicated systems, says Christopher Monroe at the Joint Quantum Institute in Maryland. The difficulty in programming error correction into quantum computers varies significantly between devices. For quantum computers that use superconducting cubits, such as Google’s Sycamore, error correction from the get-go requires devices much larger than 16 cubits, he says.
In 2021, Monroe and his colleagues used 13 trapped ions to build a single logical cubit that stored data unexpectedly accurately because of error correction.
He says the new quantum computer isn’t yet big or reliable enough to tackle any practical, real-world problem.
Different research groups are putting the pieces together on how to do that with trapped ion cubits, he says. This new experiment is another piece of that puzzle.
Quantum computers can be protected from noise by encoding the logical quantum information redundantly into multiple cubits using error-correcting codes. When manipulating the logical quantum states, it is imperative that errors caused by imperfect operations do not spread uncontrollably through the quantum register. This requires that all operations on the quantum register obey a fault-tolerant circuit design, which, in general, increases the complexity of the implementation. Here we demonstrate a fault-tolerant universal set of gates on two logical cubits in a trapped-ion quantum computer. In particular, we make use of the recently introduced paradigm of flag fault tolerance, where the absence or presence of dangerous errors is heralded by the use of auxiliary flag cubits. We perform a logical two-cubit controlled-NOT gate between two instances of the seven-cubit color code, and fault-tolerantly prepare a logical magic state. We then realize a fault-tolerant logical T gate by injecting the magic state by teleportation from one logical cubit onto the other. We observe the hallmark feature of fault tolerance-a superior performance compared with a non-fault-tolerant implementation. In combination with recently demonstrated repeated quantum error-correction cycles, these results provide a route towards error-corrected universal quantum computation.
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