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A quantum processor is like a busy intersection: the more cars, the harder it is for them to navigate without collisions. In the quantum world, qubits play the role of cars — elementary carriers of information that, unlike classical bits, can be both 0 and 1 simultaneously. Today's best qubits are superconducting; they operate at temperatures colder than outer space and require close proximity to exchange signals. But cram too many onto a single chip, and interference sets in, corrupting computations.

The solution is to spread qubits across different modules, like city blocks, and connect them with long "roads." The problem is that fluxonium qubits, favorites among many researchers for their long coherence times and precise operations, have only been able to communicate within a single chip. The authors of the new work devised a tunable bridge: a special element that links two fluxonium qubits over distances greater than a centimeter. With this bridge, a quantum operation between modules takes less than 100 nanoseconds, with an error rate no higher than one ten-thousandth. That's almost as good as having the qubits side by side.

This bridge paves the way for modular quantum computers, where hundreds or thousands of qubits are distributed across separate boards, like servers in a data center. Moreover, neighboring modules barely interfere with each other — the "noise" from one block doesn't reach the next. In the long run, this could enable complex quantum error correction codes that require qubits to communicate not only with nearest neighbors but also with distant ones. The quantum city is becoming more spacious, and at last, long routes can be laid without traffic jams.