Regardless of what makes up the innards of a quantum computer, its speedy calculations all boil down to sequences of simple instructions applied to qubits—the basic units of information inside a quantum computer. Whether that computer is built from chains of ions, junctions of superconductors, or silicon chips, it turns out that a handful of simple operations, which affect only one or two qubits at a time, can mix and match to create any quantum computer program—a feature that makes a particular handful “universal.” Scientists call these simple operations quantum gates, and they have spent years optimizing the way that gates fit together. They’ve slashed the number of gates (and qubits) required for a given computation and discovered how to do it all while ensuring that errors don’t creep in and cause a failure. Now, researchers at JQI have discovered ways to implement robust, error-resistant gates using just a constant number of simple building blocks—achieving essentially the best reduction possible in a parameter called circuit depth. Their findings, which apply to quantum computers based on topological quantum error correcting codes, were reported in two papers published recently in the journals Physical Review Letters and Physical Review B, and expanded on in a third paper published earlier in the journal Quantum.
