Solid-state Qubits: Forget About Being Clean, Embrace the Mess

The pursuit of practical quantum computing faces numerous challenges, with one major hurdle being the quest for qubits that retain quantum information long enough to be useful. Traditionally, the prevailing wisdom was that solid-state qubits needed to exist in ultra-clean, dilute environments to achieve extended lifetimes. However, recent findings from researchers at the Paul Scherrer Institute PSI, ETH Zurich, and EPFL challenge this conventional thinking, suggesting that a cluttered environment may not be as detrimental as once believed.

In the world of quantum computing, where minimalism is often considered a virtue, the researchers propose a radical shift in mindset. Instead of diluting qubits to the extreme and keeping them isolated, they demonstrated that packing rare-earth ions into a crystal can lead to the formation of pairs that act as highly coherent qubits. This approach, in contrast to the traditional minimalist stance, offers a more effective strategy for achieving long qubit lifetimes.

The researchers created solid-state qubits using the rare-earth metal terbium, embedded in crystals of yttrium lithium fluoride. Surprisingly, they found that within a densely packed crystal loaded with rare-earth ions, certain qubit pairs exhibited significantly longer coherences than expected. The strategy here was to "throw in the rare-earth ions and pick the gems from the junk" rather than attempting to separate individual ions through dilution.

Unlike conventional qubits formed from single ions, the team's qubits were derived from strongly interacting pairs of ions. These pairs, formed by chance within the crystal, exhibited unique physical properties that shielded them from decoherence. By entangling their states, these qubit pairs operated at different energy levels, rendering them immune to the disturbances caused by single terbium ions in the cluttered environment.

The researchers stumbled upon the phenomenon of qubit pairs while probing terbium-doped yttrium lithium fluoride with microwave spectroscopy. The use of light to manipulate and measure quantum effects in materials, particularly with optical laser light, is an avenue the researchers are exploring. Rare-earth metals like terbium, with optical transitions, could pave the way for utilizing X-ray light for reading out entire qubit ensembles, a prospect that holds promise for advancing quantum information processing.

Understanding the unique protective mechanisms of these qubit pairs, the researchers sought to optimize their coherence further. Applying a magnetic field to the material canceled out the effects of nuclear spins on surrounding atoms, resulting in essentially non-magnetic qubit states. This additional protection allowed the qubit pairs to exhibit lifetimes up to one hundred times longer than single ions in the same material.

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