Quantum cramming: Junk becomes gems in new qubit breakthrough

In the fast paced world of quantum computing, a study challenges the widely accepted notion that solid state qubits thrive in ultra clean environments. Contrary to popular belief, researchers from the Paul Scherrer Institute PSI, ETH Zurich, and EPFL discovered that dense arrays of qubits with long lifetimes can emerge in a seemingly messy setting.

The traditional belief in quantum design mimics the principles of minimalist interior aesthetics – keep it clean and clutter free. However, the researchers, led by Gabriel Aeppli, head of the Photon Science Division at PSI and professor at ETH Zürich and EPFL, propose a novel approach. Instead of diluting qubits to extreme levels, they densely packed , specifically terbium, into yttrium lithium fluoride crystals.

Markus Müller, providing crucial theoretical insights, explains the strategy: "For a given density of qubits, we show that it’s a much more effective strategy to throw in the rare earth ions and pick the gems from the junk, rather than trying to separate the individual ions from each other by dilution." What sets their approach apart is the formation of from strongly interacting pairs of ions instead of single ions.

Adrian Beckert, lead author of the study, highlights the rarity of these pairs within the crystal matrix. These qubit pairs, although dilute, possess significantly longer coherences due to their unique electron shell states. Surprisingly, these qubit pairs remain undisturbed amidst the quantum mess.

The physical properties of these gems shield them from the surrounding 'junk' ions. Müller elaborates, "If the excitation is on a terbium pair, its state is entangled, so it lives at a different energy and cannot hop over to the single terbiums. It’d have to find another pair, but it can’t because the next one is a long distance away." The researchers stumbled upon this phenomenon while probing terbium doped yttrium lithium fluoride with microwave spectroscopy.

They also explore the potential of using light, including light, to manipulate and measure quantum effects in materials. Aeppli expresses their ambition: "Eventually, our goal is to also use light from the X ray Free Electron Laser SwissFEL or Swiss Light Source SLS to witness quantum information processing." To further shield qubits from environmental disturbances, the team applied a magnetic field precisely tuned to cancel out the effects of nuclear spins from the terbium pairs.

This ingenious approach resulted in non magnetic qubit states with lifetimes up to one hundred times longer than single ions in the same material. Armed with a deeper understanding of protective mechanisms, the researchers now aim to optimize the matrix for even longer coherence. Aeppli emphasizes, "What this shows is how powerful this approach can be.

With the right material, the coherence could be even longer." In the realm of quantum computing, it appears that embracing a bit of mess might be the key to unlocking the full potential of solid state qubits..