Researchers from the Paul Scherrer Institute PSI, ETH Zurich, and EPFL have discovered a new method for creating qubits, the building blocks of quantum computers. Contrary to previous belief, they found that qubits can exist in a cluttered environment and still retain their quantum information for long periods. The team created solid-state qubits from the rare-earth metal terbium, doped into crystals of yttrium lithium fluoride. Some terbium ions formed pairs, acting as highly coherent qubits, protected from their environment and thus maintaining their quantum information. This discovery could significantly advance the development of practical quantum computing.
Solid-State Qubits: A New Approach
A recent study published in Nature Physics has challenged the conventional wisdom that solid-state qubits need to be super dilute in an ultra-clean material to achieve long lifetimes. Instead, the research suggests that cramming lots of rare-earth ions into a crystal can result in some forming pairs that act as highly coherent qubits. This discovery was made by researchers at the Paul Scherrer Institute PSI, ETH Zurich, and EPFL.
The researchers created solid-state qubits from the rare-earth metal terbium, doped into crystals of yttrium lithium fluoride. They found that within a crystal filled with rare-earth ions, there were qubit gems with much longer coherences than would typically be expected in such a dense system. Gabriel Aeppli, head of the Photon Science Division at PSI and professor at ETH Zürich and EPFL, who led the study, stated that this new pathway allows for squeezing qubits closer together.
The Role of Terbium Ions
“In the long run, how to make it onto a chip is a question that’s universally discussed for all types of qubits. Instead of diluting more and more, we’ve demonstrated a new pathway by which we can squeeze qubits closer together,”
Gabriel Aeppli, head of the Photon Science Division at PSI and professor at ETH Zürich and EPFL.
The team’s success with this radically different approach is because their qubits are formed from strongly interacting pairs of ions, rather than single ions. Within the matrix of the crystal, only a few of the terbium ions form pairs. These pairs, due to their physical properties, are shielded from the single terbium ions, which would otherwise cause them to lose their quantum information.
Adrian Beckert, lead author of the study, explained that these qubits are relatively rare, so they are quite dilute. Markus Müller, whose theoretical explanations were essential to understanding the observations, added that the excitation on a terbium pair lives at a different energy and cannot hop over to the single terbiums, protecting them from decoherence.
Discovering Qubit Pairs
The researchers discovered the phenomenon of qubit pairs when probing terbium-doped yttrium lithium fluoride with microwave spectroscopy. They noticed unexpected peaks during spin echo tests, which corresponded to much longer coherences than those on the single ions. With further microwave spectroscopy experiments and careful theoretical analysis, they identified these as pair states.
The Potential of Terbium
Terbium is an attractive choice of dopant as it can be easily excited by frequencies in the microwave range used for telecommunications. The researchers also expect the same kind of qubits to operate at the higher frequencies of optical laser light, which is of interest as rare-earth metals possess optical transitions, providing an easy way in with light.
“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,”
Markus Müller.
Protecting Qubits from Decoherence
Although the excitations of the terbium pairs might be well shielded from the influence of other terbium ions, the nuclear spins on other atoms in the material could still interact with the qubits and cause them to decohere. To protect the qubits further, the researchers applied a magnetic field to the material that was tuned to exactly cancel out the effect of the nuclear spin of the terbium in the pairs. This resulted in essentially non-magnetic qubit states, which were only minimally sensitive to noise from the nuclear spins of surrounding atoms.
The Future of Qubit Research
Once this level of protection was included, the qubit pairs had lifetimes of up to one hundred times longer than single ions in the same material. Aeppli believes that with the right material, the coherence could be even longer. Armed with knowledge of this phenomenon, the researchers are now focusing on optimizing the matrix.
“If you throw a lot of terbium into the crystal, by chance there are pairs of ions – our qubits. These are relatively rare, so the qubits themselves are quite dilute,”
Adrian Beckert, lead author of the study.
Implications for Quantum Computing
This research has significant implications for the field of quantum computing. The discovery of a new pathway for creating qubits that retain their quantum information for longer periods could help overcome one of the major barriers to practical quantum computing. The findings also suggest that a ‘minimalistic’ approach to qubit design may not be necessary, potentially making the scale-up of resulting technology less challenging.
“If you make an excitation on a single terbium, it can easily hop over to another terbium, causing decoherence,”
Markus Müller.
“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,”
Gabriel Aeppli.
Summary
New research has shown that solid-state qubits, the building blocks of quantum computers, can have long lifetimes even in a cluttered environment, challenging the previous belief that they needed to be in ultra-clean materials. The study found that cramming many rare-earth ions into a crystal can create pairs that act as highly coherent qubits, protected from their environment and maintaining their quantum information for surprisingly long periods.
“There was something unexpected lurking,”
Adrian Beckert.
- Researchers from the Paul Scherrer Institute PSI, ETH Zurich and EPFL have discovered a new method to create qubits, the building blocks of quantum computers, that retain their quantum information for longer periods.
- Contrary to previous belief that qubits need to be super dilute in ultra-clean material, the researchers found that cramming many rare-earth ions into a crystal can form pairs that act as highly coherent qubits.
- The team used the rare-earth metal terbium, doped into crystals of yttrium lithium fluoride, to create solid-state qubits.
- The qubits are formed from strongly interacting pairs of ions, which are shielded from their environment due to their unique physical properties. This allows them to retain their quantum information for longer periods.
- The researchers discovered this phenomenon while probing terbium doped yttrium lithium fluoride with microwave spectroscopy.
- The team, led by Gabriel Aeppli, head of the Photon Science Division at PSI and professor at ETH Zürich and EPFL, plans to optimise this approach further.
- The findings, published in Nature Physics, could help overcome one of the major barriers to practical quantum computing.
Nonequilibrium thermodynamics of uncertain stochastic processes” in Physical Review Research (January 8, 2024). doi: 10.1103/PhysRevResearch.6.013021