Collisional quantum gates created using fermionic atoms
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Illustration of interacting atoms Independent teams have created collisional quantum gates using fermionic atoms. (Courtesy: iStock/Traffic Analyzer) Collisional quantum gates based on fermionic atoms have been realized independently by researchers in Germany and Switzerland. The gates are a long-proposed building block for quantum processors, but had been very challenging to create. Both teams’ gates achieve entangling operations with a fidelity above the theoretical threshold for quantum error correction – and could potentially be particularly useful for simulations of quantum chemistry. The potential of collisional quantum gates was proposed in the late 1990s by researchers such as Peter Zoller of the University of Innsbruck in Austria and Ivan Deutsch of the University of New Mexico in the US. The underlying principle is that the states of qubits are encoded into the spin states of atoms in an optical lattice. Then, gate operations between qubits are performed by manipulating interactions between the atoms’ wavefunctions. Experimental attempts followed shortly after, but the technology of the time was insufficient to create practical gates. Early schemes “Schemes were developed to move the atoms using state dependent potentials, but the laser light was too near resonant, so it worked in principle, but in practice there was too much heating involved,” explains Konrad Viebahn of ETH Zurich and a member of the Swiss team. German-team member Petar Bojović of the Max Planck Institute for Quantum Optics in Garching adds that imaging the resulting gates was another problem: “They got some first collisional gates showing proof of principle that this could possibly be done at around the same time as they did [trapped] ions, but they couldn’t move further and scale this up or do many more things with it because there was no way to really see the individual qubits and individual gates”. Since those early days, much progress has been made in quantum-computing schemes that use neutral atoms held in optical tweezer arrays. During a gate operation, one atom is laser excited to a high-energy, large-size Rydberg state in which its wavefunction easily overlaps with the other atoms – allowing atomic qubits to interact. There are, however, challenges associated with this architecture. Rydberg states are loosely bound, so the qubits are prone to disruption by classical noise. Furthermore, ensembles of Rydberg atoms tend to be large and this is a barrier to scaling-up the architecture. Robust collisional quantum gates Bojović and colleagues at the Max Planck Institute led by Titus Franz and Viebahn’s group at ETH Zurich now unveil independent work on new, more robust collisional quantum gates using fermionic lithium-6 atoms. Lithium has the advantage of being lighter, which allows for faster gates. Most prior work on collisional quantum gates has used bosonic atoms, explains Viebahn, but using fermions makes the gates more robust because the exclusion principle guards against gate errors: “For our [collisional] implementation, the wavefunctions are allowed to overlap completely, and this amplifies the effects of quantum statistics,” he says. Both groups produced two-qubit gates, including those able to perform entangling operations, with fidelities of over 99%.
The Max Planck researchers controlled the interactions between the qubits by manipulating the potential barriers between them. They utilized an optical lattice among the most stable in the world, together with a quantum gas microscope that allowed single-site resolution. “There’s been some criticism from other communities,” says Bojović; “Once you get to a regime of ‘ninety-nine point something’ fidelity, you really need to be able to see it precisely in order to characterize it.” The researchers would like to go on to demonstrate all the other gates in a universal quantum gate set, but Bojović says that researchers in quantum chemistry are already intrigued by the potential of the platform to simulate molecular behaviour. Different protocol The ETH Zurich researchers used a different protocol involving control of the bias voltage to couple the quantum states of their fermionic atoms rather than manipulation of the barrier height. The researchers have not achieved single site resolution – they are currently working to do so – but Viebahn believes his group’s protocol should prove more robust to noise. “I would say the key novelty here is that we came up with this more robust way of doing this interaction, which was not part of the original proposals from the 90s,” says Viebahn. “We’re the first to implement this gate where these qubits form this fully overlapped quantum state.” Gauge theory could give quantum error correction a boost Read more Both groups’ two-qubit gate fidelities are well above the theoretical minimum required for quantum error correction (QEC) to be possible. However, implementing QEC will be difficult because creating the required universal gate set involves a complete set of single qubit gates as well as at least one two-qubit gate that can generate entanglement in the system. Nevertheless, Viebahn concludes, “The two-qubit gate is limiting many other quantum computing platforms, and that’s the thing that we’re very good at.” The collisional quantum gates are described in two papers in Nature: links to the Max Planck paper and the ETH Zurich paper. Quantum-computing expert Barry Sanders of University of Calgary in Canada says the papers “have two different purposes and both purposes are significant”.
The Max Planck paper, he says, is especially impressive because it opens up the potential to simulate the Fermi–Hubbard dynamics of strongly-correlated electronic systems directly in a quantum simulator. The ETH Zurich paper, meanwhile, uses Fermi dynamics to offer gate operation protection against time-dependent sources of error. “There’s a lot of rich physics available with two fermions at a site,” he says. Want to read more? Registration is free, quick and easy Note: The verification e-mail to complete your account registration should arrive immediately. However, in some cases it takes longer. Don't forget to check your spam folder. If you haven't received the e-mail in 24 hours, please contact customerservices@ioppublishing.org. E-mail Address Register