While quantum computing could be the future, it is currently plagued by finicky hardware. To make the technology practical, researchers must demonstrate that it consistently and continuously works and performs at scale.
In a new study, published in Physical Review Letters, researchers at Lawrence Livermore National Laboratory (LLNL) and the Ion Storage Group at the National Institute of Standards and Technology in Boulder, Colorado, created a robust process for entangling trapped-ion qubits. The result means better building blocks for ion-based quantum computers.
The approach creates a physical link between two trapped ions. Those ions are held in place with an electric field, although they do vibrate naturally. Because they are the same charge, they also repel each other. If one ion moves, it nudges its neighbor.
That shared motion can be used as a messenger between the qubits. In this experiment, the team used radio-frequency and microwave electromagnetic fields to apply a force to the ions. At different quantum states - say when the qubit is pointing up versus down - the force pushes the shared-ion motion in a slightly different way.
As the ions move from this controlled push, they acquire a so-called "phase shift." That phase shift, essentially an angle encoded in the qubits, depends on the combined quantum state of the two ions.
With the correct, precisely chosen timing, the motion of the ions fizzles out to end exactly where it started. And now, the ions have a phase shift that depends on their quantum states. They are linked together, or entangled.
"Entanglement is one of the key features that distinguishes quantum computers from classical computers and is central to how quantum advantage can be achieved," said author and LLNL scientist Tyler Guglielmo. "These types of non-classical correlations are what make universal quantum computation possible."
Getting the detuning - the gap between the applied electromagnetic force and the natural vibration frequency of the ions - right was the tricky and novel part of the process.
Imagine a vibrating ion like a person in a swing. If you push on the swing at exactly the right rhythm, or resonance, you can get it to speed up. But if that person wiggles, they will mess up the rhythm. The same is true for ions. Resonant driving makes qubits run faster, but the ions' own natural jiggling can throw off the process.
In contrast, pushing a swing out of rhythm barely moves it. You'd have to push for a long time before it built up any real change in motion. However, there is such a difference between your force and the swing's resonance that it doesn't really matter if the person sitting in it moves around. The system is much more robust, but slower.
The authors wanted both benefits: fast qubits, which mean fast quantum computing, and robust qubits, which mean robust quantum computing. To achieve that duality, their electromagnetic fields used a ramped detuning approach.
"We start far from resonance, move closer to resonance in the middle and then ramp back out," said Guglielmo. "This allows us to capture some of the robustness associated with far-detuned operation while still gaining speed from spending part of the gate close to resonance."
The team also ramped the strength of the applied force. Together, the amplitude and frequency ramps make the application of the state-dependent force more forgiving, which allows the ions to operate at higher temperatures, reduces calibration needs and paves the way for scale-up.
"This work is having an impact on many ion trap experiments. Ramping the detuning will surely percolate into many schemes," said Guglielmo. "Our next step will be to implement this in a new ion trap system with extra shielding hardware, allowing us to minimize errors even further."
