Short answer: The quantum charge-coupled device (QCCD) is an architecture for trapped-ion quantum computers in which many small traps are connected by physically shuttling ions between storage and interaction zones. It was proposed in 2002 as a way to scale beyond a single long ion chain. A 2021 Nature paper reports integrating the needed elements on a cryogenic surface trap, with a quantum volume of 64, and a 2022 architecture study finds that design choices matter a great deal for reliability. These are research results from the authors.
The scaling problem
A Physics magazine article describing a 2023 trapped-ion system explains that in a typical machine a linear chain of ions is confined by electric fields and laser cooled, and the motional modes are used to entangle pairs of ions. It says controlling individual ions in a long chain brings technical difficulties, and that it is unlikely a million qubits, the number the article cites as needed for a universal fault-tolerant computer, could be trapped in a single potential. In 2002 a group proposed the QCCD architecture, in which short linear ion chains are connected by physically shuttling ions between storage and interaction regions.
What QCCD proposes
The Nature paper describes the proposal as a blueprint for a universal quantum computer using mobile ions as qubits. By analogy with a CCD camera, which moves charge between pixels, quantum information is stored in the internal state of charged ions that are transported between processing zones using dynamic electric fields. The aim is to keep the low error rates seen in small trapped-ion experiments by limiting interactions to small ion crystals, then splitting and rearranging ions into new crystals. The authors list the supporting properties as transport timescales that are fast relative to qubit coherence times, qubit states that are insensitive to the transport fields, and low crosstalk between spatially separated crystals. They also note that engineering such a machine with low error across multiple zones is difficult and has slowed scaling.
A reported demonstration
The 2021 paper reports using a cryogenic surface trap to integrate a scalable trap design, parallel interaction zones and fast ion transport into a programmable trapped-ion computer. The authors report a teleported CNOT gate using mid-circuit measurement, negligible crosstalk error and a quantum volume of 64. This is the authors' own report of a small system.
Design choices
A 2022 study in an ACM journal evaluates QCCD design options, which it says include trap sizing, communication topology and gate implementation. The authors built a design toolflow that maps applications onto a target device and simulates run time, reliability and noise. Using six applications and several design points, they report that trap sizing and communication topology can change application reliability by up to three orders of magnitude, and that gate implementation choices change it by another order of magnitude. This is a simulation study based on hardware performance models.
What this does not tell you
These sources do not show that a large QCCD machine exists or that shuttling scales to the sizes needed for fault tolerance. The Nature result is a small demonstration, and the architecture study is based on models. Results from one device do not carry over automatically to another.
FAQ
What does QCCD stand for?
Quantum charge-coupled device, an analogy to the camera sensor that moves charge between pixels.
Why shuttle ions?
To keep interactions within small ion crystals while connecting many of them, as the proposal describes.
Who proposed it?
A group of scientists in 2002, according to the Physics magazine article.
What did the 2021 paper show?
A programmable trapped-ion computer on a cryogenic surface trap with a quantum volume of 64, as the authors report.
Which design choices matter most?
In the 2022 simulation study, trap sizing and communication topology affected reliability by up to three orders of magnitude.
All field notes