In a recent paper published in Nature, MIT researchers have described a process to manufacture "artificial atoms" that can be integrated to create larger-scale quantum chips. As a proof of this, they built a 128-qubit chip, the largest yet.
The qubits in the new chip are artificial atoms made from defects in diamond, which can be prodded with visible light and microwaves to emit photons that carry quantum information.
Defects in diamonds are replaced through germanium and silicon, which thus functions as photon emitters whose spin states are associated to the quantum information represented by the qubit.
Unfortunately, this approach, which is not new, does not lend itself to build system that can scale to thousands or millions of qubits, say the researchers. Hence, instead of attempting to build a larger chip in diamond, they preferred a modular approach using semiconductor fabrication techniques to integrate small chiplets of diamond into a larger hybrid and modular chip. This process uses a photonic integrated circuit, which is analogous to a traditional integrated circuit but carries information through photons instead of electrons.
Photonics provides the underlying architecture to route and switch photons between modules in the circuit with low loss. The circuit platform is aluminum nitride, rather than the traditional silicon of some integrated circuits.
As mentioned, using this approach the researchers could connect 128 qubits on one aluminium nitride platform. Thanks to photoluminescence, they showed the qubits were stable and long-lived, and could furthermore be tuned to be spectrally indistinguishable.
The significance of this achievement is easy to understand if you think that, as the paper authors explain, a key challenge on the road to quantum computers is finding a way to control entanglement across large numbers of qubits.
However, this achievement is a only step towards a new way to build multiplexed quantum repeaters and general-purpose quantum computers. For it to be a real break-through, there are still many challenges to overcome. The next step would be finding a way to control these qubits and induce interactions between them, say the researchers. Furthermore, in order to build bigger chips to be used in modular quantum computers able to transport qubits over long distances, they will need to demonstrate the possibility of integrating their hybrid quantum chips with optoelectronics components.