Research team demonstrates modular, scalable hardware architecture for a quantum computer

Quantum computers promise to quickly solve extremely complex problems that could take the world’s most powerful supercomputer decades to crack.

But achieving that performance requires building a system with millions of interconnected building blocks, called qubits. Creating and controlling so many qubits in a hardware architecture is a huge challenge that scientists around the world want to tackle.

To achieve this goal, researchers from MIT and MITER have demonstrated a scalable, modular hardware platform that integrates thousands of interconnected qubits onto a custom integrated circuit. This quantum-system-on-chip (QSoC) architecture allows researchers to precisely tune and control a dense array of qubits. Multiple chips can be connected together using optical networks to create a large-scale quantum communications network.

By tuning qubits across eleven frequency channels, this QSoC architecture enables a newly proposed protocol of “entanglement multiplexing” for large-scale quantum computers.

The team spent years perfecting a complex process for fabricating two-dimensional arrays of atom-sized qubit microchiplets and transferring thousands of them to a carefully prepared complementary metal oxide semiconductor (CMOS) chip. This transfer can be done in one step.

“We will need a large number of qubits, and great control over them, to really harness the power of a quantum system and make it usable. We propose a completely new architecture and a manufacturing technology that can support the scalability requirements of a hardware. system for a quantum computer,” says Linsen Li, an electrical engineering and computer science (EECS) graduate student and lead author of a paper on this architecture.

Li’s co-authors include Ruonan Han, associate professor at EECS, leader of the Terahertz Integrated Electronics Group and member of the Research Laboratory of Electronics (RLE); senior author Dirk Englund, professor of EECS, principal investigator of the Quantum Photonics and Artificial Intelligence Group and of RLE; as well as others at MIT, Cornell University, the Delft Institute of Technology, the Army Research Laboratory and the MITER Corporation. The paper appears Nature.

Diamond microchiplets

Although there are many types of qubits, the researchers chose to use diamond color centers because of their scalability benefits. They previously used such qubits to produce integrated quantum chips with photonic circuits.

Qubits made from diamond color centers are ‘artificial atoms’ that contain quantum information. Because diamond color centers are solid-state systems, qubit production is compatible with modern semiconductor manufacturing processes. They are also compact and have relatively long coherence times, which refers to the amount of time a qubit’s state remains stable, thanks to the clean environment provided by the diamond material.

Furthermore, diamond color centers have photonic interfaces that allow them to be remotely entangled or connected to other qubits that are not adjacent to them.

“The conventional assumption in the field is that the inhomogeneity of the diamond color center is a disadvantage compared to identical quantum memory such as ions and neutral atoms. However, we turn this challenge into an advantage by embracing the diversity of the artificial atoms: each atom has its own spectral frequency. This allows us to communicate with individual atoms by bringing them into resonance with a laser, just like tuning the dial on a small radio,” says Englund.

This is especially difficult because researchers need to realize this on a large scale to compensate for qubit inhomogeneity in a large system.

To communicate via qubits, several such ‘quantum radios’ must be connected to the same channel. Achieving this condition becomes almost certain when scaling to thousands of qubits.

To that end, the researchers overcame that challenge by integrating a large number of diamond color center qubits onto a CMOS chip that provides the control buttons. The chip integrates with built-in digital logic that quickly and automatically reconfigures voltages, allowing the qubits to achieve full connectivity.

“This compensates for the inhomogeneous nature of the system. With the CMOS platform, we can quickly and dynamically tune all qubit frequencies,” Li explains.

Lock and release manufacturing

To build this QSoC, the researchers developed a fabrication process to transfer ‘microchiplets’ with diamond color centers to a CMOS backplane on a large scale.

They began by fabricating a series of diamond color center microchiplets from a solid block of diamond. They also designed and fabricated nanoscale optical antennas that enable more efficient collection of the photons emitted by these color center qubits into free space.

They then designed and mapped the semiconductor foundry chip. In MIT.nano’s cleanroom, they post-processed a CMOS chip to add microscale sockets that correspond to the diamond microchiplet array.

They built an internal transfer setup in the lab and applied a lock-and-release process to integrate the two layers by locking the diamond microchiplets into the sockets on the CMOS chip. Because the diamond microchiplets are weakly bonded to the diamond surface, the microchiplets remain in the sockets when they release the bulk diamond horizontally.

“Because we can control the manufacturing of both the diamond and the CMOS chip, we can create a complementary pattern. In this way, we can transfer thousands of diamond chiplets into the corresponding sockets at the same time,” says Li.

The researchers demonstrated an area transfer of 500 by 500 microns for an array of 1,024 diamond nanoantennas, but they could use larger diamond arrays and a larger CMOS chip to further scale up the system. In fact, they found that tuning the frequencies with more qubits actually requires less voltage for this architecture.

“In this case, our architecture will work even better if you have more qubits,” says Li.

The team tested many nanostructures before determining the ideal microchiplet array for the lock-and-release process. However, creating quantum microchiplets is not an easy task, and the process has taken years to perfect.

“We iterated and developed the recipe to fabricate these diamond nanostructures in MIT’s cleanroom, but it is a very complicated process. It took 19 nanofabrication steps to obtain the diamond quantum microchiplets, and the steps were not simple,” he adds to it.

In addition to their QSoC, the researchers developed an approach to characterize the system and measure its performance on a large scale. To do this, they built a custom cryo-optical metrology setup.

Using this technique, they demonstrated an entire chip with more than 4,000 qubits that could be tuned to the same frequency while retaining their spin and optical properties. They also built a digital twin simulation that connects the experiment with digitized modeling, allowing them to understand the root causes of the observed phenomenon and determine how to efficiently implement the architecture.

In the future, the researchers could improve the performance of their system by refining the materials they used to make qubits or by developing more precise control processes. They could also apply this architecture to other solid-state quantum systems.

This story is republished courtesy of MIT News (web.mit.edu/newsoffice/), a popular site covering news about MIT research, innovation and education.