Research team demonstrates modular, scalable hardware architecture for quantum computer

Quantum computers promise to be able to quickly solve extremely complex problems that could take decades for the world’s most powerful supercomputers.

But achieving this performance requires building a system made up of 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 are working to overcome.

To achieve this goal, researchers from MIT and MITER demonstrated a modular and scalable hardware platform that integrates thousands of interconnected qubits on 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 could be connected using an optical network to create a large-scale quantum communications network.

By tuning the qubits over 11 frequency channels, this QSoC architecture makes it possible to propose a new “entanglement multiplexing” protocol for large-scale quantum computing.

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

“We will need large numbers of qubits and great control over them to truly harness the power of a quantum system and make it useful. We are proposing an entirely new architecture and manufacturing technology capable of supporting system hardware scalability requirements for a quantum computer,” says Linsen Li, a graduate student in electrical and computer engineering (EECS) and lead author of a paper on this architecture.

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

Diamond microchips

Although there are many types of qubits, researchers have chosen to use diamond-colored centers because of their scalability advantages. They previously used these qubits to produce quantum chips integrated with photonic circuits.

Qubits made from diamond-colored centers are “artificial atoms” that carry quantum information. Since the color centers of diamonds are semiconductor systems, manufacturing qubits is compatible with modern semiconductor manufacturing processes. They are also compact and have relatively long coherence times, which refer to the length of time a qubit’s state remains stable, due to the clean environment provided by the diamond material.

Additionally, the color centers of diamonds have photonic interfaces that allow them to be entangled or remotely connected with other qubits that are not adjacent to them.

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

This is particularly difficult because researchers must achieve this on a large scale to compensate for the inhomogeneity of qubits in a large system.

To communicate between qubits, they must have several “quantum radios” connected to the same channel. The realization of this condition becomes almost certain when scaling to thousands of qubits.

To this end, the researchers overcame this challenge by integrating a wide array of diamond-colored central qubits onto a CMOS chip that provides the control dials. The chip can be integrated with embedded digital logic that quickly and automatically reconfigures voltages, allowing 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 the frequencies of the qubits,” explains Li.

Lock and unlock construction

To build this QSoC, researchers developed a manufacturing process to transfer diamond-colored center “microchips” onto a CMOS backplane at scale.

They started by making a range of diamond-colored core microchips from a solid block of diamond. They also designed and fabricated nanoscale optical antennas that enable more efficient collection of photons emitted by these color center qubits into free space.

Next, they designed and mapped the semiconductor foundry chip. Working in the MIT.nano clean room, they post-processed a CMOS chip to add microscale supports that match the diamond microchip 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 microchips into the CMOS chip carriers. Since diamond microchips are weakly bonded to the diamond surface, when they release the loose diamond horizontally, the microchips remain in the dimples.

“Since we can control the fabrication of the diamond and the CMOS chip, we can create a complementary pattern. This way, we can transfer thousands of diamond chiplets into their corresponding carriers at the same time,” Li explains.

The researchers demonstrated an area transfer of 500 microns 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 the system. In fact, they found that with more qubits, tuning frequencies actually requires less voltage for this architecture.

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

The team tested numerous nanostructures before determining the ideal microchip array for the lock-and-release process. However, making quantum microchips is not an easy task and the process has taken years to perfect.

“We repeated and developed the recipe for making these diamond nanostructures in the MIT clean room, but it is a very complicated process. It took 19 nanofabrication steps to obtain the diamond quantum microchips, and the steps do not were not simple,” he adds.

Alongside 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, which helps them understand the root causes of the observed phenomenon and determine how to effectively implement the architecture.

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

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