Diamond-studded quantum computer chips
Metal Tech News - October 16, 2023
Last updated 11/6/2023 at 6:52pm
Trapping qubits inside diamonds, MIT researchers create memory nodes that may unlock quantum computing realm.
Quantum computers have the potential to carry out calculations millions of times faster than today's most advanced supercomputers, but this game-changing computing power has yet to be realized due to the fragile nature of qubits, the quantum realm equivalent of the bits that store and transfer data in today's computers. Diamonds, however, could provide the armor that protects these subatomic particles in the big world of quantum computing and internet.
The classical computers, smartphones, and the other computing devices we use today convey information with bits, with each bit representing a zero (off) or one (on). Qubits, on the other hand, are both zero and one and neither of these states at the same time. This quantum phenomenon, known as superposition, is the foundation of the potential mind-blowing speed of quantum computers and networks.
The downside to qubits is they do not do well in the larger-than-atom-sized world in which we live.
As qubits make their way through our world, their quantum state can degrade or be lost – the longer the distance, the greater the degradation.
Scientists use the children's game of telephone to demonstrate what they are dealing with when it comes to qubit transmission. As each child whispers a phrase in the next one's ear, the information is often altered slightly. By the time this phrase has been passed along 20 times, the child at the end of the line declares a phrase that is often quite different than what was whispered at the start of the line.
While this gradual change in data creates a good laugh for schoolchildren, the start-to-end-point transmission errors in quantum computers leave only a very small percentage of useful data.
Even with this noisy data, IBM's 127-qubit Eagle quantum computer held its own when pitted against a conventional supercomputer at the Lawrence Berkeley National Laboratory in California.
Considering that a quantum computer running at its full potential is theorized to be able to carry out as many calculations in four minutes as it would take a traditional supercomputer 10,000 years, Eagle's ability to slightly outperform the LBNL supercomputer is considered only a small win.
Scientists at the Massachusetts Institute of Technology believe diamonds could offer a brilliant solution to the degradation and data loss that is holding back quantum computers and networks.
"One of the big challenges in quantum networking is how to effectively move these delicate quantum states between multiple quantum systems," says Scott Hamilton, leader of MIT Lincoln Laboratory's Optical and Quantum Communications Technology Group. "That's a question we're actively exploring in our group."
Critical quantum repeaters
While today's quantum computers such as Eagle have somewhere around 100 qubits, to realize the full quantum computing potential will require chips, computers, and networks with thousands or millions of these subatomic particles.
Such large quantum systems, however, cannot be connected with today's technologies. The methods currently used to detect and amplify normal computing bits destroy the quantum state of qubits.
To retain the quantum integrity of qubits while transmitting them, researchers are working to develop the quantum equivalents of classical amplifiers, which divide the transmission distance into smaller and more manageable segments to lessen losses.
"Quantum repeaters are a critical technology for quantum networks to successfully send information over lossy links," Hamilton says. "But nobody has made a fully functional quantum repeater yet."
Developing a quantum repeater is not as simple as replicating the "copy and paste" method used for classical computers.
Instead, quantum repeaters will leverage quantum entanglement, a strange phenomenon where two particles become strongly connected and correlated across space, no matter the distance between them. If you know the state of one particle in an entangled pair, then you automatically know the state of the other.
With entangled qubits, quantum information can be sent between distant systems without moving actual particles – the information vanishes at one location and reappears at another. This teleportation skips the physical journey along fiber-optic cables and eliminates the associated risk of information loss.
Quantum repeaters are what tie everything together – they enable the end-to-end generation of quantum entanglement, and, with quantum teleportation, the transmission of qubits.
Diamond qubit traps
To retain the entangled states, the quantum repeater needs a way to store them – in essence, a memory. This is where diamonds come into play.
In 2020, quantum computing collaborators at Harvard University demonstrated the ability to hold a qubit in a single silicon atom in empty spaces that are left behind by removing two carbon atoms from a diamond.
Like other individual electrons, the outermost (valence) electron on the silicon atom trapped in the diamond can point either up or down, similar to a bar magnet with north and south poles. The direction that the electron points is known as its spin, and the two possible spin states, spin up or spin down, are akin to the ones and zeros used by bits in classical computers to represent, process, and store information.
The Harvard researchers demonstrated that the silicon's valence electron can be manipulated with visible light to transfer and store a photonic qubit in the electron spin state.
Building on this discovery, scientists at MIT's Lincoln Laboratory have created a quantum memory module equipped with additional capabilities to operate as a quantum repeater. This includes growing custom diamonds for the module, a chip to control the silicon-vacancy qubit, and integration of the components into a system that can be cooled to the cryogenic temperatures needed for long-term memory storage.
Lincoln Laboratory's current system has two memory modules, each capable of holding eight optical qubits.
To test the technologies, the team is leveraging a 50-kilometer (31 miles) optical-fiber telecommunications network connecting three nodes – Lincoln Laboratory, MIT campus, and Harvard.
Using this test bed, the Lincoln Laboratory team, in collaboration with MIT and Harvard researchers, became the first in the world to demonstrate a quantum interaction with a nanophotonic quantum memory across a deployed telecommunications fiber.
"Our distance is longer than anybody else has shown; our efficiency is decent, and we think we can further improve it by optimizing some of our test bed components; the read-out qubit from memory matches the qubit we sent with 87.5% fidelity; and diamond has an inherent lithographic patterning scalability in which you can imagine putting thousands of qubits onto one small chip," said Ben Dixon, a researcher in the Optical and Quantum Communications Technology Group at Lincoln Laboratory.
The Lincoln Laboratory team is now focusing on combining multiple quantum memories at each node and incorporating additional nodes into the quantum network test bed. They also look forward to materials science investigations by Harvard and MIT collaborators that may identify other types of atoms trapped in diamonds that are capable of operating at slightly warmer temperatures for more practical operations.
For now, it seems that no matter the atoms trapped inside, diamond-studded chips are the future for quantum computing.