The popular children’s telephone game is based on a simple premise: the starting player whispers a message into the next player’s ear. The second player then passes the message to the third person, and so on until the message reaches the final recipient, who forwards it aloud to the group. Often, what the first person says and what the last person hears will be very different. Information becomes muddled throughout the chain.
Such transmission errors from start to finish are also common in the quantum world. When bits of quantum information, or qubits (analogues of classical bits in traditional digital electronics) pass through a channel, their quantum state can be degraded or lost entirely. This kind of decoherence is especially common at increasing distances because qubits—whether in the form of light particles (photons), electrons, atoms, or other forms—are inherently fragile and subject to the laws of quantum physics or extraordinary Control of physical laws of small objects. At such tiny scales (nanoscale), even slight interactions with the environment can cause a qubit to lose its quantum properties and alter the information it stores. Just like the game of telephone, the original message and the received message may not be the same.
“One of the big challenges in quantum networks is how to efficiently move these delicate quantum states between multiple quantum systems,” said Michael Si, director of the Optical and Quantum Communications Technology Group at MIT Lincoln Laboratory, part of the Communications Systems Research and Development Area. Curt Hamilton said. “This is an issue our team is actively exploring.”
As Hamilton explained, today’s quantum computing chips contain about 100 qubits. But building a fully functional quantum computer requires thousands or even billions of qubits, which promises to unlock unprecedented computing power for applications ranging from artificial intelligence and cybersecurity to healthcare and manufacturing. Interconnecting chips to create a large computer may offer a viable path forward. On the sensing side, connecting quantum sensors to share quantum information could lead to new capabilities and performance improvements beyond those of a single sensor. For example, a quantum reference shared between multiple sensors could be used to more precisely locate the source of radio frequency emissions. Space and defense agencies are also interested in interconnecting quantum sensors separated by large distances for use in satellite positioning, navigation and timing systems or in atomic clock networks between satellites. For communications, quantum satellites could be used as part of a quantum network architecture connecting local ground stations, creating a truly global quantum internet.
However, quantum systems cannot be interconnected with existing technologies. Today’s communications systems used to transmit information over networks and connect devices rely on detectors to measure bits and amplifiers to copy them. These techniques do not work in quantum networks because qubits cannot be measured or replicated without destroying the quantum state; qubits exist in a superposition of states between zero and one, as opposed to classical bits, which are at zero (off) or one (on) setting state. Therefore, researchers have been trying to develop quantum equivalents of classical amplifiers to overcome transmission and interconnect losses. These equivalents are called quantum repeaters, and they work similarly to amplifiers, dividing transmission distances into smaller, more manageable segments to reduce losses.
“Quantum repeaters are a key technology for quantum networks to successfully send information over lossy links,” Hamilton said. “But no one has built a fully functional quantum repeater yet.”
The complexity lies in how quantum repeaters operate. Rather than a simple “copy and paste” like a classical repeater, quantum repeaters work using a strange quantum phenomenon called entanglement. In quantum entanglement, two particles become closely connected and related in space, regardless of 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 particle. Entangled qubits can serve as a resource for quantum teleportation, in which quantum information is sent between distant systems without moving the actual particles; the information disappears in one location and reappears in another. Teleportation skips the physical journey along fiber optic cables and therefore eliminates the associated risk of information loss. Quantum repeaters tie everything together: they enable the end-to-end generation of quantum entanglement and, ultimately, the end-to-end transmission of qubits via quantum teleportation.
Ben Dixon, a researcher in the Optical and Quantum Communications Technologies Group, explains how the process works: “First, you generate pairs of specific entangled qubits, called Bell states, and transmit them in different directions over a network link to Two independent qubits. A quantum repeater, captures and stores these qubits. Then one of the quantum repeaters transmits and stores the qubits and any of the qubits that we want to send over the link to interconnect remote quantum systems. Two qubit measurements are made between qubits. The measurement results are transmitted to a quantum repeater at the other end of the link; the repeater uses these results to convert the stored Bell state qubit into an arbitrary qubit. Finally, the repeater Arbitrary qubits can be sent into a quantum system, linking two remote quantum systems.”
To preserve entangled states, quantum repeaters need a way to store them—memory, essentially. In 2020, collaborators at Harvard University demonstrated housing a qubit (trapped between two empty spaces left after removing two carbon atoms) within a single silicon atom in diamond. Silicon “vacancy” centers in diamond are an attractive option for quantum memory. Like other individual electrons, the outermost (valence) electrons on silicon atoms can point up or down, similar to a bar magnet with a north and south pole. The direction an electron points is called its spin, and the two possible spin states (spin up or spin down) are analogous to the 1s and 0s computers use to represent, process and store information. Additionally, silicon’s valence electrons can be manipulated with visible light to transport and store photon qubits in electron spin states. Researchers at Harvard University did just that. They designed an optical waveguide (a structure that guides light in the desired direction) surrounded by a nanophotonic optical cavity so that the photons interact strongly with the silicon atom and transfer their quantum state to the atom. The MIT collaborators then showed that this basic functionality could be used with multiple waveguides. They patterned eight waveguides and successfully created silicon vacancies inside them.
Lincoln Laboratory has since been applying quantum engineering to create a quantum memory module equipped with the additional functionality of operating as a quantum repeater. This engineering work includes on-site custom diamond growth (in collaboration with the Quantum Information and Integrated Nanosystems group); development of a scalable silicon nanophotonic interposer (a chip that fuses photonic and electronic capabilities) to control silicon vacancy qubits; and the integration of components into Integrated and packaged into a system that can be cooled to the low temperatures required for long-term storage. The current system has two memory modules, each capable of holding eight optical qubits.
To test these technologies, the team has been utilizing the lab’s rented fiber optic test benches. The test bed features 50 kilometers of telecommunications network fiber and currently connects three nodes: Lincoln Laboratory to the MIT campus and the MIT campus to Harvard University. Local industrial partners can also leverage the fiber as part of the Boston Area Quantum Network (BARQNET).
“Our goal is to take the state-of-the-art research being done by our academic partners and translate it into something we can take beyond the lab to test on real channels and bring A real loss.” “All of this infrastructure is critical to conducting baseline experiments to get entanglement onto fiber optic systems and moving between parties.”
Using this testbed, the team, working with researchers at MIT and Harvard, became the first in the world to demonstrate quantum interactions with nanophotonic quantum memory on deployed telecommunications fibers. They used Harvard’s quantum repeater to send photons encoded with quantum states through optical fibers from the lab and connected them to silicon vacancy quantum memories that capture and store the transmitted quantum states. They measured the electrons on the silicon atoms to determine the extent to which the quantum state shifted to either the spin-up or spin-down position of the silicon atoms.
“We examined the testbed’s performance on relevant quantum repeater metrics such as distance, efficiency (loss error), fidelity and scalability and found that we performed better on all of these metrics compared to other leading studies around the world. Got the best or close to the best results,” Dixon said. “Our distance is longer than others have shown; our efficiency is good, and we think we can improve it further by optimizing some of the testbed components; the qubits read out from memory match the qubits we sent, with fidelity is 87.5%; diamond has inherent lithographic patterning scalability, and you can imagine putting thousands of qubits onto a small chip.”
The Lincoln Laboratory team is now focused on combining multiple quantum memories at each node and incorporating other nodes into a quantum network testbed. These advances will allow the team to explore quantum network protocols at the system level. They also look forward to ongoing materials science research by collaborators at Harvard and MIT. These studies may identify other types of atoms in diamond that are capable of operating at slightly higher temperatures, allowing for more practical manipulations.
Nanophotonic quantum memory module wins 2023 R&D 100 Award.