Quantum computing architecture using quantum nodes connected by photonic links has been proposed. The architecture could provide a way to connect large-scale quantum devices and improve quantum communication. Quantum computers hold the promise of performing certain tasks that are intractable even on the world’s most powerful supercomputers.
Scientists believe that in the future, quantum computing will be used to replicate quantum chemistry, emulate materials systems, and optimize difficult processes, with possible applications ranging from banking to pharmaceuticals.
However, hardware that is resilient and extensible is needed to fulfill this promise. Finding a practical method to connect quantum information nodes to smaller-scale processing nodes spread throughout a computer chip is one of the challenges in developing a large-scale quantum computer.
Conventional methods for transmitting electronic information do not readily translate to quantum devices because quantum computers fundamentally differ from classical computers. However, one requirement is certain: Whether via a classical or a quantum interconnect, the carried information must be transmitted and received.
To this end, MIT researchers have developed a quantum computing architecture that will enable extensible, high-fidelity communication between superconducting quantum processors.
In work published in Nature Physics, MIT researchers demonstrate step one, the deterministic emission of single photons information carriers in a user-specified direction. Their approach guarantees that quantum information flows correctly more than 96% of the time.
It is possible to create a bigger network of interconnected quantum processors on a computer chip by connecting a number of these modules together.
“Quantum interconnects are a crucial step toward modular implementations of larger-scale machines built from smaller individual components,” says Bharath Kannan PhD ’22, co-lead author of a research paper describing this technique.
“The ability to communicate between smaller subsystems will enable a modular architecture for quantum processors, and this may be a simpler way of scaling to larger system sizes compared to the brute-force approach of using a single large and complicated chip,” Kannan adds.
Kannan wrote the paper with co-lead author Aziza Almanakly, an electrical engineering and computer science graduate student in the Engineering Quantum Systems group of the Research Laboratory of Electronics (RLE) at MIT. The senior author is William D. Oliver, a professor of electrical engineering and computer science and of physics, an MIT Lincoln Laboratory Fellow, director of the Center for Quantum Engineering, and associate director of RLE.
Moving quantum information
In a conventional classical computer, various components perform different functions, such as memory, computation, etc. Electronic information, encoded and stored as bits (which take the value of 1s or 0s), is shuttled between these components using interconnects, which are wires that move electrons around on a computer processor.
Quantum interconnects are a crucial step toward modular implementations of larger-scale machines built from smaller individual components. The ability to communicate between smaller subsystems will enable a modular architecture for quantum processors, and this may be a simpler way of scaling to larger system sizes compared to the brute-force approach of using a single large and complicated chip.
Bharath Kannan
But quantum information is more complex. Instead of only holding a value of 0 or 1, quantum information can also be both 0 and 1 simultaneously (a phenomenon known as superposition). Also, quantum information can be carried by particles of light, called photons. Quantum information is fragile due to these additional complications, and it can’t be transmitted just by utilizing standard protocols.
A quantum network connects processing nodes by employing photons that go through waveguides, a specialized type of connection. A waveguide can be unidirectional, moving a photon solely left or right, or it can be bidirectional, moving a photon in both directions.
The majority of existing systems employ unidirectional waveguides, which are simpler to install because it is simple to determine the direction in which photons move. This method is challenging to scale since each waveguide can only move photons in one direction, necessitating the use of more waveguides as the quantum network grows. Additionally, additional components are frequently added to unidirectional waveguides to enforce directionality, which creates communication problems.
“We can get rid of these lossy components if we have a waveguide that can support propagation in both the left and right directions, and a means to choose the direction at will. This ‘directional transmission’ is what we demonstrated, and it is the first step toward bidirectional communication with much higher fidelities,” says Kannan.
Using their architecture, multiple processing modules can be strung along one waveguide. A remarkable feature the architecture design is that the same module can be used as both a transmitter and a receiver, he says. And photons can be sent and captured by any two modules along a common waveguide.
“We have just one physical connection that can have any number of modules along the way. This is what makes it scalable. Having demonstrated directional photon emission from one module, we are now working on capturing that photon downstream at a second module,” Almanakly adds.
Leveraging quantum properties
To accomplish this, the researchers built a module comprising four qubits.
The fundamental components of quantum computers, qubits are utilized to store and manipulate quantum information. But qubits can also be used as photon emitters. When energy is added to a qubit, the qubit becomes excited, and when it de-excites, the energy is released as a photon.
However, simply connecting one qubit to a waveguide does not ensure directionality. Even though a single qubit emits a photon, its direction of travel is completely arbitrary. The researchers use two qubits and a phenomenon called quantum interference to make sure the emitted photon travels in the right direction in order to get around this issue.
The process entails getting the two qubits into a Bell state, which is an entangled state of single excitation. The left qubit is excited as well as the right qubit are both components of this quantum mechanical state. Both features coexist, but it is uncertain which qubit is excited at any given moment.
When the qubits are in this entangled Bell state, the photon is effectively emitted to the waveguide at the two qubit locations simultaneously, and these two “emission paths” interfere with each other.
The ensuing photon emission must move to the left or to the right depending on the relative phase within the Bell state. The researchers decide which direction the photon will travel through the waveguide by setting the Bell state with the appropriate phase.
They can use this same technique, but in reverse, to receive the photon at another module.
“The photon has a certain frequency, a certain energy, and you can prepare a module to receive it by tuning it to the same frequency. If they are not at the same frequency, then the photon will just pass by. It’s analogous to tuning a radio to a particular station. If we choose the right radio frequency, we’ll pick up the music transmitted at that frequency,” Almanakly says.
The researchers discovered that their method had a fidelity of over 96%, which indicates that if they wanted to emit a photon to the right, it did so 96% of the time.
The researchers intend to connect many modules and use the procedure to emit and absorb photons now that they have successfully employed this technology to emit photons in a certain direction. The creation of a modular architecture that combines numerous smaller-scale processors into a single, larger-scale, and more potent quantum processor would be greatly aided by this.
The research is funded, in part, by the AWS Center for Quantum Computing, the U.S. Army Research Office, the Department of Energy Office of Science National Quantum Information Science Research Centers, the Co-design Center for Quantum Advantage, and the Department of Defense.
