Quantum computers promise to perform some intractable tasks even on the world’s most powerful supercomputers. In the future, scientists plan to use quantum computing to emulate materials systems, simulate quantum chemistry, and optimize difficult tasks, with impacts that may extend from finance to pharmaceuticals.
However, delivering on this promise requires resilient and stretchy hardware. One of the challenges of building a large-scale quantum computer is that researchers must find an efficient way to interconnect quantum information nodes – separate smaller-scale processing nodes on a computer chip. Because quantum computers are fundamentally different from classical computers, the conventional techniques used to communicate electronic information do not translate directly into quantum devices. However, one requirement is certain: whether via a classical or quantum interconnect, the information carried must be transmitted and received.
To this end, MIT researchers have developed a quantum computing architecture that will enable scalable, high-fidelity communication between superconducting quantum processors. In a book published in Natural Physics, MIT researchers demonstrate the first step, the deterministic emission of single photons – carriers of information – in a direction specified by the user. Their method ensures that quantum information is flowing in the right direction more than 96% of the time.
Linking several of these modules allows for a larger network of quantum processors that are interconnected with each other, regardless of their physical separation on a computer chip.
“Quantum interconnects are a crucial step towards larger-scale, modular machine implementations built from smaller, individual components,” says Bharath Kannan PhD ’22, co-lead author of a research paper describing the technique.
“The ability to communicate between smaller subsystems will enable a modular architecture for quantum processors, and this may be an easier way to scale to larger system sizes compared to the brute force approach of use a single large and complicated chip,” adds Kannan.
Kannan authored the paper with co-lead author Aziza Almanakly, a graduate student in electrical engineering and computer science in the Engineering Quantum Systems group at MIT’s Research Laboratory of Electronics (RLE). The lead author is William D. Oliver, Professor of Electrical Engineering, Computer Science, and Physics, Fellow of MIT Lincoln Laboratory, Director of the Center for Quantum Engineering, and Associate Director of RLE.
Displacement of quantum information
In a typical classical computer, various components perform different functions, such as memory, computation, etc. that move electrons on a computer processor.
But quantum information is more complex. Instead of only containing a value of 0 or 1, quantum information can also be both 0 and 1 simultaneously (a phenomenon known as superposition). Moreover, quantum information can be carried by particles of light, called photons. These additional complexities make quantum information fragile and cannot be transported simply using conventional protocols.
A quantum network connects processing nodes using photons that travel through special interconnects called waveguides. A waveguide can either be unidirectional, and move a photon only left or right, or it can be bidirectional.
Most existing architectures use unidirectional waveguides, which are easier to implement because the direction of photon travel is easily established. But since each waveguide only moves photons in one direction, more waveguides become necessary as the quantum network grows, making this approach difficult to scale. Additionally, unidirectional waveguides typically incorporate additional components to enhance directivity, which introduces communication errors.
“We can get rid of these lossy components if we have a waveguide that can support propagation in both left and right directions, and a way to choose the direction at will. This ‘directional transmission’ is what we’ve demonstrated, and it’s the first step towards two-way communication with much higher fidelities,” says Kannan.
Thanks to their architecture, several processing modules can be chained along a waveguide. A standout feature of 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 only have one physical connection that can have any number of modules along the way. That’s what makes it scalable. Having demonstrated directional photon emission from one module, we are now working on capturing that photon downstream on a second module,” Almanakly adds.
Take advantage of quantum properties
To do this, the researchers built a module comprising four qubits.
Qubits are the building blocks of quantum computers and are used to store and process quantum information. But qubits can also be used as photon emitters. Adding energy to a qubit causes the qubit to excite, then when it de-excites, the qubit will emit the energy in the form of a photon.
However, simply connecting a qubit to a waveguide does not guarantee directivity. A single qubit emits a photon, but whether it moves left or right is completely random. To circumvent this problem, the researchers use two qubits and a property known as quantum interference to ensure that the emitted photon travels in the correct direction.
The technique consists of preparing the two qubits in a single entangled state of excitation called the Bell state. This quantum mechanical state has two aspects: the left qubit being excited and the right qubit being excited. Both aspects exist simultaneously, but which qubit is excited at any given time is unknown.
When the qubits are in this entangled bell state, the photon is effectively emitted towards the waveguide at both qubit locations simultaneously, and these two “emission pathways” interfere with each other. Depending on the relative phase in the Bell state, the resulting photon emission must move left or right. By preparing the Bell state with the correct phase, researchers choose the direction in which the photon travels through the waveguide.
They can use this same technique, but in reverse, to receive the photon on 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 simply pass. This is analogous to tuning a radio to a particular station. If we choose the right radio frequency, we will pick up the music transmitted on that frequency,” explains Almanakly.
The researchers found that their technique achieved over 96% fidelity, meaning that if they intended to emit a photon to the right, 96% of the time it was going to the right.
Now that they’ve used this technique to efficiently emit photons in a specific direction, the researchers want to connect multiple modules and use the process to emit and absorb photons. This would be a major step towards developing a modular architecture that combines many smaller-scale processors into one larger-scale and more powerful quantum processor.
The research is funded, in part, by the AWS Center for Quantum Computing, the US 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. .
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Researchers have demonstrated directional photon emission, the first step towards stretchable quantum interconnects – News Physics and Quantum Computing
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