The new quantum computing architecture could be used to interconnect large-scale devices

Quantum computers promise to perform certain tasks that would be difficult for even the world’s most powerful supercomputers to solve. In the future, scientists expect to use quantum computing to simulate systems of materials, simulate quantum chemistry, and improve challenging tasks, with potential impacts extending from finance to pharmaceuticals.

However, realizing this promise requires flexible and expandable hardware. One of the challenges in building a large scale quantum computer is that the researchers must find an effective way to connect the quantum Information Nodes — Smaller processing nodes separated by a computer chip. Because quantum computers are fundamentally different from classical computers, the traditional techniques used to communicate electronic information do not translate directly to quantum devices. However, one requirement is certain: whether by classical or quantum coherence, the transmitted information must be sent 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 the work published in nature physicsMIT researchers demonstrate the first step, the deterministic emission of single photons—carriers of information—in a user-specified direction. Their method ensures that quantum information flows in the right direction more than 96 percent of the time.

Connecting many of these modules enables a larger network of interconnected quantum processors, no matter what physical separation on a computer chip.

“Quantum interfaces are a critical step towards modular implementation of large scale machines built from smaller individual components,” says Bharath Kannan Ph.D. 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 to scale to larger system sizes compared to a brute-force approach using a single large, complex chip,” Kannan adds.

Kanaan co-wrote the paper with co-lead author Aziza Al-Manakli, Ms electrical engineering and a graduate student in computer science in the Quantum Systems Engineering group at the Research Laboratory of Electronics (RLE) at MIT. The lead author is William D. Oliver, professor of electrical engineering, computer science, and physics, fellow at the MIT Lincoln Laboratory, director of the Center for Quantum Engineering, and co-director of RLE.

Quantum information transmission

In a conventional classical computer, different components perform different functions, such as memory, arithmetic, etc. Electronic information, encoded and stored in the form of bits (which takes the value of 1s or 0s), is transmitted between these components using interconnections, which are wires that move Electrons around a computer processor.

But quantum information is much more complex. Instead of holding only a value of 0 or 1, quantum information can also be 0 and 1 at the same time (a phenomenon known as superposition). Also, quantum information can be transmitted by particles of light, called photons. These added complexities make quantum information fragile, and simply cannot be transmitted using conventional protocols.

A quantum network connects processing nodes using photons that travel through special interfaces known as waveguides. a waveguide It can be either unidirectional, moving a Photon Only left or right, or it can be bi-directional.

Most current architectures use unidirectional waveguides, which are easy to implement since the direction in which the photons travel can be easily determined. But since each waveguide only moves the photons in one direction, more waveguides become necessary as the quantum lattice expands, making this approach difficult. In addition, unidirectional waveguides usually include additional components to force direction, which leads to communication errors.

“We can eliminate these missing components if we have a waveguide that can support propagation in both left and right directions, and a means to choose the direction at will. This ‘directional transmission’ is what we have demonstrated, and it is the first step towards two-way communication with much higher fidelity,” Kanan says. “.

Using its architecture, multiple processing units can be linked along a single waveguide. A great feature of the architecture design, he adds, is that the same unit can be used as a transmitter and receiver. Photons can be sent and captured by any two units along a common waveguide.

“We only have one physical connection that can have any number of units along the way. That’s what makes it scalable. Having demonstrated directional photon emission from one unit, we’re now working on capturing that photon downstream in a second unit,” Add.

Take advantage of quantum properties

To achieve this, the researchers built a unit consisting of 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 emitters of photons. Adding energy to a qubit It causes the qubit to be excited, and then when it de-excites, the qubit will emit energy in the form of a photon.

However, simply connecting a single qubit to a waveguide does not guarantee directivity. An individual qubit emits a photon, but whether it goes left or right is completely random. To circumvent this problem, the researchers used two qubits and a property known as quantum interference to ensure that the emitted photon traveled in the correct direction.

This technique involves preparing two qubits in a single excitation entangled state called the Bell state. This quantum mechanical state consists of two sides: the left qubit is excited and the right qubit is excited. Both sides are present simultaneously, but which qubit is excited at a given time is unknown.

When the qubits are in this entangled Bell state, the photon is effectively emitted to the waveguide at the two qubit sites simultaneously, and the two “emission paths” interfere with each other. Depending on the relativistic phase within the Bell state, the resulting photon emission must shift to the left or right. By setting the bell state to the correct phase, the researchers choose the direction in which the photon travels through the waveguide.

They can use the same technology, but in reverse, to receive the photon into another unit.

“A photon has a certain frequency, a certain energy, and you can prepare a unit to receive it by tuning it to the same frequency. If it’s not on the same frequency, the photon will just pass through. It’s the same as tuning in to a radio to a specific station. If we choose the right frequency for the radio, we’ll pick up the music transmitted at that frequency.” Manakli says.

The researchers found that their method achieved an accuracy of more than 96% – meaning that if they intended to emit a photon to the right, 96% of the time it went to the right.

Now that they have used this technology to effectively emit photons in a specific direction, the researchers want to connect multiple units and use the process to emit and absorb photons. This will be a major step towards developing a modular architecture that combines many smaller processors into a single, larger, more powerful quantum processor.