Connected! Important progress in interconnection of superconducting quantum computers
As with classical computers and the Internet, quantum computers cannot exploit their full capabilities if they are not interconnected. Recently, researchers at MIT have developed a quantum computing architecture that will enable scalable, high-fidelity communication between superconducting quantum processors.
Fidelity up to 96%, modularity of quantum processors achieved
Quantum computers hold the promise of performing certain tasks that are difficult to accomplish even on the world's most powerful supercomputers. In the future, scientists expect to use quantum computing to model materials systems, simulate quantum chemistry, and optimize hard tasks with implications that could span finance and pharmaceuticals.
However, realizing this promise will require resilient and scalable hardware. One challenge in building large-scale quantum computers is that researchers must find an efficient way to interconnect quantum information nodes - smaller-scale processing nodes separated on a computer chip. Because quantum computers are fundamentally different from classical computers, traditional technologies used to communicate electronic information do not translate directly into quantum devices. However, one requirement is certain: the information carried must be transmitted and received, whether through classical or quantum interconnects.
To this end, the MIT team demonstrated the first step, the deterministic emission of a single photon - the information carrier - in a direction specified by the user, and their method ensures that quantum information flows in the right direction more than 96% of the time. Connecting several of these modules creates a larger network of quantum processors that can be interconnected regardless of their physical spacing on a computer chip.
"Quantum interconnections are a key step toward modularity for larger machines built from smaller, individual components. "The ability to communicate between smaller subsystems will enable modular architectures for quantum processors, which may be a simpler way to scale to larger system sizes than the brute force approach of using a single large, complex chip," said Dr. Bharath Kannan, co-lead author of the research paper. way. "
Using bidirectional waveguides to move quantum information
In a traditional classical computer, various components perform different functions, such as memory, computation, etc. Electronic information is encoded and stored as bits (taking the value of 1 or 0) that travel between these components via interconnects; interconnects are wires that move electrons around the computer processor.
But quantum information is more complex. Rather than having only the value 0 or 1, quantum information can be both 0 and 1, depending on the phenomenon of superposition. furthermore, quantum information can be carried by particles of light (photons). These additional complexities make quantum information fragile and cannot be simply transmitted using traditional protocols.
A quantum network uses photons to connect processing nodes, and these photons are transmitted through waveguides. Waveguides can be unidirectional, moving only one photon to the left or right, or bi-directional.
Most existing architectures use unidirectional waveguides, which are easier to implement because the direction in which the photons move is easily determined. However, because each waveguide can only move photons in one direction, more waveguides are needed as the quantum network expands, making it difficult to scale this approach. In addition, one-way waveguides often contain additional components to enforce directionality, which introduces communication errors.
"If we had a waveguide that could support propagation in both left and right directions, and a means to choose the direction at will, we could get rid of these lossy components. This 'directional transmission' is what we have demonstrated, and it is the first step toward two-way communication with higher fidelity. "With their architecture, multiple processing modules can be strung together along a single waveguide, Kannan said. Also, a distinguishing feature of the architecture's design is that the same module can be used as both a transmitter and receiver; and photons can be sent and captured by any two modules along a common waveguide.
Experimental setup
Almanakly adds, "We have only one physical connection and can have any number of modules along the way. That's how scalable it is. Having demonstrated directed photon emission from one module, we are now working on capturing that photon downstream of a second module. "
Leveraging quantum properties, a step closer to modular architecture
To achieve this goal, the researchers built a module consisting of four quantum bits.
Quantum bits are the building blocks of quantum computers, used to store and process quantum information. But quantum bits can also be used as photon emitters; adding energy to a quantum bit causes the quantum bit to be excited, and then when it de-excites, the quantum bit will emit energy as a photon.
Directional photon emission in a waveguide-based QED architecture.
However, simply attaching a quantum bit to a waveguide does not ensure directionality. A single quantum bit emits a photon, but whether it moves to the left or to the right is completely random. To circumvent this problem, the researchers used the properties of two quantum bits and quantum interference to ensure that the emitted photon travels in the correct direction.
The technique involves preparing two quantum bits in a single excited entangled state of the Bell state. This quantum mechanical state consists of two aspects: the quantum bit on the left is excited, and the quantum bit on the right is excited. Both aspects exist simultaneously, but it is unknown which quantum bit is excited at a particular time.
When the quantum bits are in this entangled Bell state, photons are actually emitted into the waveguide at the same time at the positions of the two quantum bits, and the two "emission paths" are interfering with each other. Depending on the relative phase within the Bell state, the resulting photon emission must be shifted to the left or to the right. By preparing the Bell state with the correct phase, the researchers chose the direction in which the photons would cross the waveguide.
They could use this same technique, but in the opposite way, to receive photons in 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 on the same frequency, then the photons will just pass through. It's like tuning a radio to a specific station. If we choose the right radio frequency, we will receive music transmitted at that frequency." Almanakly said.
Their technique achieves more than 96 percent fidelity: this means that if they intend to transmit a photon to the right, 96 percent of the time it will transmit to the right.
Now that they have used this technique to effectively emit photons in a specific direction, the researchers hope to connect multiple modules and use this process to emit and absorb photons. This would be an important step toward developing a modular architecture that combines many smaller-scale processors into a larger, more powerful quantum processor.
https://www.nature.com/articles/s41567-022-01869-5
https://phys.org/news/2023-01-quantum-architecture-large-scale-devices.html
