In general, photons traveling over random nodes or resonant coupling of neighboring nodes facilitate the transfer of quantum information between them. The type of transmitter, propagation channel, and receiver all affect utility.

Conventional methods of transmitting microwave photons have low fidelity due to photon loss and are often unidirectional. Direct resonance coupling systems, on the other hand, are theoretically bidirectional, but can typically only support a small number of local nodes.

In a new study, MIT scientists have demonstrated high-fidelity, on-demand, directional microwave photon emission. They have developed a quantum computing architecture that enables extensible, high-fidelity communication between superconducting quantum processors. Their approach guarantees that quantum information flows effectively more than 96% of the time.

A quantum network connects processing nodes using photons traveling through waveguides. A waveguide can be unidirectional, moving a photon only to the left or right, or it can be bidirectional. This method is challenging at scale, because each waveguide can only move photons in one direction, requiring more waveguides as the quantum network grows. In addition, additional components are often added to unidirectional waveguides to enforce directionality, causing communication problems.

The solution is a waveguide that can support propagation in both left and right directions and a means to choose the direction at will.

Scientists in this study demonstrated directional transmission, the first step towards bidirectional communication with much higher fidelity.

Bharat Kannan, Ph.D. ’22, co-lead author of a research paper describing this technique, said: “Using the architecture, multiple processing modules can be strung along one waveguide. A notable feature of the architectural design is that the same module can be used as both a transmitter and a receiver. And photons can be sent and received by any two modules along a common waveguide.”

Aziza Almanakly, a graduate student in electrical engineering and computer science in the Engineering Quantum Systems group at the Research Laboratory of Electronics (RLE) at MIT, said: “We only have one physical connection that can have an unlimited number of modules along the way. This makes it scalable. Having demonstrated the directional photon emission from one module, we are now working on capturing that photon downstream at a second module.”

To achieve this, scientists are developing a module consisting of four qubits.

Qubits can be used as photon emitters. Adding energy to qubits makes them excited, and in an unexcited state they emit the energy in the form of a photon.

However, direction is not guaranteed by just attaching one qubit to a waveguide. Even though a single qubit emits a photon, its direction of travel is completely random. Scientists use two qubits and a phenomenon called quantum interference to make sure the emitted photon is going in the right direction to get around this problem.

The process involves putting the two qubits into a Bell state, which is an entangled state of single excitation. The left and right qubits are both excited in this quantum mechanical state, which consists of two components. Both functions coexist, but it is uncertain which qubit is excited at any given time.

Qubits in an entangled Bell state effectively cause the photon to emit simultaneously to the waveguide at the two qubit sites. These two “emission paths” interfere with each other.

The resulting photon emission should move left or right depending on the relative phase within the Bell state. The scientists decide which direction the photon travels through the waveguide by adjusting the Bell state to the correct phase.

almanacly says, “They can use the same technique, but in reverse, to receive the photon on a different module.”

“The photon has a certain frequency and energy, and you can prepare a module to receive it by tuning it to the same frequency. If they are not of the same frequency, the photon will pass. It is analogous to tuning a radio to a particular station. We pick up the music broadcast on that frequency if we choose the right radio frequency.”

Scientists now want to connect multiple modules together and use the process to emit and absorb photons. This would be a major step toward developing a modular architecture that combines much smaller processors into one larger, more powerful quantum processor.

Yasunobu Nakamura, director of the RIKEN Center for Quantum Computing, who was not involved in this research, said: “The work demonstrates an on-demand quantum transmitter, in which the interference of the emitted photon from an entangled state determines its direction, beautifully manifesting the power of waveguide quantum electrodynamics. It can be used as a fully programmable quantum node that can emit/absorb quantum information /pass/store on a quantum network and as an interface to a bus connecting multiple quantum computer chips.”

Magazine reference:

  1. Kannan, B., Almanakly, A., Sung, Y. et al. On-demand directional microwave photon emission using waveguide quantum electrodynamics. Wet. Physical. (2023). DOI: 10.1038/s41567-022-01869-5