Device enables direct communication between multiple quantum processors

Quantum computers have the potential to solve complex problems, which is impossible for the most powerful classic supercomputers.

Just as a classical computer has separate but interconnected components that must work together, such as memory chips and CPUs on motherboards, quantum computers will need to convey quantum information between multiple processors.

The current architectures for interconnecting superconducting quantum processors are “point-to-point” in connectivity, meaning they require a series of transfers between network nodes and have more complex error rates.

In overcoming these challenges, MIT researchers have developed a new interconnect device that can support scalable “all-around” communications so that all superconducting quantum processors in the network can communicate directly with each other.

They created a network of two quantum processors and used their interconnections to send microwave photons back and forth in user-defined directions. Photons are light particles that can carry quantum information.

The device includes a superconducting wire or waveguide that shuttles photons between processors and can be routed as needed. Researchers can pair any number of modules with them, effectively transmitting information between scalable processor networks.

They use this interconnect to demonstrate remote entanglement, a kind of correlation between unphysically connected quantum processors. Remote entanglement is a critical step in developing powerful, distributed networks for many quantum processors.

“In the future, a quantum computer will probably need both local and nonlocal interconnects. Local interconnects are natural in arrays of superconducting qubits. Ours allow for more nonlocal connections. We can send photos at different frequency, times, and in two propagation directions, which gives our network more flexibility and throughput,” says Aziza Almanakly, an electric engineering and computer Science graduate student in the Engineering Electronic Research Laboratory (RLE) group of quantum systems and lead authors of the paper on interconnection.

Her co-authors include Beatriz Yankelevich, a graduate student at the Equs Group; senior writer William D. Oliver, Henry Ellis Warren (1894), Professor of Electrical Engineering and Computer Science (EEC), Professor of Physics, Director of the Center for Quantum Engineering and Deputy Director of RLE; and others from the MIT and Lincoln Laboratory. The study appears in natural physics today.

Scalable architecture

The researchers previously developed a quantum computing module, which allows them to send microwave photons carrying information along the waveguide in either direction.

In the new work, they take the architecture a step further by connecting two modules to the waveguide to emit photons in the desired direction and then absorbing them at the other end.

Each module consists of four qubits, which serve as the interface between waveguides with photons and a larger number of subprocessors.

The quantum weight is emitted and absorbed photons with the waveguide, and then transferred to nearby data digits.

The researchers used a series of microwave pulses to add energy to the quantum and then emit photons. Careful control of the phase of these pulses can produce quantum interference effects, allowing them to emit photons along the waveguide in either direction. Timely reverse pulses can allow values ​​in another module to absorb photons at any arbitrary distance.

“Pitching and capturing photons allow us to create ‘quantum interconnects’ between non-native quantum processors, and with the range of quantum interconnects, remote entanglement is far away,” Oliver explained.

“Generating distant entanglements is a critical step in building large quantum processors from smaller-scale modules. Even after the photons disappear, we have correlations between two distant or “non-local” quebits. Remote remote entanglement allows us to operate parallel between these correlations and between the two quebits, even if they are not long, and eSTELS extended and extremely and extly and extly””””””””””””””””””””””””””””””””””””””””””””””””””””””””””””””””””””””””””””””””””””””””””””””””””””””””””””””””””””””””””””””””””””””””””””””””””””””””””””””””””””””””””””””””””””””””””””””””””””””””””

However, transmitting photons between the two modules is not enough to create remote entanglement. Researchers need to prepare quantum and photons so that the modules “share” the photons at the end of the protocol.

Cause entanglement

The team did this by stopping photon emissions midway through its duration. In quantum mechanical terms, photons are both retained and emitted. Classically speaking, one can think that it has been retained for half a month and sent out half.

Once the receiver module absorbs the “half-photon”, the two modules will be entangled.

However, as the photons propagate, joints, wire bonds and connections, the waveguide distorts the photons and limits the absorption efficiency of the receiving module.

To generate remote entanglement with sufficient fidelity or accuracy, researchers need to absorb the frequency at which photons absorb at the other end to the maximum extent.

“The challenge of this work is to properly shape photons so that we can maximize absorption efficiency,” Almanakly said.

They use reinforcement learning algorithms to “pre-introduce” photons. The algorithm optimizes protocol pulses to form photons for maximum absorption efficiency.

When they implement this optimized absorption scheme, they are able to show photon absorption efficiency greater than 60%.

This absorption efficiency is high enough to prove that the result state at the end of the protocol is entangled, a major milestone in this demonstration.

“We can use this architecture to create a network with all connections. This means we can have multiple modules on the same bus, and we can create remote entanglements in any pair of our choice,” Yankelevich said.

In the future, they can improve absorption efficiency by optimizing the path of photon propagation, perhaps by integrating modules in 3D, rather than using superconducting wires connected to separate microwave packaging. They can also make the protocol faster and therefore have fewer chances of accumulating errors.

“In principle, our remote entanglement generation protocol can also be extended to other types of quantum computers and larger quantum Internet systems,” Almanakly said.

This work is funded in part by the U.S. Army Research Office, the AWS Center for Quantum Computing, and the U.S. Air Force Office of Scientific Research.