Quantum computers have the potential to solve complex problems that most powerful classic supercomputers can’t crack.
Just as classic computers have separate components that need to work together, such as memory chips and CPUs on motherboards, quantum computers must communicate quantum information between multiple processors.
The current architecture used to interconnect superconducting quantum processors is “point-to-point” of connectivity. This means that a series of transfers between network nodes is required, resulting in a combined error rate.
On the way to overcome these challenges, MIT researchers have developed new interconnect devices that can support scalable “all” communications so that all superconducting quantum processors in a network can communicate directly with each other.
They created a network of two quantum processors and used interconnects to transmit microwave photons on demand in user-defined directions. Photons are particles of light that can carry quantum information.
The device includes superconducting wires, or waveguides, that can shuttle photons between processors and route them if necessary. Researchers can combine any number of modules to efficiently transmit information across networks of scalable processors.
They used this interconnect to demonstrate remote entanglement, a type of correlation between un-physically connected quantum processors. Remote entanglement is a critical step in developing powerful, distributed networks of many quantum processors.
“In the future, quantum computers will likely need both local and non-local interconnects. Local interconnects are natural in an array of superconducting Qubits. Ours allows for more non-local connections. Research Institute of Electronics (RLE) and the lead author of the paper on interconnects.
Her co-authors include Beatriz Yankelevich, a graduate student at the EQUS Group. Senior authors William D. Oliver, Henry Ellis Warren (1894) Professor of Electrical Engineering and Computer Science (EECS) and Professor of Physics, Director of the Quantum Engineering Center, and Associate Director of RLE. Others at MIT and Lincoln Laboratory. This study is published today in Natural Physics.
Scalable architecture
Researchers previously developed quantum computing modules to allow microwave photons to be transmitted along the waveguides that carry information in either direction.
In the new work, we took its architecture a step further by connecting two modules to a waveguide, emitting photons in the desired direction and absorbing them on the opposite side.
Each module consists of four qubits that act as an interface between the photon-carrying waveguide and the larger quantum processor.
The Qubits coupled to the waveguide emit and absorb photons, then are transmitted to nearby datakitz.
Researchers use a series of microwave pulses to apply energy to the chikubit and emit photons. Carefully controlling the phase of these pulses provides quantum interference effects that allow photons to be emitted in either direction along the waveguide. Reverse the pulse in time allows for qubits of another module at any distance to absorb photons.
“Pitching and catch photons allow you to create ‘quantum interconnects’ between non-local quantum processors, and there is remote entanglement with quantum interconnects,” explains Oliver.
“Generating remote entanglements is an important step in building large quantum processors from small modules. Even after their photons are gone, they are still correlated with two far or “non-local” Qubits.
However, simply transferring photons between two modules is not sufficient to generate remote entanglement. Researchers need to prepare qubits and photons so that the module “shares” the photons at the end of the protocol.
Generating Entanglement
The team did this by stopping the photon emission pulse midway through the period. In quantum mechanical terms, photons are retained and emitted. Classically, we can think of half of the photons being held and half being emitted.
When the receiver module absorbs “half photons”, the two modules become entangled.
However, as photons move, junctions, wire couplings, and connections within the waveguide distort the photons, limiting the absorption efficiency of the receiver module.
To generate remote entanglement with high fidelity or accuracy, researchers were required to maximize the frequency at which photons were absorbed on the opposite side.
“The challenge with this task was to properly shape the photons and maximize absorption efficiency,” says Almanakly.
They used a reinforcement learning algorithm to “plan” the photons. The algorithm optimized the protocol pulses to shape photons for maximum absorption efficiency.
When this optimized absorption protocol was implemented, it was able to exhibit photon absorption efficiency above 60%.
This absorption efficiency is high enough to prove that the resulting states are intertwined at the end of the protocol, and is a major milestone in this demonstration.
“We can use this architecture to create a network with all the connectivity. This means that we can create multiple modules along the same bus, and we can create remote entanglements between any pair of choices,” says Yankelevich.
In the future, absorption efficiency could be improved by optimizing the pathway through which photons propagate, perhaps by integrating the modules into 3D instead of having superconducting wires connecting separate microwave packages. Additionally, the protocol can be made faster, which reduces the chances of errors building up.
“In principle, our remote entanglement generation protocol can also be extended to other types of quantum computers and larger quantum internet systems,” says Almanakly.
This work was funded in part by the U.S. Army Investigation Bureau, the AWS Quantum Computing Center, and the U.S. Air Force Bureau of Scientific Research.
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