Quantum computers possess the capability to tackle intricate issues that would be unfeasible for even the most robust classical supercomputer to resolve.
In a similar fashion to how a classical computer comprises distinct yet interconnected elements that must function collaboratively, such as a memory module and a CPU on a motherboard, a quantum computer must relay quantum data among various processors.
The existing designs utilized to connect superconducting quantum processors exhibit a “point-to-point” connectivity, signifying that they necessitate a succession of transfers between network nodes, leading to accumulating error rates.
To address these obstacles, MIT researchers invented a novel interconnect device capable of facilitating scalable, “all-to-all” communication, enabling all superconducting quantum processors within a network to communicate directly with one another.
They assembled a network comprising two quantum processors and employed their interconnect to transmit microwave photons back and forth on request in a user-specified direction. Photons are light particles capable of conveying quantum information.
The device features a superconducting wire, or waveguide, that transports photons between processors and can be directed over any required distance. The researchers can link any number of modules to it, effectively relaying information throughout a scalable processor network.
Utilizing this interconnect, they showcased remote entanglement, which refers to a type of correlation between quantum processors that lack physical connections. Remote entanglement represents a crucial progression towards establishing a potent, distributed network comprising multiple quantum processors.
“In the future, a quantum computer will likely require both local and nonlocal interconnects. Local interconnects are inherent in arrays of superconducting qubits. Our design permits increased nonlocal connections. We can transmit photons at various frequencies, timings, and in two propagation directions, which enhances our network’s flexibility and capacity,” states Aziza Almanakly, a graduate student in electrical engineering and computer science who is part of the Engineering Quantum Systems group at the Research Laboratory of Electronics (RLE) and the primary author of a paper on the interconnect.
Her co-authors comprise Beatriz Yankelevich, a graduate student in the EQuS Group; senior author William D. Oliver, a Professor at MIT in electrical engineering and computer science as well as physics, an MIT Lincoln Laboratory Fellow, director of the Center for Quantum Engineering, and associate director of RLE; along with other MIT and Lincoln Laboratory collaborators. The research is published today in Nature Physics.
An expandable architecture
The researchers earlier developed a quantum computing module, enabling the transmission of information-laden microwave photons in both directions along a waveguide.
In this latest research, they advanced that architecture further by interconnecting two modules to a waveguide to emit photons in a chosen direction and subsequently absorb them at the opposite end.
Every module comprises four qubits, which function as an interface between the waveguide carrying photons and the larger quantum processors.
The qubits linked to the waveguide emit and absorb photons, which are later transferred to adjacent data qubits.
The researchers apply a sequence of microwave pulses to infuse energy into a qubit, which, in turn, emits a photon. Thoughtful control over the phase of these pulses enables a quantum interference effect that permits them to emit the photon in either direction along the waveguide. Reversing the pulses within time allows a qubit in another module, regardless of distance, to absorb the photon.
“Transmitting and capturing photons permits us to form a ‘quantum interconnect’ between nonlocal quantum processors, and remote entanglement emerges together with quantum interconnects,” clarifies Oliver.
“The generation of remote entanglement is an essential step towards constructing a large-scale quantum processor from smaller-scale components. Even after the photon departs, we maintain a correlation between two distant, or ‘nonlocal,’ qubits. Remote entanglement enables us to capitalize on these correlations and execute parallel operations between two qubits, even though they are no longer linked and may be widely separated,” Yankelevich elaborates.
However, simply transmitting a photon between two modules does not suffice for generating remote entanglement. The researchers must prepare the qubits and the photon so that the modules “share” the photon at the conclusion of the process.
Creating entanglement
The team achieved this by pausing the photon emission pulses midway through their duration. In quantum mechanical parlance, the photon is both conserved and emitted. Classically, one might visualize this as keeping half a photon and emitting the other half.
Once the receiving module absorbs that “half-photon,” the two modules enter an entangled state.
However, as the photon travels, joints, wire connections, and links within the waveguide distort the photon and constrain the absorption efficiency of the receiving module.
To create remote entanglement with sufficient fidelity, or precision, the researchers needed to enhance how frequently the photon is absorbed at the other end.
“The key challenge in this endeavor was precisely shaping the photon to optimize the absorption efficiency,” Almanakly states.
They utilized a reinforcement learning algorithm to “predistort” the photon. The algorithm fine-tuned the protocol pulses to shape the photon for maximal absorption efficiency.
Upon implementing this optimized absorption protocol, they demonstrated photon absorption efficiency exceeding 60 percent.
This level of absorption efficiency is adequate to confirm that the resultant state at the completion of the process is entangled, marking a significant milestone in this demonstration.
“We can leverage this architecture to establish a network with comprehensive connectivity. This allows us to incorporate multiple modules, all along the same bus, and facilitate remote entanglement between any pair of our selection,” Yankelevich states.
Looking ahead, they may enhance absorption efficiency by optimizing the path along which the photons travel, potentially by integrating modules in 3D rather than using a superconducting wire to link separate microwave packages. They could also accelerate the protocol to reduce the likelihood of error accumulation.
“In principle, our remote entanglement generation protocol could also be adapted for other types of quantum computers and larger quantum internet frameworks,” Almanakly states.
This research received partial funding from the U.S. Army Research Office, the AWS Center for Quantum Computing, and the U.S. Air Force Office of Scientific Research.