Despite the fact that quantum computers are still a new technology that isn’t ready for widespread usage, researchers have been looking into the theoretical limits that would constrain quantum technologies.
Researchers have revealed that the speed at which quantum information may be transmitted via any quantum device has a limit.
Lieb-Robinson boundaries are the names given to these speed restrictions, and they’ve been taunting scholars for years: There was a gap between the theoretically optimal speeds and the greatest algorithms anyone has created for particular tasks.
It’s as if no automaker could come up with a model that could go over the speed limit on the local highway.
However, unlike speed restrictions on highways, information speed limits can’t be ignored when you’re in a hurry because they’re the unavoidable result of physics’ fundamental principles.
There is a limit to how rapidly quantum interactions may make their effect felt (and so convey information) from a particular distance away for any quantum endeavor. The fundamental rules establish the best potential performance.
In this sense, information speed constraints are more akin to the highest score on an old arcade game than traffic laws, and getting the highest score is a tempting prize for scientists.
At each step of a new quantum protocol, groups of quantum entangled qubits (red dots) recruit more qubits (blue dots) to assist transport information quickly from one location to another. The technique creates a snowball effect that achieves the maximum information transfer speed allowed by theory since additional qubits are involved at each phase.
Now, a group of researchers led by Adjunct Associate Professor Alexey Gorshkov has developed a quantum protocol that achieves theoretical speed limits for specific quantum jobs. Their discovery adds to our understanding of how to develop optimal quantum algorithms and indicates that there hasn’t been a lower, undiscovered limit that has stymied attempts to create better ones.
Gorshkov, a physicist at the National Institute of Standards and Technology and a Fellow of the Joint Quantum Institute and the Joint Center for Quantum Information and Computer Science (QuICS), and his colleagues recently published a paper in the journal Physical Review X describing their new protocol.
Minh Tran, a JQI and QuICS graduate student who was the lead author on the research, says, “This gap between maximum speeds and achievable speeds had been worrying us because we didn’t know if it was the bound that was loose, or if we weren’t smart enough to improve the protocol.” “We weren’t anticipating this idea to have such a strong impact. And, despite our best efforts, we were unable to improve the bound. As a result, we’re very pleased with the outcome.”
The theoretical speed limit for transferring information in a quantum device (such as a quantum computer) is, unsurprisingly, determined by the device’s underlying structure. The new protocol is intended for quantum devices in which the fundamental building blocks—qubits—influence one another even when they are not physically present. The technique was specifically created for qubits with weakening interactions as the distance between them increases.
The new protocol covers interactions in many practical building elements of quantum technologies, such as nitrogen-vacancy centers, Rydberg atoms, polar molecules, and trapped ions, that do not weaken too quickly.
Importantly, the protocol can send information from an unknown quantum state to a remote qubit, which is a requirement for many of the quantum computer’s touted benefits. This restricts the methods for transferring data and eliminates some straightforward approaches, such as simply copying the data to a new location. (Knowing what quantum state you’re transmitting is required.)
Quantum entanglement is used in the new protocol to communicate data stored on one qubit with its neighbors. Then, because all of those qubits are involved in carrying the data, they collaborate to disseminate it to additional qubit sets. They transport information significantly faster since there are more qubits involved.
This technique can be done indefinitely to generate larger blocks of qubits that pass data more quickly. Rather than passing information one by one like a basketball team passing the ball down the court, the qubits are more like snowflakes that merge into a larger and faster rolling snowball at each step. With each revolution, more flakes stick to the snowball as it grows larger.
The resemblance to snowballs, however, may end there. The quantum collection, unlike a genuine snowball, may unfurl on its own. When the process is reversed, the information is left on the far qubit while the other qubits are returned to their original states.
The snowballing qubits speed along with the information at the theoretical limits allowed by physics, according to the researchers who studied the phenomenon. No new protocol should be able to surpass the protocol since it reaches the previously demonstrated limit.
The method we entangle two blocks of qubits is a novel component, according to Tran. “Previously, there was a mechanism that entangled data into one block and then attempted to integrate the qubits from the second block one by one into it. The boost will be bigger now because the qubits in the second block are also entangled before merging them into the first block.”
The protocol is the product of the team’s research on the potential of transporting data stored on many qubits at the same time. They realized that moving information in blocks of qubits might improve the performance of a protocol.
“On the practical side,” Tran explains, “the protocol allows us to not only propagate information but also entangle particles faster.” “We also know that entangled particles may be used to conduct a variety of intriguing things, such as measuring and detecting with greater precision. Moving information quickly also allows you to process it more quickly. There are numerous more obstacles in the construction of quantum computers, but at least in terms of fundamental constraints, we know what’s possible.
There are many additional barriers in the development of quantum computers, but we know what’s doable and what’s not.”
The team’s mathematical calculations disclose fresh information about how vast a quantum processor must be in order to model particles with interactions like the qubits in the new protocol, in addition to theoretical insights and possible technological applications. The researchers want to test the protocol’s limits with different types of interactions and look into other parts of it, such as how resistant it is to noise disruption.
Main paper: Minh C. Tran et al, Optimal State Transfer and Entanglement Generation in Power-Law Interacting Systems, Physical Review X (2021). DOI: 10.1103/PhysRevX.11.031016