Quantum mechanics promise ultra-secure communications and will provide the building blocks for a large-scale quantum network. However, quantum information systems are sensitive to environmental factors, including energy loss and heat, or thermal noise, which can cause these systems to collapse.

In quantum communication, this can occur as the quantum information, in the form of light particles, or photons, travel through a channel, preventing them from reaching their target.

Now, researchers at the University of Chicago and the University of York have detailed a new strategy for securely boosting the successful transmission rates of quantum information. They reported their strategy in Nature Communications.

“When we send light through a channel some part of it might be lost along the way,” said the paper’s lead author, Kyungjoo Noh, a new Ph.D. graduate from Yale University and a visiting researcher at UChicago’s Pritzker School of Molecular Engineering. “You might get only, say, 80 or 90 percent of it. Those kinds of attenuations are what we’re dealing with here.”

The UChicago-Yale-York team seeks to design an information coding scheme that will operate in the presence of imperfections such as energy loss. The team’s new paper assesses the limitations presented by factors such as energy loss and thermal noise and identifies strategies to negate these impacts.

Their feat duplicates for quantum information theory what Claude Shannon, the inventor of information theory, did for classical information theory by addressing the question of how much information can be sent through a noisy communication channel.

However, the world of quantum communication depends upon quantum physics, the exotic and non-intuitive laws that govern the behavior of subatomic particles. In the quantum communications system that Noh considered, these subatomic particles are photons that carry information from sender to recipient.

In particular, the team investigated quantum communication in a low-energy environment, one that transmits a relatively small number of light particles, as this has been shown to successfully increase communication rates. While, in principle, highly energetic states might allow researchers to send more photons and therefore more information, previous work has shown reduced communication rates as photon numbers increase due to limited controllability of high-energy states. The researchers therefore opted for a practical scenario: sending communications with a lower-energy state and fewer photons that they could reliably control.

In low-energy environments, quantum researchers typically opt to spread their information evenly across multiple unlinked communications channels to get the best results.

The new paper introduces a new, more secure strategy, where the UChicago researchers allocate larger amounts of photons at a time, and then distribute them unequally across channels to increase their odds of success.

Noh explained that if they have limited photons to send through the noisy channel each time, the photons may not get through. But if they choose instead to group more photons together,  they increase their odds of seeing a positive return. Then, before sending the information to a recipient, “we uniformly shuffle this energy imbalance,” Noh said. In the quantum world, this new strategy boosts the chances of getting a positive “energy return.”

Because of the odd nature of quantum physics, distributing the photons unevenly across multiple channels causes them to become linked when the energy imbalance is uniformly shuffled.

Storing information over multiple linked quantum information channels also is a good way to increase its security, said co-author Liang Jiang, a professor at the Pritzker School of Molecular Engineering. Thieves trying to steal from such a system would get away with very little information. Breaking into multiple channels would be harder than breaking into one, and each of those multiple channels would carry less information than a single, more comprehensive information channel. “Kyungjoo came up with this. It’s a very original and clever construction,” Jiang said.

For quantum communications to be practical for private information exchange, users of a quantum channel need to know how many secret keys they can generate for coding and decoding secure messages. But assessing the upper limits of quantum information data transmission has plagued researchers for 20 years, ever since quantum information theory began emerging as a serious scientific field.

The Nature Communications paper has for the first time raised the theoretical quantum communication limits to a class of realistic quantum information channels (in both information loss and added noise) that are practically relevant. While significant progress has been made in simplified, highly idealized theoretical models, it is also important to investgate models closer to reality, Noh said. In his latest work, he said, “I took inspiration from these idealized mathematical models, extracted the key idea there and then adapted it to this more realistic scenario.”

Today’s 4G LTE and 5G communications technology works within the fundamentally bounded limit that Shannon defined in the setting of classical physics. The fundamental limit in the quantum setting, however, remains incompletely understood. Despite the new advance described in their latest paper, “We’re not claiming we closed the question,” Jiang said. “There’s much work to be done along this direction.”