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“Qutrit” Experiments Are a First in Quantum Teleportation

The proof-of-concept demonstrations herald a major step forward in quantum communications

For the first time, researchers have teleported a qutrit, a tripartite unit of quantum information. The independent results from two teams are an important advance for the field of quantum teleportation, which has long been limited to qubits—units of quantum information akin to the binary “bits” used in classical computing.

These proof-of-concept experiments demonstrate that qutrits, which can carry more information and have greater resistance to noise than qubits, may be used in future quantum networks.

Chinese physicist Guang-Can Guo and his colleagues at the University of Science and Technology of China (USTC) reported their results in a preprint paper on April 28, although that work remains to be published in a peer-reviewed journal. On June 24 the other team, an international collaboration headed by Anton Zeilinger of the Austrian Academy of Sciences and Jian-Wei Pan of USTC, reported its results in a preprint paper that has been accepted for publication in Physical Review Letters. That close timing—as well as the significance of the result—has each team vying for credit and making critiques of the other’s work.

“Each of these [experiments] is an important advance in the technology of teleportation,” says William Wootters, a physicist at Williams College, who was not involved with either study.

BEAM ME UP?
The name quantum teleportation brings to mind a technology out of Star Trek, where “transporters” can “beam” macroscale objects—even living humans—between far-distant points in space. Reality is less glamorous. In quantum teleportation, the states of two entangled particles are what is transported—for instance, the spin of an electron. Even when far apart, entangled particles share a mysterious connection; in the case of two entangled electrons, whatever happens to one’s spin influences that of the other, instantaneously.

“Teleportation” also conjures visions of faster-than-light communication, but that picture is wrong, too. If Alice wants to send Bob a message via quantum teleportation, she has to accompany it with classical information transported via photons—at the speed of light but no faster. So what good is it?

Oddly enough, quantum teleportation may also have important utility for secure communications in the future, and much of the research is funded with cybersecurity applications in mind. In 2017 Pan, Zeilinger and their colleagues used China’s Micius satellite to perform the world’s longest communication experiment, across 7,600 kilometers. Two photons—each acting as a qubit—were beamed to Vienna and China. By taking information about the state of the photons, the researchers in each location were able to effectively construct an unhackable password, which they used to conduct a secure video call. The technique acts like a wax seal on a letter: any eavesdropping would interfere and leave a detectable mark.

Researchers have attempted to teleport more complicated states of particles with some success. In a study published in 2015 Pan and his colleagues managed to teleport two states of a photon: its spin and orbital angular momentum. Still, each of these states was binary—the system was still using qubits. Until now, scientists had never teleported any more complicated state.

MAKING THE IMPOSSIBLE
A classical bit can be a 0 or 1. Its quantum counterpart, a qubit, is often said to be 0 and 1—the superposition of both states. Consider, for instance, a photon, which can exhibit either horizontal or vertical polarization. Such qubits are breezily easy for researchers to construct.

A classical trit can be a 0, 1 or 2—meaning a qutrit must embody the superposition of all three states. This makes qutrits considerably more difficult to make than qubits.

To create their qutrits, both teams used the triple-branching path of a photon, expressed in carefully orchestrated optical systems of lasers, beam splitters and barium borate crystals. One way to think about this arcane arrangement is the famous double-slit experiment, says physicist Chao-Yang Lu, a co-author of the new paper by Pan and Zeilinger’s team. In that classic experiment, a photon goes through two slits at the same time, creating a wavelike interference pattern. Each slit is a state of 0 and 1, because a photon goes through both. Add a third slit for a photon to traverse, and the result is a qutrit—a quantum system defined by the superposition of three states in which a photon’s path effectively encodes information.

Creating a qutrit from a photon was only the opening skirmish in a greater battle. Both teams also had to entangle two qutrits together—no mean feat, because light rarely interacts with itself.

Crucially, they had to confirm the qutrits’ entanglement, also known as the Bell state. Bell states, named after John Stewart Bell, a pioneer of quantum information theory, are the conditions in which particles are maximally entangled. Determining which Bell state qutrits are in is necessary to extract information from them and to prove that they conveyed that information with high fidelity.

What constitutes “fidelity” in this case? Imagine a pair of weighted dice, Wootters says: If Alice has a die that always lands on 3, but after she sends it to Bob, it only lands on 3 half of the time, the fidelity of the system is low—the odds are high it will corrupt the information it transmits. Accurately transmitting a message is important, whether the communication is quantum or not. Here, the teams are in dispute about the fidelity. Guo and his colleagues believe that their Bell state measurement, taken over 10 states, is sufficient for a proof-of-concept experiment. But Zeilinger and Pan’s group contends that Guo’s team failed to measure a sufficient number of Bell states to definitively prove that it has high enough fidelity.

Despite mild sniping, the rivalry between the groups remains relatively friendly, even though provenance for the first quantum teleportation of a qutrit hangs in the balance. Both teams agree that each has teleported a qutrit, and they both have plans to go beyond qutrits: to four level systems—ququarts—or even higher.

Some researchers are less convinced, though. Akira Furusawa, a physicist at the University of Tokyo, says that the method used by the two teams is ill-suited for practical applications because it is slow and inefficient. The researchers acknowledge the criticism but defend their results as a work in progress.

“Science is step by step. First, you make the impossible thing possible,” Lu says. “Then you work to make it more perfect.””

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