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Creating quantum entanglement from scratch can be tricky business, so researchers are exploring ways that entangled pairs can “share” these states amongst themselves.
A new study shows how an entangled pair can pass on their entangled state to another pair by using particle interactions.
Theoretically, this sharing could be done indefinitely, but eventually, not enough entanglement is shared to be particularly useful.
Superposition and entanglement together form the foundation of quantum computing. The former describes how qubits—the building blocks of quantum computers—can be in multiple states (in this case, 0 or 1) until measured, and the latter links qubits together so that they share the same fate, no matter the distance between them. Being able to form entanglement between particles is crucial, as this is what allows the separate qubits to function as a system.
Creating entangled pairs is an inherently fragile process, so scientists from the Harish-Chandra Research Institute in India and the Université libre de Bruxelles in Belgium wondered if there was another way to create entangled pairs rather than just from scratch. In a study published in the journal Physical Review A, the researchers walk through how an entangled qubit pair named Alice and Bob (the common couple name used for quantum thought experiments) can actually share their entangled state with a second quantum pair (named Charu and Debu). In fact, the entangled state can be transferred to an indefinite number of quantum pairs.
“We find joint unitaries which, when applied by Alice and a Charu, and by Bob and the corresponding Debu, can transfer entanglement from the Alice-Bob pair to an indefinite number of pairs of Charus and Debus,” the authors wrote. “Given the valuable role played by the costly resource, entanglement, in quantum information processing and communication, we expect that our results will have significant impact in quantum technologies.”
This means that while Charu and Debu can’t form an entangled pair on their own, they can tap into the same “entanglement bank” with particles that communicated with the other entangled pair—Alice to Charu, and Bob to Debu.
“We thought of a scenario where someone, like money or sweets, has a lot of it and is willing to share it with children or subordinates or just some others,” Harish-Chandra Research Institute’s Ujjwal Sen told New Scientist.
This metaphor is an apt one, as the original entangled pair isn’t keen to give away all of its “sweets”—or, in this case, its entangled state. When entanglement passes to Charu and Debu, the pair receives a slightly smaller amount. This means that while this entanglement sharing could theoretically go on forever, it does eventually stop, because the amount of entanglement is no longer useful.
While quantum computers have made slow-yet-steady progress in the last decade, their computational promise still remains out of reach—largely due to the finicky nature of qubits, as noise (thermal or otherwise) causes them to decohere. Any process that can increase methods of entanglement, which can improve a quantum computer’s error correction, is a welcomed one.
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