Caltech researchers have demonstrated a four-way quantum memory, successfully entangling the spins of four caesium atoms stored close to absolute zero in a magnetic grid. They succeeded in storing information, and reading it out again.
Clearly, the conditions are somewhat sub-optimal for immediate deployment in the real world, but the experiment was an important proof of principle, the researchers say.
"Our work introduces new sets of experimental capabilities to generate, store, and transfer multipartite entanglement from matter to light in quantum networks," explained Caltech graduate student Kyung Soo Choi, the lead author of the Nature paper. "It signifies the ever-increasing degree of exquisite quantum control to study and manipulate entangled states of matter and light."
The research group has transferred information in and out of entangled memories before, but never with four particles.
From the release: For such two-component—or bipartite—entanglement, the subsystems are either entangled or not. But for multi-component entanglement with more than two subsystems—or multipartite entanglement—there are many possible ways to entangle the subsystems. For example, with four subsystems, all of the possible pair combinations could be bipartite entangled but not be entangled over all four components; alternatively, they could share a "global" quadripartite (four-part) entanglement.
Hence, multipartite entanglement is accompanied by increased complexity in the system. While this makes the creation and characterization of these quantum states substantially more difficult, it also makes the entangled states more valuable for tasks in quantum information science.
The research is published in the November 18th edition of Nature, and you can browse the Caltech press release here.