A quantum version of a random access memory can read and write information 1000 times, and could eventually become a key component in long-distance quantum networks.
In conventional computers, random access memory (RAM) is essential for short-term information storage. Random access quantum memory (RAQM) is similar, and the expectation is that it will be vital for the smooth running of an unhackable quantum Internet connecting cities. This is because quantum information degrades easily as it travels 鈥� adding RAQMs at regular intervals along the network connection between two cities could allow network users to create backups and salvage failed information transmissions of quantum data.
at Tsinghua University in China and his colleagues have now built a RAQM that is a step towards such technology. It can operate effectively even if information is written into or read out of it 1000 times, making it far more stable than any RAQMs built before.
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The researchers built the RAQM using around a billion extremely cold rubidium atoms confined in a small, airless chamber. They cooled the atoms to a few hundred millionths of a degree above absolute zero by hitting them with lasers. At this temperature, the atoms鈥� quantum states and behaviour could be very precisely controlled with lasers and magnetic fields. The researchers then arranged the atoms into 144 groups, each containing a few million atoms.
Saving a piece of information into the memory involved adding a quantum bit, or qubit, the basic unit of information in quantum computing. The team first encoded that qubit into a single particle of light 鈥� a photon 鈥� and then shone it onto the atoms. This action changed the quantum properties of two of the 144 groups of atoms, and meant that the qubit was effectively saved 鈥� or written into the RAQM.
To read it out again, the researchers shone a specially tuned laser onto the RAQM, which made the groups of atoms encoding the qubit emit a particle of light in a quantum state that matched the saved information. This light particle could then be sent to another device, like a quantum computer, through an optical fibre.
The team can store 72 qubits in its RAQM at any one time, and the device stores them for 0.5 milliseconds. By tuning the light that they used to control the RAQM鈥檚 atoms, the researchers could also program the memory to store and release information in multiple different sequences, just as is possible with memories in conventional computing.
鈥淥ur memory can function in a nearly 鈥榰niversal鈥� style, which means there are almost no limitations in the way to use it, and it can support multiple different tasks depending on the demand,鈥� says Pu.
He says that previous attempts to build RAQM devices have been stymied by the fact that they could only support about a dozen consecutive read and write operations before the information would start to degrade, but the new device vastly outperforms them because it can withstand up to 1000 such operations.
at the Max Planck Institute of Quantum Optics in Germany says this is an 鈥渋mpressively large number鈥� of operations that could make the memory a useful component for quantum communication networks, including those that would span up to 100 kilometres.
Going forward, the researchers want to make their memory last longer so that it could support even wider-ranging quantum networks in the future, and they want to make it more efficient, says Pu.
Journal reference:
Physical Review X,
