Record-long storage of entangled telecom-wavelength photons
Image credit: Group of Professor Xiao-Song Ma at Nanjing University
A new paper in Nature Communications has reported record-long storage of entangled photons at telecom wavelengths on a platform that can be deployed in extended networks, an achievement that the researchers say could pave the way for practical large-scale quantum networks.
The physical fabric of the Internet is woven from optical fibres. The glass fibres that make up these vast networks are famously pure. A common example is that you could see clearly through a kilometre-thick window made of such glass. Nonetheless, some losses are unavoidable, and the optical signals that travel through telecommunications networks need to be 'refreshed' at regular intervals once distances exceed a few hundreds of kilometres. For classical signals, there exist well-established and routinely used techniques based on repeated signal amplification.
However, these amplification processes are not suitable for quantum states of light, because they would cause the entangled photons to lose their quantum correlations – a key ingredient that makes quantum technologies so powerful.
One solution is to use so-called quantum repeaters. In a nutshell, quantum repeaters store the fragile entangled state and transform it into another quantum state that shares entanglement with the next node down the line. In other words, instead of amplifying the signal, the nodes are 'stitched together', exploiting their unique quantum properties. At the heart of such quantum-repeater networks are quantum memories in which quantum states of light can be stored. Realising these memories with a sufficiently long storage time is an outstanding challenge, especially for photons at telecom wavelengths.
However, researchers in the group of Xiao-Song Ma at Nanjing University have now reported storage and retrieval of the entangled state of two telecom photons with a storage time of close to two microseconds. The team says that this is almost 400 times longer than what had been demonstrated before in this field and therefore is a decisive step towards practical devices.
The memories, developed by Ming-Hao Jiang, Wenyi Xue and their colleagues, are based on yttrium orthosilicate (Y2SiO5) crystals doped with erbium ions. These ions have optical properties that are almost perfect for use in existing fibre networks, matching the wavelength of around 1.5 μm. The suitability of erbium ions for quantum storage has been known for some years, and the researchers say that embedding them in a crystal makes them particularly attractive with a view to large-scale applications. However, practical implementations of erbium-ion-based quantum memories have proved relatively inefficient so far, hindering further progress towards quantum repeaters.
According to the paper, Ma's group has now made significant advances in perfecting the techniques and has shown that even after storing the photon for 1936 ns, the entanglement of the photon pair is preserved. This means that the quantum state can be manipulated during this time, as is required in a quantum repeater.
The researchers added that they also combined their quantum memory with a novel source of entangled photons on an integrated chip, demonstrating the ability to both generate high-quality entangled photons at telecom frequencies and store the entangled state, all on a solid-state platform suitable for low-cost mass production. This could be a promising building block that might, together with existing large-scale fibre networks, enable a future quantum internet.
