Quantum computers could soon connect over longer distances

A new study discusses how high-fidelity quantum information could be sent through fiber optic networks by a novel atomic device.
Jijo Malayil
Representational image of a quantum computer in a white room.
Representational image of a quantum computer in a white room.

Bartlomiej Wroblewski/iStock 

Did you know quantum transmissions can't be amplified over a city or an ocean like conventional data signals? Instead, they have to be periodically repeated using specialized devices called quantum repeaters.

For the technology to be used in future communications networks, researchers have developed a novel method of connecting quantum devices over great distances.

Since the repeaters are poised to become crucial in connecting distant quantum computers and enhancing security in communication networks in the future, a team of researchers at Princeton has detailed a new approach to building quantum repeaters in their study published in the journal Nature on August 30.

The idea involves repeaters that transmit light that is telecom-ready thanks to one ion that has been inserted into a crystal. 

Efficient transmission

The visible spectrum, which is emitted by certain other popular quantum repeater systems, degrades fast through optical fiber and needs to be transformed before being sent over great distances.

According to Jeff Thompson, the lead author of the paper, a single rare Earth ion implanted in a host crystal serves as the foundation of the new gadget. Additionally, since this ion produces light at a perfect infrared wavelength, it does not need to convert signals, which can result in networks that are simpler and more reliable.

"The effort was many years in the making. The work combined advances in photonic design and materials science," said Thompson in a press statement

The design of the device has two components, a nanoscopic slice of silicon that has been etched into a J-shaped channel and a calcium tungstate crystal, which is doped with a small number of erbium ions. The ion emits light through the crystal when pulsed by a unique laser.

However, the silicon component, a tiny semiconductor whisp attached to the crystal's tip, traps and directs individual photons into the fiber optic cable.

The team explains that, ideally, the information from the ion would be embedded into this photon. Or, to be more precise, from the ion's spin, a quantum attribute. Entanglement between the spins of distant nodes would be created in a quantum repeater by gathering and interfering with the signals from those nodes, permitting end-to-end transmission of quantum states despite transmission losses.

Thorough testing

The team started off with its work involving erbium ions several years before, but the crystals used in their earlier version yielded significant noise. "This noise caused the frequency of the emitted photons to jump around randomly in a process known as spectral diffusion."

From hundreds of thousands of prospective materials, they whittled the list down to a few hundred, then a dozen, then three. Testing for each of the three finalists took six months. The team zeroed in on calcium tungstate crystal for ideal results.

The team used an interferometer, which merges two or more sources of light to create an interference pattern, to prove that erbium ions in the new material emit indistinguishable photons and that "puts the signal well above the hi-fi threshold."

While this study passes a significant threshold, the team is now working to extend the period that quantum states may be stored in the spin of the erbium ion. The group is now striving to produce calcium tungstate that is more thoroughly purified and has fewer contaminants that interfere with the quantum spin states.

The complete study was published in Nature on August 30 and can be found here.


Atomic defects in the solid state are a key component of quantum repeater networks for long-distance quantum communication1. Recently, there has been significant interest in rare earth ions in particular Er3+ for its telecom band optical transition that allows long-distance transmission in optical fibres. However, the development of repeater nodes based on rare earth ions has been hampered by optical spectral diffusion, precluding indistinguishable single-photon generation. Here, we implant Er3+ into CaWO4, a material that combines a non-polar site symmetry, low decoherence from nuclear spins and is free of background rare earth ions, to realize significantly reduced optical spectral diffusion. For shallow implanted ions coupled to nanophotonic cavities with large Purcell factor, we observe single-scan optical linewidths of 150 kHz and long-term spectral diffusion of 63 kHz, both close to the Purcell-enhanced radiative linewidth of 21 kHz. This enables the observation of Hong–Ou–Mandel interference between successively emitted photons with a visibility of V = 80(4)%, measured after a 36 km delay line. We also observe spin relaxation times T1,s = 3.7 s and T2,s > 200 μs, with the latter limited by paramagnetic impurities in the crystal instead of nuclear spins. This represents a notable step towards the construction of telecom band quantum repeater networks with single Er3+ ions.

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