A step closer to repeaters for quantum networks

Loss of photons over long-distance connections limits the development of quantum networks, necessitating the use of quantum ‘repeater’ systems to boost signals between network nodes. 

Researchers have a new way to connect quantum devices over long distances, a necessary step toward allowing the technology to play a role in future communications systems.  Erbium ions incorporated into calcium tungstate crystals have been found to emit photons in the telecommunications frequency band that are indistinguishable from each other, and thus show promise for use in such repeaters.

The study "Indistinguishable telecom band photons from a single Er ion in the solid state" details the basis for a new approach to building quantum Repeaters.

Quantum Repeaters can overcome transmission losses

Long-distance quantum networks have many applications in quantum information science, including ultra-secure communication, modular quantum computing, and distributed quantum sensing. Such networks operate by distributing entanglement (correlations between quantum memory states) between quantum particles at distant nodes. Nodes can be connected by sending photons. However, photons are lost over long distances; Even at the optimum wavelength of the fiber (1.5 microns), half of the photons are lost every 20 kilometers.

This loss can be overcome with an intermediate network node called a quantum repeater, which stores entanglement using long-lived quantum memory on each node.

Repeaters are manufactured using atoms or atom-like systems whose optical transitions (changes in their energy state that result in the emission of a single photon) are connected to internal degrees of freedom (such as spin angular momentum) and can be used as quantum memory. In this way, they can produce single photons entangled with quantum memory. A pair of distant memories can be entangled by taking joint measurements of the photons they emit. This measurement requires that the two photons are indistinguishable: not affected by noise or random fluctuations, which weaken entanglement during decoherence.

Emit almost indistinguishable photons at telecommunication band frequencies

However, very little atomic energy naturally emits photons of the optimal wavelength. Previous work had considered erbium ions (Er³⁺), which emit photons with a wavelength of 1.5 µm. These ions can be added to crystals as crystal defects and individually controlled using nanophotonics devices, but so far noise in these devices has hindered the emission of undistinguishable photons.

The amount of decoherence experienced by an atomic defect in a solid-state crystal depends on the size of the noise (generated by other spins, charges, and lattice vibrations in the main crystal) and the sensitivity of the defect to the noise. A particular advantage of rare earth ions, such as Er³⁺, is that they can be incorporated into a variety of host materials, thus providing space for the design of these determinants. Therefore, the experimental team found a host material with two properties that the traditional host crystals of rare earth ions do not possess: low nuclear spin abundance (to reduce magnetic noise) and non-polar symmetry of the Er³⁺ site (to reduce charge sensitivity); It is also expected that suppressing the sensitivity to charge noise is particularly important for Er³⁺ ions near the surface and interface of nanophotonics.

After exploring various implantable Er³⁺ ionic materials, the team finally chose calcium tungstate (CaWO₄). Er³⁺ ions are implanted in non-polar sites of this material with a low nuclear spin abundance and therefore a long spin coherence time (the available time before decoherence occurs).

In the experiment, the scientists glued silicon nanophotonics to the top surface of the crystal to enhance and efficiently collect photons emitted by shallowly embedded Er³⁺ ions. When excited by the laser, the Er³⁺ ions produced a light emission peak with a linewidth of 150 kilohertz - the narrowest linewidth ever recorded for solid-state defects in nanophotonic devices, and a 100-fold improvement over shallow Er³⁺ ions in other host materials. This allowed the team to emit almost indistinguishable photons at telecommunications band frequencies, which they confirmed using a phenomenon called "Hong-Ou-Mandel interference."

Er³⁺:CaWO₄ device structure.

Indistinguishable photon source. Photons can be used to encode quantum information in quantum networks. In such networks, the effects of photon loss can be mitigated by devices called quantum Repeaters. Quantum Repeaters can be based on rare earth metal ions, such as erbium ions (Er³⁺), and incorporated into nanophotonic devices. a) Here, Er³⁺ ions are implanted in a shallow layer of calcium tungstate (CaWO₄ ), and silicon nanophotonic resonators are bonded to the top surface of the crystal to enhance the collection of photons emitted by Er³⁺ ions. b) The emission of inseparable photons is characterized by the "Hong-Ehr-Mandel effect" (HOM). This suppresses the probability that two photons (HOM coincidence) will be detected at the same time with zero delay (t1-t2 = 0) in the output of the splitter.

By observing the strong suppression of individual photons in the interferometer output (up to 80%), the team finally demonstrated that erbium ions in the new material emit photons that are difficult to distinguish. Salim Ourari, a graduate student who co-led the study, said this puts the signal well above the high-fidelity threshold.

Further extension of the spin coherence time is required

While this work crosses an important threshold, additional work is needed to improve the storage time of quantum states in the spin of erbium ions.

Spin mechanics

The high inseparability of Er³⁺ emitted photons in CaWO₄ and the long spin coherence time create conditions for subsequent experiments investigating spin-photon and spin-spin entanglement. The team said that at present, the laboratory is conducting these related experiments.

However, to achieve a truly long-distance, multi-node quantum relay network, further extension of the spin coherence time is required: this can be achieved by increasing the purity of the bulk CaWO₄ material or the surface of the nanophotonics, or by lowering the operating temperature to "freeze" the noise.

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