Quantum Repeater Node Development

Advanced trapped-ion technologies for quantum networks

Photon-mediated entanglement generation between remote trapped-ion qubits

Entanglement is the fundamental building blocks of quantum networks. Trapped ions have excellent coherence properties for quantum information processing. The QUANT-NET research team utilizes this feature for storing entanglement between distant matter systems relying on long-lived optical qubit transitions of Calcium ions.

Meanwhile, high-rate and high-fidelity two-photon BSM is necessary for distributing entanglement across remote matter platforms and it requires complete indistinguishability between the interfering photons emerging from the distant matter nodes. In this sense, trapped ions are perfect candidates for high-fidelity BSM as they are exactly identical, guaranteeing the spectral indistinguishability of generated photons.

Cavity-enhanced ion-photon interface for high-rate entanglement generation 

The recent advances have shown the possibility of reaching high-rate remote entanglement with ion-cavity interfaces, allowing near unity photon extraction efficiencies. The QUANT-NET research team is advancing this technology further by implementing a novel ion-cavity interaction scheme, which brings two advantages: (1) high photon collection efficiency; and (2) enhanced ion-photon coupling. Both advantages facilitate high-rate entanglement generation.

Figure 3: Advanced ion-cavity interaction scheme

Cavity-mediated Raman scheme for high-fidelity entanglement generation

Innsbruck University researchers have developed a highly-efficient quantum node using Calcium-40 ions. Their system features near deterministic photon generation allowing for close to optimal performance mostly limited by fundamental laws of physics. However, one drawback of the current scheme is that off-resonant excitation causes time jitter, decreasing the fidelity by about 0.1. While fidelities below 0.9 are acceptable for fundamental physics demonstrations such as violation of Bell inequalities, higher fidelites of order 0.99 are highly desirable for most quantum information applications. In current networking experiments at Innsbruck that demonstrate entanglement over 510 m, the loss of fidelity due to the time jitter was mitigated by using a short coincidence interval, at the cost of reduction in the entanglement rate by more than two-orders of magnitude. 

The QUANT-NET research team is tackling this problem by switching to a scheme where the Calcium ion is excited from the D3/2 state instead of from the S1/2 state. This scheme is expected to reduce re-excitation of the ion from 10% to 0.1% thereby nearly mitigating the fidelity loss due to the time jitter.

Figure 4: New Raman excitation scheme

Scalability and integration

Scalable and integrated devices are one of the key ingredients for practical applications. The quant-net research team is developing novel miniaturized ion-trap technologies that combine quantum networking capabilities with scalable quantum processors

Inherent support of quantum computing and quantum networking

One of the “killer” applications of quantum communication is distributed quantum computing ,which may also overcome the scalability problem of the existing quantum computing technologies. In this regard, trapped ions are one of the best quantum computing platforms while being inherently suitable for quantum communication. Incorporating the networking capability to the existing trapped ion processors makes these systems native candidates for distributed quantum computers. The QUANT-NET research team is exploring this avenue by developing networked and scalable quantum processors, and demonstrating the basic ingredients of distributed quantum information processing. 

High-efficiency and low-noise quantum frequency conversion

To support the low-loss transmission of photons in fiber infrastructure, the QUANT-NET research team is developing a high-efficiency quantum frequency conversion module for telecom operations based on the difference frequency process using the established integrated quantum photonic technologies. In this approach, an input 854 nm-photon (emitted from a Ca+ ion) is coherently mixed with a strong pump field at 1900 nm in a non-linear medium leading to an output telecom-photon at 1550 nm. In conjunction with a proper arrangement of the crystal geometry and filtering, this QFC will preserve the quantum states of the input photons while providing high conversion efficiencies and low-noise.

High-rate single photon sources 

The QUANT-NET research team is developing a telecom-wavelength single-photon source from an artificial atom in silicon nanophotonics. Future improvement of this source will allow an interface for quantum teleportation from a single photon to a trapped-ion qubit.