Quantum Networks

Technical University of Munich

TUM School of Natural Sciences - Physics Department

James-Franck-Str. 1

85748 Garching


Research Webpage

Typically, fascinating research is either intellectually stimulating (“fundamental science”) or it can lead to applications in everyday life (“applied science”). What I like about quantum networks is that it combines both aspects: On the fundamental side, we seek to answer the question if the maximum size that a system can have without losing its quantum properties is bounded by unknown laws of nature. On the applied side, we aim to build devices for provably secure communication, for the connection of quantum processors to form more powerful quantum computers, and for the sensing of distributed quantities with unprecedented accuracy.


Research focus: quantum networks, quantum information, solid-state quantum optics

Towards a quantum internet

A future quantum network will consist of quantum processors that are connected by quantum channels, just like conventional computers are wired up to form the Internet. In contrast to classical devices, however, the information that can be encoded in a quantum network grows exponentially with the number of nodes, and entanglement of remote particles gives rise to non-local correlations. Exploring these effects facilitates fundamental tests of quantum theory and the quantum-to-classical transition. In addition, quantum networks will enable applications in precision sensing and in distributed quantum information processing, which will fundamentally enhance computational power and ensure unbreakable encryption over global distances.

Pioneering experiments with atomic ensembles, single trapped atoms and solid-state spins have demonstrated the connection and entanglement of two quantum nodes separated by up to 1.3 km. However, accessing the full potential of quantum networks requires scaling of these prototypes to more network nodes and even larger distances. To this end, a new technology that overcomes the bottlenecks of existing physical systems has to be developed.

In this context, we employ the spin of individual rare-earth ions embedded into thin crystals. In contrast to other impurities, rare-earth ions exhibit optical transitions between inner-shell 4f levels. Like in a Faraday cage, the electrons in these levels are largely shielded from the electric fields of the crystal by the outer shell 5s and 5p electrons. When operating at a specific magnetic field, also the spin transition frequencies can be made insensitive to magnetic fluctuations. In this way, the current world record of six hours of quantum coherence time has been achieved (using Eu3+ in Y2SiO5 crystals in M. Sellars’ group, ANU Canberra).

The remaining challenge in the context of quantum networks is that the long-lived optical transitions of rare-earth impurities typically lead to a small fluorescence rate, which makes optical detection and readout challenging. To solve this task, we will embed rare-earth-doped crystals into optical resonators. In this setting, the excited state lifetime can be strongly reduced by the Purcell effect, and the emission of individual ions is efficiently channeled into an optical output mode.

Our main focus lies on the rare-earth element Erbium because it is the only known impurity that has coherent optical transitions within the “telecommunications” wavelength regime. This allows us to reduce the experimental overhead by harnessing existing photonic technologies. In addition, loss in optical fibers is minimal at these frequencies: compared to the visible, the transmission over 100km of optical fiber is improved by about 40 orders of magnitude. This is mandatory to realize quantum networks that span global distances.

Selected Publications

  • Narrow Optical Transitions in Erbium-Implanted Silicon Waveguides. A. Gritsch, L. Weiss, J. Früh, S. Rinner, A. Reiserer, Physical Review X 12, 041009 (2022).
  • Spectral multiplexing of telecom emitters with stable transition frequency. A. Ulanowski, B. Merkel, A. Reiserer, Science Advances 8, eabo4538 (2022).
  • Dynamical Decoupling of Spin Ensembles with Strong Anisotropic Interactions. B. Merkel, P. Cova Fariña, A. Reiserer; Physical Review Letters 127 (2021).
  • Coherent and Purcell-Enhanced Emission from Erbium Dopants in a Cryogenic High-Q Resonator. B. Merkel, A. Ulanowski, A. Reiserer; Physical Review X 10, 041025 (2020).
  • Cavity-enhanced quantum network nodes, A. Reiserer, arXiv:2205.15380, Rev. Mod. Phys. (2022).



Spectral multiplexing of telecom emitters with stable transition frequency

A. Ulanowski, B.Merkel, A. Reiserer.

Science Advances 8, Issue 43 (2022).

Show Abstract

In a quantum network, coherent emitters can be entangled over large distances using photonic channels. In solid-state devices, the required efficient light-emitter interface can be implemented by confining the light in nanophotonic structures. However, fluctuating charges and magnetic moments at the nearby interface then lead to spectral instability of the emitters. Here, we avoid this limitation when enhancing the photon emission up to 70(12)-fold using a Fabry-Perot resonator with an embedded 19-micrometer-thin crystalline membrane, in which we observe around 100 individual erbium emitters. In long-term measurements, they exhibit an exceptional spectral stability of <0.2 megahertz that is limited by the coupling to surrounding nuclear spins. We further implement spectrally multiplexed coherent control and find an optical coherence time of 0.11(1) milliseconds, approaching the lifetime limit of 0.3 milliseconds for the strongest-coupled emitters. Our results constitute an important step toward frequency-multiplexed quantum-network nodes operating directly at a telecommunication wavelength.

DOI: 10.1126/sciadv.abo4538

Narrow optical transitions in erbium-implanted silicon waveguides

A. Gritsch, L. Weiss, J. Früh, S. Rinner, A. Reiserer

Physical Review X 12, 041009 (2022).

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The realization of a scalable architecture for quantum information processing is a major challenge of quantum science. A promising approach is based on emitters in nanostructures that are coupled by light. Here, we show that erbium dopants in high-purity silicon-on-insulator chips combine the ease of low-loss waveguide fabrication with < 1 GHz inhomogeneous broadening, strong optical transitions, and an outstanding optical coherence even at temperatures of 8 K, with an upper bound to the homogeneous linewidth of 20 kHz. Our study thus introduces a promising materials platform for the implementation of on-chip quantum memories, microwave-to-optical conversion, and distributed quantum information processing.

DOI: 10.1103/PhysRevX.12.041009

Cavity-enhanced quantum network nodes

A. Reiserer

arXiv:2205.15380 (2022).

Show Abstract

A future quantum network will consist of quantum processors that are connected by quantum channels, just like conventional computers are wired up to form the Internet. In contrast to classical devices, however, the entanglement and non-local correlations available in a quantum-controlled system facilitate novel fundamental tests of quantum theory. In addition, they enable numerous applications in distributed quantum information processing, quantum communication, and precision measurement.

While pioneering experiments have demonstrated the entanglement of two quantum nodes separated by up to 1.3 km, and three nodes in the same laboratory, accessing the full potential of quantum networks requires scaling of these prototypes to many more nodes and global distances. This is an outstanding challenge, posing high demands on qubit control fidelity, qubit coherence time, and coupling efficiency between stationary and flying qubits.

In this work, I will describe how optical resonators facilitate quantum network nodes that achieve the above-mentioned prerequisites in different physical systems -- trapped atoms, defect centers in wide-bandgap semiconductors, and rare-earth dopants -- by enabling high-fidelity qubit initialization and readout, efficient generation of qubit-photon and remote qubit-qubit entanglement, as well as quantum gates between stationary and flying qubits. These advances open a realistic perspective towards the implementation of global-scale quantum networks in the near future.

DOI: 10.48550/arXiv.2205.15380

Dynamical decoupling of spin ensembles with strong anisotropic interactions

B. Merkel, P. Cova Fariña, A. Reiserer

Physical Review Letters 127, 030501 (2021).

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Ensembles of dopants have widespread applications in quantum technology. The miniaturization of corresponding devices is however hampered by dipolar interactions that reduce the coherence at increased dopant density. We theoretically and experimentally investigate this limitation. We find that dynamical decoupling can alleviate, but not fully eliminate, the decoherence in crystals with strong anisotropic spin-spin interactions that originate from an anisotropic g tensor. Our findings can be generalized to many quantum systems used for quantum sensing, microwave-to-optical conversion, and quantum memory.

DOI: 10.1103/PhysRevLett.127.030501

Coherent Control in the Ground and Optically Excited States of an Ensemble of Erbium Dopants

P. Cova Fariña, B. Merkel, N. Herrera Valencia, P. Yu, A. Ulanowski, and A. Reiserer

Physical Review Applied 15, 64028 (2021).

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Ensembles of erbium dopants can realize quantum memories and frequency converters that operate in the minimal-loss wavelength band of fiber optical communication. Their operation requires the initialization, coherent control, and readout of the electronic spin state. In this work, we use a split-ring microwave resonator to demonstrate such control in both the ground and optically excited state. The presented techniques can also be applied to other combinations of dopant and host and may facilitate the further development of quantum memory protocols and sensing schemes.

DOI: 10.1103/PhysRevApplied.15.064028

Erbium dopants in nanophotonic silicon waveguides

L. Weiss, A. Gritsch, B. Merkel, A. Reiserer

Optica 8, 40–41 (2021).

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We perform resonant spectroscopy of erbium implanted into nanophotonic silicon waveguides, finding 1 GHz inhomogeneous broadening and homogeneous linewidths below 0.1 GHz. Our study thus introduces a promising materials platform for on-chip quantum information processing.

DOI: 10.1364/OPTICA.413330

Laser stabilization to a cryogenic fiber ring resonator

B. Merkel, D. Repp, A. Reiserer

Optics Letters 46, 444-447 (2021).

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The frequency stability of lasers is limited by thermal noise in state-of-the-art frequency references. Further improvement requires operation at cryogenic temperature. In this context, we investigate a fiber-based ring resonator. Our system exhibits a first-order temperature-insensitive point around 3.55K, much lower than that of crystalline silicon. The observed low sensitivity with respect to vibrations (<5⋅10−11m−1s2), temperature (−22(1)⋅10−9K−2), and pressure changes (4.2(2)⋅10−11mbar−2) makes our approach promising for future precision experiments.

DOI: 10.1364/OL.413847

Coherent and Purcell-Enhanced Emission from Erbium Dopants in a Cryogenic High-Q Resonator

B. Merkel, A. Ulanowski, A. Reiserer

Physical Review X 10, 041025 (2020).

Show Abstract

The stability and outstanding coherence of dopants and other atomlike defects in tailored host crystals make them a leading platform for the implementation of distributed quantum information processing and sensing in quantum networks. Albeit the required efficient light-matter coupling can be achieved via the integration into nanoscale resonators, in this approach the proximity of interfaces is detrimental to the coherence of even the least-sensitive emitters. Here, we establish an alternative: By integrating a 19 μm thin crystal into a cryogenic Fabry-Perot resonator with a quality factor of 9×106, we achieve a two-level Purcell factor of 530(50). In our specific system, erbium-doped yttrium orthosilicate, this leads to a 59(6)-fold enhancement of the emission rate with an out-coupling efficiency of 46(8)%. At the same time, we demonstrate that the emitter properties are not degraded in our approach. We thus observe ensemble-averaged optical coherence up to 0.54(1) ms, which exceeds the 0.19(2) ms lifetime of dopants at the cavity field maximum. While our approach is also applicable to other solid-state quantum emitters, such as color centers in diamond, our system emits at the minimal-loss wavelength of optical fibers and thus enables coherent and efficient nodes for long-distance quantum networks.

DOI: 10.1103/PhysRevX.10.041025

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