Research

We work on developing methods to establish entanglement between remote quantum systems and using that capability to realize new scientific and technological applications. Our remote quantum systems are equivalent to registers of spins, in which entanglement can be stored and processed, that are linked together using photons.  We would like to develop such quantum networks over distances from a few meters, in order to study scalable many-body quantum systems, up to thousands of kilometers, for global quantum communication and sensing networks. Our key research questions are:
 

• How can entanglement be distributed, stored and grown between remote locations?
• What are the most promising applications of distributed entanglement and how can they be realized in the near future?

Cavity-integrated ion-traps

In our lab we focus on developing spin registers encoded into arrays of trapped atomic ions. The singularly-ionized atoms are held in a radio-frequency linear Paul traps. Each atom encodes a quantum spin, or quantum bit, and universal quantum processing can be realized over the spin register using laser-driven interactions. An optical cavity integrated around the trap allows for efficient interfacing of the spins with single photons.  The image in Figure 1 below shows our first cavity-integrated ion-trap, which we refer to as DQS1.

 

Figure 1. Cavity-integrated ion trap known as DQS1. View through the front optical viewport of its vacuum chamber. Vertical gold-coloured metal electrodes form the linear-Paul ion-trap. The two light blue-coloured titanium rings, one either side of the ion trap, each contain a mirror and together form a 20mm-long cavity for collecting 854 nm photons. More details are provided in the theses of Josef Schupp [1] and Vojtech Krcmarsky [2]. This system was used to e.g., demonstrate a repeater node [3], entanglement of remote ions over 230m [4] and a ten-qubit network node [5]. References are listed at the bottom of this page.

 

We are currently developing two more cavity-integrated ion-trap systems, which we refer to as DQS2 and Deployable respectively. The DQS2 system aims to realise a second multi-qubit quantum processing node to investigate applications in distributed quantum computing and sensing. Figure 2 shows the design and construction of DQS2.  The Deployable system is rack-mounted cavity-integrated ion-trap network node being developed in a close collaboration between our group, the Quantum Interfaces group led by Prof. Tracy Northup and the company Alpine Quantum Technologies. The Deployable system aims to be transportable to remote locations and serve as a prototype network node that can function outside of the laboratory environment.

 

Figure 2. Design and construction of the cavity-integrated ion trap known as DQS2. Left: system design, showing central vacuum chamber, assorted vacuum pumps and dual helical resonators.  Top right: Yash and Viktor preparing to perform a leak test of the vacuum chamber and optical viewports. Bottom right: Vacuum chamber with optical viewports and custom feedthrough flange.

Non-linear optics

We develop non-linear optical systems for interconverting the wavelengths of photons from ion-compatible values to the special value of 1550nm. The 1550nm `Telecom C-band’ wavelength is used by the global classical telecommunications industry, suffers minimal loss whilst travelling through optical fibers and is the ideal value for a future quantum communication standard. Our photon conversion systems exploit the difference frequency generation (DFG) processes to convert single photons between the ion-compatible wavelength of 854nm and the telecom C band at 1550nm, using a pump laser at 1901nm. Our primary approach realises DFG in a crystal of periodically-pole lithium niobate (PPLN) with integrated ridge waveguides. More details about our first photon converter systems can be found in the thesis of Martin Meraner [6].

 

Figure 3. Single photon wavelength converter. Centre: a 50mm-long periodically-poled lithium-niobate chip with micron-scale integrated ridge waveguides. Left: a gold-coated parabolic mirror in-couples both ion-emitted single 854nm photons and a 1901 nm laser into the waveguide. Right: an aspehric lens collimates the converted single 1550nm single photons. The system shown converts ion-emitted photons at 854nm to the telecom-C band at 1550nm and is one of two sequential waveguides that was used to obtain ion-photon entanglement over 50km [7], and demonstrate both multi-mode ion-photon entanglement over 101km [8] and a telecom-wavelength quantum repeater node [3]. 

Off-campus quantum network

Distributing entanglement using photons at telecom wavelengths opens up the possibility of building optical-fiber based quantum networks that span cities, countries and even continents. A key next step is to start sending networking photons out of the laboratory environment, into existing optical fiber networks that span cities and investigate the various noise processes introduced in such a`real-world' scenario. In this context, and in close collaboration with the Quantum Interfaces Group, our lab is connected via optical fibers to a remote building in a village outside of Innsbruck. The 17km-long fiber link consists of two fibers rented from the Innsbruck Municipal Company (IKB), who path traverses across the city of Innsbruck before traveling partway up into the Brennerpass before terminating at a building in Gärberbach Mutters. The remote-building is home to the University of Innsbruck's library central depot, where we have installed systems to detect single photons and characetise their polarisation, allowing e.g., for the establish of entanglement between a trapped-ion on campus and a photon detected remotely. Figure 4 below shows a map of the off-campus fiber network. In addition, our lab is connected via optical fiber bundles to several other labs spread out over the Technik campus of the University of Innsbruck, including the labs of the Quantum Interfaces Group and labs at the IQOQI (Institute for Quantum Optics and Quantum Information).

 

Figure 4. Off-campus quantum network. Sattelite image showing the city of Innsbruck at the top and the entrace to the Brenner pass at the bottom. Point A is an ion-trap lab on the Univeristy of Innsbruck's Technik Campus. Point B is an office containing singe photon detectors at the University of Innsbruck's Library Central Depot in a business park, on the outskirts of the village of Mutters. Yellow line shows the path of two co-bundled telecom-wavelength optical fibers rented from the local services company. Red dots show known fiber-fiber connection points. Sattelite image from Google, Landsat/Copernicus.

 

References

[1] Josef Schupp, Interface between trapped-ion qubits and travelling photons with close-to-optimal efficiency, PhD thesis, University of Innsbruck (2022)
[2] Vojtěch Krčmarský, A trapped-ion quantum network over 230 m, PhD thesis, University of Innsbruck (2024)
[3] Victor Krutyanskiy et al "A telecom-wavelength quantum repeater node based on a trapped-ion processor", Phys. Rev. Lett. 130, 213601 (2023)
[4] V. Krutyanskiy, M. Galli et al,  "Entanglement of trapped-ion qubits separated by 230 meters", Phys. Rev. Lett. 130, 050803 (2023)
[5] M. Canteri et al, "A photon-interfaced ten qubit quantum network node", arXiv:2406.09480 (2024)
[6] Martin Meraner, "A photonic quantum interface between trapped ions and the telecom C-band", PhD thesis, University of Innsbruck (2022)
[7] V. Krutyanskiy et al, npj Quantum Information 5, 72 (2019)
[8] V. Krutyanskiy et al, "Multimode Ion-Photon Entanglement over 101 Kilometers", PRX Quantum 5, 020308 (2024)