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Error-Proof Optical Bell-State Analyzer

Periodic Reporting for period 2 - ErBeStA (Error-Proof Optical Bell-State Analyzer)

Reporting period: 2019-07-01 to 2021-06-30

ErBeStA's overall objective is to make a decisive contribution towards realizing the "quantum internet" by providing a hitherto lacking key component for long-distance quantum communication: an error-proof optical Bell-state-analyzer. Such a device is highly sought after because it is essential for realizing universal optical quantum computers as well as for building efficient quantum repeaters. With the latter, it will become possible to establish secure intercity quantum links and, as a long-term goal, an internet-wide quantum-safe security. This unconditional communication security will be highly beneficial for consumers, enterprises, and governments alike.

The envisioned Bell-state-analyzer directly processes an incoming two-photon-state and is error-proof in the sense that every detection event unambiguously projects the photon state onto a Bell state and imperfections only result in reduced success probability, not in wrong results. ErBeStA goes beyond the proof of concept demonstration and aims at delivering a robust and integrated optical-chip-based device that is directly interfaced with optical fibers. The optical chip is a necessary requirement for the implementation of the optical circuitry that underlies the Bell-state-analyzer and is essential to ensure scalability.

In the course of the project, ErBeStA develops advanced optical chips and quantum-enabled integrated optical devices, such as nondestructive photon detectors, photon-number-resolving detectors, as well as configurable photon-number-specific filters and sorters. All these technologies and devices constitute intermediate scientific breakthrough objectives that also offer profound technological implications.
The ErBeStA project has now completed the first year of its three-year duration. So far, the research effort of the ErBeStA consortium concentrated on providing the prerequisites for the realization of the error-proof Bell-state analyzer. This endeavor led to significant advances in technology development, accompanied by ground-breaking basic research results.

In our quest of giant optical nonlinearities based on strong coupling between quantum emitters and photons in nanophotonic structures, we explored different candidate systems for matter-light interfaces. This included ground state atoms coupled to optical nanofibers or to optical resonators, single molecules on nanofibers, and so-called Rydberg superatoms, i.e. ensembles of interacting atoms that absorb and emit light like one single atom while featuring a collectively enhanced coupling strength. We achieved large coupling and giant nonlinearities in multiple of these systems, thereby paving the way for the next relevant step - the integration into optical chips. We also started exploiting the different matter-light interfaces for applications: We demonstrated a fiber-coupled photon number-dependent router and we are currently working on a photon-number resolving detector.

Towards coupling atoms to chip-guided light, we managed to etch a microscopic transverse hole through a waveguide attached to a glass chip. This allowed us to controllably insert laser-trapped atoms into the waveguide mode and to interface them with the guided light. Using an ultra-sensitive camera, we also imaged and resolved, in real time, up to four atoms that are trapped in a nanofiber-based optical dipole trap and located only 300 nm away from the fiber surface. This result takes cold-atom based nanophotonic systems to a new experimental level. It will allow us to precisely position atoms inside optical waveguide modes, facilitate post-selection, and enable us to implement feedback schemes.

With regard to the fabrication of optical chips that are optimized for the coupling to emitters, we are following two complementary approaches - direct femtosecond laser-writing of waveguides into substrates and a lithography-based process. While the former method was not yet fully operational, lithographically fabricated chips with excellent optical properties were available and, in close collaboration between the consortium members, customized to the experimental needs. With a view to practical applications, we also put significant effort into optimizing the interface between our optical chips and standard optical fibers.

On the theory side, we made significant progress concerning the optimization of the coupling of atoms to waveguides or fibers. In particular, we worked out a theoretical framework that will allow us to optimize light propagation through Rydberg superatoms. For certain applications of Rydberg superatoms, we also found a way to use room temperature ensembles of atoms rather than having to resort to laser cooled atoms. This may enable much more robust and less complex set-ups and devices. Finally, studying the collective chiral coupling and decay of a waveguide-coupled ensemble of atoms, we were able to show that dynamical features of such systems can be probed, characterized, and classified by monitoring the record of photons emitted into the waveguide modes.
Quantum technology is now unfolding worldwide and brings transformative advances to science, industry, and society. With the realization of an error-proof Bell-state analyzer, ErBeStA contributes to this development in the area of quantum communication and computation. Furthermore, the developed Bell-state analyzer provides a general means of distributing entanglement.

Sharing entanglement between remote quantum systems will enable, for example, high-precision sensing, more precise atomic clocks, and quantum (cloud) computing. Possible applications in these areas go far beyond the state-of-the-art in science and technology and will provide significant societal benefits. These include low-frequency gravitational wave detection, the exploration of natural resources, improved precision in geodesy and navigation services such as GPS or Galileo, and progress in computationally hard problems such as computational drug development or climate simulations.

Several achieved and anticipated breakthroughs of ErBeStA will trigger new lines of scientific and/or technological research and applications. For example, non-destructive photon detection is widely considered a disruptive technology, which will, e.g. enable the implementation of practical photonic quantum gates, the step-by-step engineered generation of photonic cluster states, as well as scalable photonic quantum computation. As another example, the optical chips developed by ErBeStA allow one to interface various types of optical emitters - such as atoms, (bio-) molecules, quantum emitters in nanocrystals, quantum dots, and plasmonic nanoparticles - with complex optical circuits. Thus, they represent an enabling technology for fundamental quantum science research as well as applications in chemistry and biology.
Picture of Quantum Kate explaining entanglement
Photo of the fiber-based system used for observing an ultra-strong optical response
Group photo of the ErBeStA team, taken on the occasion of the kick-off meeting 2018.