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Error-Proof Optical Bell-State Analyzer

Periodic Reporting for period 3 - ErBeStA (Error-Proof Optical Bell-State Analyzer)

Reporting period: 2021-07-01 to 2022-06-30

ErBeStA's overall objective was to make a decisive contribution towards realizing the "quantum internet". A key component for realizing universal optical quantum computers as well as for building efficient quantum repeaters is an error-proof optical Bell-state analyser. This device can measure the state of a pair of photons in an entangled state basis.

Bell-state measurements require an optical non-linearity at the level of single photons, achieved by the strong coupling of light to individual single photon emitters. Three different systems, paired with nanophotonics, were used: cold atoms, Rydberg superatoms and solid-state quantum emitters.

ErBeStA gave rise to the development of novel and improved types of quantum light sources, novel non-reciprocal optical elements, novel photon number-resolving photodetectors, and to the study and control of novel single-photon emitters in 2D materials. Further it led to the fabrication of nano-sized waveguides for strong light-matter interactions and to portable quantum technologies, which are crucial for miniaturization and scalability.
In ErBeStA, we did show that for media composed of saturable emitters, the transmitted light also exhibits a modified photon statistics. This experimental breakthrough, challenges the established picture (Beer-Lambert law) of something as fundamental as the transmission of light and will permit the development of novel single photon sources.
A breakthrough was made concerning non-reciprocal optical elements., which treat light differently when propagating forwards or backwards, thereby enabling, e.g. diodes and circulators for light. The established ways to break reciprocity are overcome and a non-reciprocal device is experimentally demonstrated that is controlled by the atomic spin. The amplifier for light is implemented by using cold atoms coupled to light in a nanophotonic waveguide, which is essential for photonic quantum networks and single photon detection. By reducing the size of the ensemble, such that only a single photon can be absorbed collectively, due to a Rydberg blockade, a strongly coupled two-level system, a so-called Rydberg superatom is realized. They are studied as photon-detectors, by absorbing exactly one photon per ensemble. The scaling of such a detector up to 3 photons, by chaining three independent superatoms, was demonstrated. As a follow up, we have theoretically analyzed the further scaling of this scheme.
In a collaboration with the University of Cambridge, we have shown that large arrays of solid-state quantum emitters in 2D materials that are well localized in all three dimensions can be fabricated and have made progress in determining the origin of these emitters.
Semiconductor processing technology was used to produce silica on silicon waveguides suspended in a membrane, which will permit repetitive manufacturing. Waveguides were manufactured that could be interrupted on distances up to 20 µm and still transmit up to 60% of the incoupled light at the facets. To bring waveguides laser written in glass blocks close to the surface, excessive glass was removed by wafer thinning technology. A new dicing technology was developed, to cut a spot in the laser written waveguide for probing photonic interactions. These waveguides allow for new ways of exploring light matter interactions on a fundamental level. Single photon sources are a fundamental ingredient of quantum networks and we have developed and optimized a narrow-band photon pair source. The emitted four-photons were used as a base for the probabilistic generation of narrow-band photonic Bell states, in addition to other potential entangled states such as Dicke states that are interesting for quantum information processing.
Within ErBeStA, we have theoretically investigated the dynamics of atomic emitters coupled to a waveguide. In the single-excitation regime we have shown that the collective nature of the atom-light coupling can be used to enhances the chiral, i.e. directional, emission of photons. This is of importance for developing an efficient interface between atoms and waveguide structures with unidirectional coupling, with applications in quantum computing and communication such as the development of nonreciprocal photon devices or quantum information transfer channels. In the many-body regime, we demonstrated that a laser-driven atom-waveguide system features a phase transition from a stationary to a non-stationary phase. In the stationary phase one obtains a steady-state that is squeezed, while the non-stationary phase can be interpreted as a dissipative time-crystal.
In the project, a range of studies of light propagation and nonlinearities in Rydberg ensembles and waveguide QED settings were deployed, which include: (i) investigations of emerging photon interactions in chains of waveguide-coupled three-level atoms, (ii) collective and motional effects on light propagation in dense three-level media, and (iii) photon-photon interactions due to dipole-dipole and van der Waals interactions between Rydberg states. A superatom ensembles can be used to perform photon-sorting operations and the unidirectional nature of the ensemble-photon interaction, will allow a Bell state analyser without the need of optical circulators.
Superatoms and two-dimensional atomic arrays do achieve deterministic photon sorting and Bell-state analysis, which currently presents the only deterministic method that can be applied in passive operation.
In total, ErBeStA has resulted in 63 peer-reviewed publications, 8 preprints and 3 patents (RP3: 14 publications and 8 preprints).
The realization of an error-proof Bell-state analyzer, ErBeStA contributes to this development in the area of quantum communication and computation.
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, the new types of photon sources developed allow the generation of single photons for a wide frequency range and the new types of non-reciprocal elements will enable applications such as spin-controlled directional lasers and potentially open new synergies between the fields of photonics and spintronics. As another example, the optical chips developed allow 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.