arXiv subjects

Stefanie Barz

Publications and source records attributed to Stefanie Barz.

At least 19 recordsLinked to original sources

arXiv2026

Demonstration of a logical Bell-state measurement beyond the linear-optical limit

Fault tolerance is essential for scalable quantum technologies and is enabled by quantum error-correction codes. Bell-state measurements (BSMs) are a fundamental building block for modern quantum technologies such as measurement-based quantum computation and fusion-based quantum computation, as well as quantum networks. Therefore, performing BSMs on error-corrected qubits is a necessary step for achieving fault tolerance in these applications. In this work, we realise a logical BSM using linear optics, based on a two-qubit repetition code, an instance of a quantum parity code that allows detection of bit-flip errors, and experimentally achieve a mean success probability of (70.8 +/- 0.4)%. While standard linear-optical BSMs are fundamentally limited to a maximum success probability of 50%, this increased success probability enables higher secure key rates in quantum communication and facilitates the generation of large graph states for quantum computation. Since fault-tolerant schemes require error-correction codes regardless, this improvement comes at no additional resource overhead. Our results demonstrate that error-correction codes can be used to surpass the linear-optics limit of BSMs, which is an important step towards practical, fault-tolerant, and scalable photonic quantum technologies.

quant-ph↗

arXiv2026

The QTF-Backbone: Proposal for a Nationwide Optical Fibre Backbone in Germany for Quantum Technology and Time and Frequency Metrology

The recent breakthroughs in the distribution of quantum information and high-precision time and frequency (T&F) signals over long-haul optical fibre networks have transformative potential for physically secure communications, resilience of timing infrastructure (such as that supporting Global Navigation Satellite Systems (GNSS)) and fundamental physics. To date, these capabilities remain confined to isolated testbeds, with quantum and T&F signals accessible, for example in Germany, to only a few institutions. In this white paper we propose the QTF Backbone: a dedicated national fibre-optic infrastructure in Germany for the networked distribution of Quantum and T&F signals using dark fibres and specialised hardware. The QTF Backbone is planned as a four-phase deployment over ten years to ensure scalable, sustainable access for research institutions and industry. The concept builds on successful demonstrations of time and frequency distribution at high Technology Readiness Levels (TRLs) across Europe, including PTB-MPQ links in Germany, REFIMEVE in France, and the Italian LIFT network. The QTF Backbone will enable transformative Research and Development (R&D), support a nationwide QTF ecosystem, and ensure the transition from innovation to deployment. As a national and European hub, it will position Germany and Europe at the forefront of quantum networking, as well as T&F transfer.

physics.ins-det↗

arXiv2026

Measurement-Based Quantum Computing on a Photonic Chip

Integrated photonics provides a scalable platform for quantum information processing. In this context, measurement-based quantum computing (MBQC) offers an attractive approach in which quantum computation is realised by adaptive measurements on highly entangled graph states, circumventing the need for deterministic photon-photon interactions. Here, we demonstrate MBQC on an integrated silicon photonic chip capable of generating photonic graph states with up to four qubits. We achieve fidelities of $F_{Star} = (83.5 \pm 1.8)\,\%$ and $F_{Lin} = (75.6 \pm 1.1)\,\%$ for four-photon star and linear graph states, respectively. We use these resource states to implement MBQC-based single- and two-qubit gates and to demonstrate Grover's search algorithm and the Deutsch-Jozsa algorithm. These results establish the feasibility of reconfigurable four-photon MBQC on an integrated photonic platform and provide a foundation for future larger-scale implementations.

quant-ph↗

arXiv2025

Exchange Symmetry in Multiphoton Quantum Interference

Photons are bosons, and yet, when prepared in specific entangled states, they can exhibit non-bosonic behaviour. While this phenomenon has so far been studied in two-photon systems, exchange symmetries and interference effects in multi-photon scenarios remain largely unexplored. In this work, we show that multi-photon states uncover a rich landscape of exchange symmetries. With three photons already, multiple pairwise combinations are possible, where each pair of photons can exhibit either bosonic, fermionic, or anyonic exchange symmetry. This gives rise to mixed symmetry systems that are not possible to achieve with two photon alone. We experimentally investigate how these symmetry configurations manifest themselves in the observed interference of three photons. We show that multi-photon interference can be effectively turned on and off by tuning the symmetry of the constituent pairs. The possibility of accessing and tuning new quantum statistics in a scalable photonic platform not only deepens our understanding of quantum systems, but is also highly relevant for quantum technologies that rely on quantum interference.

quant-ph↗

arXiv2025

Ultracompact Low-Loss Grating Couplers

Fiber-to-chip couplers play a crucial role in interfacing on-chip photonic circuits with other optical systems or off-chip devices. Downsizing the couplers via topology optimization addresses the demand for high-density integration and improves the scalability of photonic integrated systems. However, these optimized couplers have yet to reach the performance level demonstrated by their conventional counterparts, leaving room for further improvement. In this work, we apply topology optimization to design single-polarization 1D and dual-polarization 2D grating couplers incorporating bottom reflectors and achieve sub-decibel coupling efficiency. Both types of couplers are fabricated on the silicon-on-insulator platform with dimensions of mere 14 $μ$m $\times$ 14 $μ$m and are compatible with standard single-mode fibers at normal incidence. From our experimental characterization, the measured peak coupling efficiency of the topology-optimized 1D and 2D couplers is -0.92(1) dB and -0.86(13) dB, respectively, within the telecom C-band. Our demonstration provides a coupling solution for photonic applications requiring high efficiency and high-density integration, such as spatial division multiplexing and photonic quantum technologies.

physics.optics↗

arXiv2025

Integrated high-fidelity preparation and analysis of photonic two-qubit states for quantum network nodes

The realisation of quantum networks requires local quantum information processing at the network nodes and highly efficient transmission of quantum information across the network. Integrated photonics, based on silicon-on-insulator, is a promising platform for quantum network nodes, as it supports low-loss propagation of telecom wavelength photons, making it compatible with existing optical fibre networks. Here, we present a silicon-on-insulator integrated photonic chip, capable of bidirectional operation, enabling the preparation of arbitrary single- and two-qubit states, and performing full quantum state tomography on up to two qubits. Using our chip, we obtain preparation fidelities above 97% for on-chip prepared Bell states coupled into optical fibres. Furthermore, we demonstrate that we can distribute entanglement between network nodes by preparing a two-qubit cluster state on the first node and performing full quantum state tomography on the second node, achieving a fidelity of 90.0(16)%. This result proves that our approach allows the distribution of entanglement from one chip to another. The potential of bidirectional operation makes our circuit a versatile node in telecom quantum networks, both functioning as a sender and receiver unit, a key element for the deployment of fully photonic multi-purpose quantum networks.

quant-ph↗

arXiv2025

Deterministic and highly indistinguishable single photons in the telecom C-band

Quantum dots are promising candidates for deterministic single-photon sources, yet achieving high photon indistinguishability at telecom wavelengths remains a critical challenge. Here, we report a quantum dot-based single-photon source operating in the telecommunications C-band that achieves a raw two-photon interference visibility of up to (91.7+-0.2)%, thus setting a new benchmark for indistinguishability in this spectral range. The device consists of an indium arsenide (InAs) quantum dot embedded within indium aluminum gallium arsenide (InAlGaAs) and integrated into a circular Bragg grating resonator. We explore multiple optical excitation schemes to optimize coherence and source performance. To our knowledge, this is the first demonstration of two-photon interference visibility exceeding 90% from a quantum-dot emitter in the telecommunications C-band, advancing the viability of solid-state sources for quantum communication and photonic networks.

quant-ph↗

arXiv2025

Multiphoton Quantum Interference at Ultracompact Inverse-Designed Multiport Beam Splitter

Photonic quantum technologies enter a new phase when realized in photonic integrated circuits, leading to a great advance in practical applications. In the pursuit of high integration density and low circuit complexity, ultracompact devices delivered by topology optimization offer a promising solution to miniaturize these photonic systems even further. However, their potential for quantum experiments has not yet been fully explored despite the constant development. In this work, we demonstrate multiphoton quantum interference using a topology-optimized tritter with a size of 8.0 $\rmμ$m $\times$ 4.5 $\rmμ$m. We characterize the tritter and reconstruct its transfer matrix by means of single- and two-photon statistics. We also perform heralded three-photon quantum interference with the tritter. The measured four-fold coincidence features a peak with visibility of (-47.9$\pm$ 8.6)%, which is in fair agreement with the prediction of -55.8% estimated from the reconstructed transfer matrix. Our work confirms successful multiphoton quantum interference at an ultracompact interferometer and demonstrates the possibility of utilizing topology-optimized multiport interferometers in various fields of quantum technologies.

quant-ph↗

arXiv2024

Boosted Bell-state measurements for photonic quantum computation

Fault-tolerant fusion-based photonic quantum computing (FBQC) greatly relies on entangling two-photon measurements, called fusions. These fusions can be realized using linear-optical projective Bell-state measurements (BSMs). These linear-optical BSMs are limited to a success probability of 50%, greatly reducing the performance of FBQC schemes. To improve the performance of FBQC architectures, a boosted BSM scheme taking advantage of ancillary entangled photon pairs and a 4x4 multiport interferometer has been proposed. This scheme allows the success probability to be increased up to 75%. In this work, we experimentally demonstrate this boosted BSM by using two Sagnac photon-pair sources and a fibre-based 4x4 multiport beam splitter. A boosted BSM success probability of $(69.3\pm0.3)\%$ has been achieved, exceeding the 50% limit. Furthermore, based on our BSMs, we calculate photon-loss thresholds for a fusion network using encoded six-ring resource states. We show that with this boosted BSM scheme an individual photon loss probability of 1.4% can be tolerated, while the non-boosted BSM leads to a photon-loss threshold of 0.45%.

quant-ph↗

arXiv2024

Hardware-Efficient Preparation of Graph States on Near-Term Quantum Computers

Highly entangled quantum states are an ingredient in numerous applications in quantum computing. However, preparing these highly entangled quantum states on currently available quantum computers at high fidelity is limited by ubiquitous errors. Besides improving the underlying technology of a quantum computer, the scale and fidelity of these entangled states in near-term quantum computers can be improved by specialized compilation methods. In this work, the compilation of quantum circuits for the preparation of highly entangled architecture-specific graph states is addressed by defining and solving a formal model. Our model incorporates information about gate cancellations, gate commutations, and accurate gate timing to determine an optimized graph state preparation circuit. Up to now, these aspects have only been considered independently of each other, typically applied to arbitrary quantum circuits. We quantify the quality of a generated state by performing stabilizer measurements and determining its fidelity. We show that our new method reduces the error when preparing a seven-qubit graph state by 3.5x on average compared to the state-of-the-art Qiskit solution. For a linear eight-qubit graph state, the error is reduced by 6.4x on average. The presented results highlight the ability of our approach to prepare higher fidelity or larger-scale graph states on gate-based quantum computing hardware.

quant-ph↗

arXiv2024

Compact Inversely-Designed Vertical Coupler with Bottom Reflector for Sub-Decibel Fiber-to-Chip Coupling on Silicon-on-Insulator Platform

Inverse design via topology optimization has led to innovations in integrated photonics and offers a promising way for designing high-efficiency on-chip couplers with a minimal footprint. In this work, we exploit topology optimization to design a compact vertical coupler incorporating a bottom reflector, which achieves sub-decibel coupling efficiency on the 220-nm silicon-on-insulator platform. The final design of the vertical coupler yields a predicted coupling efficiency of -0.35 dB at the wavelength of 1550 nm with a footprint of 14 μm x 14 μm, which is considerably smaller than conventional grating couplers. Its topology-optimized geometry can be realized by applying one full etch and one 70-nm shallow-etch process and the fabricability is also guaranteed by a minimum feature size around 150 nm. The proposed vertical coupler can further miniaturize photonic integrated circuits and enable highly-efficient networks between optical fibers and other photonic devices.

physics.optics↗

arXiv2024

Heralding Higher-Dimensional Bell and Greenberger-Horne-Zeilinger States Using Multiport Splitters

One of the most important resources for quantum optical experiments and applications are on-demand highly entangled multiphoton quantum states. A promising way of generating them is heralding entanglement generation at a high rate from letting independent photons interfere. However, such schemes often work for a specific internal degree of freedom of the interfering photons only. Going to higher numbers of entangled photons, the success probabilities decrease while the number of necessary resources, e.g., auxiliary photons and optical elements, increases. To make probabilistic schemes feasible also for larger quantum states, it is therefore important to find resource-efficient generation schemes with high success probabilities. In this work, we introduce easily implementable schemes to herald qubit Greenberger-Horne-Zeilinger (GHZ) states, higher-dimensional Bell states and higher-dimensional three-party GHZ states. Our schemes solely rely on multiphoton interference, i.e., they can be adjusted to work for arbitrary degrees of freedom. Furthermore, they demonstrate high success probabilities and need comparably few auxiliary photons.

quant-ph↗

arXiv2024

Boosted quantum teleportation

Quantum teleportation has proven to be fundamental for many quantum information and communication processes. The core concept can be exploited in many tasks, from the transmission of quantum states, quantum repeaters, to quantum computing. However, for linear-optical systems, the efficiency of teleportation is directly linked to the success probability of the involved Bell-state measurement. In most implementations, this is realized by linear optics with an intrinsically limited success probability of 50%. Here, we demonstrate quantum teleportation surpassing this limit. We achieve an average fidelity of the teleported states of $0.8677\pm0.0024$, leading to an overall acceptance rate of the teleportation of $69.71\pm0.75\%$. We obtain this boosted success probability by generating ancillary photonic states that are interfered with the Bell states. Thus, our work demonstrates the boosting Bell-state measurements in quantum-technology applications and our scheme could directly be applied to e.g. quantum repeaters.

quant-ph↗

arXiv2023

Non-adaptive measurement-based quantum computation on IBM Q

We test the quantumness of IBM's quantum computer IBM Quantum System One in Ehningen, Germany. We generate generalised n-qubit GHZ states and measure Bell inequalities to investigate the n-party entanglement of the GHZ states. The implemented Bell inequalities are derived from non-adaptive measurement-based quantum computation (NMQC), a type of quantum computing that links the successful computation of a non-linear function to the violation of a multipartite Bell-inequality. The goal is to compute a multivariate Boolean function that clearly differentiates non-local correlations from local hidden variables (LHVs). Since it has been shown that LHVs can only compute linear functions, whereas quantum correlations are capable of outputting every possible Boolean function it thus serves as an indicator of multipartite entanglement. Here, we compute various non-linear functions with NMQC on IBM's quantum computer IBM Quantum System One and thereby demonstrate that the presented method can be used to characterize quantum devices. We find a violation for a maximum of seven qubits and compare our results to an existing implementation of NMQC using photons.

quant-ph↗

arXiv2023

Experimental entanglement generation using multiport beam splitters

Multi-photon entanglement plays a central role in optical quantum technologies. One way to entangle two photons is to prepare them in orthogonal internal states, for example, in two polarisations, and then send them through a balanced beam splitter. Post-selecting on the cases where there is one photon in each output port results in a maximally entangled state. This idea can be extended to schemes for the post-selected generation of larger entangled states. Typically, switching between different types of entangled states require different arrangements of beam splitters and so a new experimental setup. Here, we demonstrate a simple and versatile scheme to generate different types of genuine tripartite entangled states with only one experimental setup. We send three photons through a three-port splitter and vary their internal states before post-selecting on certain output distributions. This results in the generation of tripartite W, G and GHZ states. We obtain fidelities of up to $(87.3 \pm 1.1)\%$ with regard to the respective ideal states, confirming a successful generation of genuine tripartite entanglement.

quant-ph↗

arXiv2023

Generating Greenberger-Horne-Zeilinger states using multiport splitters

Symmetric multiport splitters are versatile tools in optical quantum information processing. They can be used for studying multiparticle scattering, studying distinguishability and mixedness, and also for the generation of multipartite entangled quantum states. Here, we show that N-photon N-mode Greenberger-Horne-Zeilinger (GHZ) states can be generated using symmetric multiport beam splitters. Varying the input states' internal degrees of freedom and post-selecting onto certain photon-number distributions allows the probabilistic generation of GHZ states with arbitrary photon numbers. We present two novel schemes, one for odd and one for even numbers of photons, to generate GHZ states using symmetric multiport splitters and compare them to a strategy utilizing a 2N-port network as well as the standard post-selection method.

quant-ph↗

arXiv2022

Bell-state measurement exceeding 50% success probability with linear optics

Bell-state projections serve as a fundamental basis for most quantum communication and computing protocols today. However, with current Bell-state measurement schemes based on linear optics, only two of four Bell states can be identified, which means that the maximum success probability of this vital step cannot exceed $50\%$. Here, we experimentally demonstrate a scheme that amends the original measurement with additional modes in the form of ancillary photons, which leads to a more complex measurement pattern, and ultimately a higher success probability of $62.5\%$. Experimentally, we achieve a success probability of $(57.9 \pm 1.4)\%$, a significant improvement over the conventional scheme. With the possibility of extending the protocol to a larger number of ancillary photons, our work paves the way towards more efficient realisations of quantum technologies based on Bell-state measurements.

quant-ph↗

arXiv2022

Experimental anonymous conference key agreement using linear cluster states

Multipartite entanglement enables secure and anonymous key exchange between multiple parties in a network. In particular Greenberger-Horne-Zeilinger (GHZ) states have been introduced as resource states for anonymous key exchange protocols, in which an anonymous subset of parties within a larger network establishes a secret key. However, the use of other types of multipartite entanglement for such protocols remains relatively unexplored. Here we demonstrate that linear cluster states can serve as a versatile and potentially scalable resource in such applications. We implemented an anonymous key exchange protocol with four photons in a linear cluster state and established a shared key between three parties in our network. We show how to optimize the protocol parameters to account for noise and to maximize the finite key rate under realistic conditions. As cluster states have been established as a flexible resource in quantum computation, we expect that our demonstration provides a first step towards their hybrid use for networked computing and communication.

quant-ph↗