arXiv subjects

Sören Wengerowsky

Publications and source records attributed to Sören Wengerowsky.

14 recordsLinked to original sources

arXiv2024

Efficient cavity-assisted storage of photonic qubits in a solid-state quantum memory

We report on the high-efficiency storage and retrieval of weak coherent optical pulses and photonic qubits in a cavity-enhanced solid-state quantum memory. By using an atomic frequency comb (AFC) memory in a $Pr^{3+}:Y_2 SO_5$ crystal embedded in a low-finesse impedance-matched cavity, we stored weak coherent pulses at the single photon level with up to 62% efficiency for a pre-determined storage time of 2 $μ$s. We also confirmed that the impedance-matched cavity enhances the efficiency for longer storage times up to 70 $μ$s. Taking advantage of the temporal multimodality of the AFC scheme, we then store weak coherent time-bin qubits with (51+-2)% efficiency and a measurement-device limited fidelity over (94.8+-1.4)% for the retrieved qubits. These results represent the most efficient storage in a single photon level AFC memory and the most efficient qubit storage in a solid-state quantum memory up-to-date.

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arXiv2024

Implementation of space-division multiplexed entanglement-based quantum cryptography over multicore fiber

Quantum communication implementations require efficient and reliable quantum channels. Optical fibers have proven to be an ideal candidate for distributing quantum states. Thus, today's efforts address overcoming issues towards high data transmission and long-distance implementations. Here, we experimentally demonstrate the secret key rate enhancement via space-division multiplexing using a multicore fiber. Our multiplexing technique exploits the momentum correlation of photon pairs generated by spontaneous parametric down-conversion. We distributed polarization-entangled photon pairs into opposite cores within a 19-core multicore fiber. We estimated the secret key rates in a configuration with 6 and 12 cores from the entanglement visibility after transmission through 411 m long multicore fiber.

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arXiv2023

Scalable authentication and optimal flooding in a quantum network

The global interest in quantum networks stems from the security guaranteed by the laws of physics. Deploying quantum networks means facing the challenges of scaling up the physical hardware and, more importantly, of scaling up all other network layers and optimally utilising network resources. Here we consider two related protocols, their experimental demonstrations on an 8-user quantum network test-bed, and discuss their usefulness with the aid of example use cases. First, an authentication transfer protocol to manage a fundamental limitation of quantum communication -- the need for a pre-shared key between every pair of users linked together on the quantum network. By temporarily trusting some intermediary nodes for a short period of time (<35 min in our network), we can generate and distribute these initial authentication keys with a very high level of security. Second, when end users quantify their trust in intermediary nodes, our flooding protocol can be used to improve both end-to-end communication speeds and increase security against malicious nodes.

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arXiv2023

Transmission of light-matter entanglement over a metropolitan network

We report on the transmission of telecom photons entangled with a multimode solid-state quantum memory over a deployed optical fiber in a metropolitan area. Photon pairs were generated through spontaneous parametric down-conversion, with one photon stored in a rare earth-based quantum memory, and the other, at telecommunication wavelengths, traveling through increasing distances of optical fibre, first in the laboratory and then outside in a deployed fibre loop. We measured highly-non-classical correlations between the stored and the telecom photons for storage times up to 25 $μ$s and for a fibre separation up to 50 km. We also report light-matter entanglement with a two-qubit fidelity up to 88$\%$, which remains constant within error bars for all fibre lengths, showing that the telecom qubit does not suffer decoherence during the transmission. Finally, we moved the detection stage of the telecom photons to a different location placed 17 km away, and confirmed the non-classical correlations between the two photons. Our system was adapted to provide the transmission of precise detection times and synchronization signals over long quantum communication channels, providing the first steps for a future quantum network involving quantum memories and non-classical states.

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arXiv2022

Unconditionally secure digital signatures implemented in an 8-user quantum network

The ability to know and verifiably demonstrate the origins of messages can often be as important as encrypting the message itself. Here we present an experimental demonstration of an unconditionally secure digital signature (USS) protocol implemented for the first time, to the best of our knowledge, on a fully connected quantum network without trusted nodes. Our USS protocol is secure against forging, repudiation and messages are transferrable. We show the feasibility of unconditionally secure signatures using only bi-partite entangled states distributed throughout the network and experimentally evaluate the performance of the protocol in real world scenarios with varying message lengths.

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arXiv2022

Certifying position-momentum entanglement at telecommunication wavelengths

TheThe successful employment of high-dimensional quantum correlations and its integration in telecommunication infrastructures is vital in cutting-edge quantum technologies for increasing robustness and key generation rate. Position-momentum Einstein-Podolsky-Rosen (EPR) entanglement of photon pairs are a promising resource of such high-dimensional quantum correlations. Here, we experimentally certify EPR correlations of photon pairs generated by spontaneous parametric down-conversion (SPDC) in a nonlinear crystal with type-0 phase-matching at telecommunication wavelength for the first time. To experimentally observe EPR entanglement, we perform scanning measurements in the near- and far-field planes of the signal and idler modes. We certify EPR correlations with high statistical significance of up to 45 standard deviations. Furthermore, we determine the entanglement of formation of our source to be greater than one, indicating a dimensionality of greater than 2. Operating at telecommunication wavelengths around 1550 nm, our source is compatible with today's deployed telecommunication infrastructure, thus paving the way for integrating sources of high-dimensional entanglement into quantum-communication infrastructures.

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arXiv2020

Experimental implementation of secure anonymous protocols on an eight-user quantum network

Anonymity in networked communication is vital for many privacy-preserving tasks. Secure key distribution alone is insufficient for high-security communications, often knowing who transmits a message to whom and when must also be kept hidden from an adversary. Here we experimentally demonstrate 5 information-theoretically secure anonymity protocols on an 8 user city-wide quantum network using polarisation-entangled photon pairs. At the heart of these protocols is anonymous broadcasting, which is a cryptographic primitive that allows one user to reveal one bit of information while keeping her identity anonymous. For a network of $n$ users, the protocols retain anonymity for the sender, given less than $n-2$ users are dishonest. This is one of the earliest implementations of genuine multi-user cryptographic protocols beyond standard QKD. Our anonymous protocols enhance the functionality of any fully-connected Quantum Key Distribution network without trusted nodes.

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arXiv2020

A trusted-node-free eight-user metropolitan quantum communication network

Quantum communication is rapidly gaining popularity due to its high security and technological maturity. However, most implementations are limited to just two communicating parties (users). Quantum communication networks aim to connect a multitude of users. Here we present a fully connected quantum communication network on a city wide scale without active switching or trusted nodes. We demonstrate simultaneous and secure connections between all 28 pairings of 8 users. Our novel network topology is easily scalable to many users, allows traffic management features and minimises the infrastructure as well as the user hardware needed.

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arXiv2019

Polarization entanglement by time-reversed Hong-Ou-Mandel interference

Sources of entanglement are an enabling resource in quantum technology, and pushing the limits of generation rate and quality of entanglement is a necessary pre-requisite towards practical applications. Here, we present an ultra-bright source of polarization-entangled photon pairs based on time-reversed Hong-Ou-Mandel interference. By superimposing four pair-creation possibilities on a polarization beam splitter, pairs of identical photons are separated into two spatial modes without the usual requirement for wavelength distinguishability or non-collinear emission angles. Our source yields high-fidelity polarization entanglement and high pair-generation rates without any requirement for active interferometric stabilization, which makes it an ideal candidate for a variety of applications, in particular those requiring indistinguishable photons.

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arXiv2019

Passively stable distribution of polarisation entanglement over 192 km of deployed optical fibre

Quantum key distribution (QKD) based on entangled photon pairs holds the potential for repeater-based quantum networks connecting clients over long distance. We demonstrate long-distance entanglement distribution by means of polarisation-entangled photon pairs through two successive deployed 96 km-long telecommunications fibres in the same submarine cable. One photon of each pair was detected directly after the source, while the other travelled the fibre cable in both directions for a total distance of 192 km and attenuation of 48 dB. The observed two-photon Bell state exhibited a fidelity 85% $\pm$ 2% and was stable over several hours. We employed neither active stabilisation of the quantum state nor chromatic dispersion compensation for the fibre.

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arXiv2018

In-field entanglement distribution over a 96 km-long submarine optical fibre

Techniques for the distribution of quantum-secured cryptographic keys have reached a level of maturity allowing them to be implemented in all kinds of environments, away from any form of laboratory infrastructure. Here, we detail the distribution of entanglement between Malta and Sicily over a 96 km-long submarine telecommunications optical fibre cable. We used this standard telecommunications fibre as a quantum channel to distribute polarisation-entangled photons and were able to observe around 257 photon pairs per second, with a polarisation visibility above 90%. Our experiment demonstrates the feasibility of using deployed submarine telecommunications optical fibres as long-distance quantum channels for polarisation-entangled photons. This opens up a plethora of possibilities for future experiments and technological applications using existing infrastructure.

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arXiv2018

Entanglement-based wavelength multiplexed quantum communication network

Quantum networks scale the advantages of quantum communication protocols to more than just two distant users. Here we present a fully connected quantum network architecture in which a single entangled photon source distributes quantum states to a multitude of users. Our network architecture thus minimizes the resources required of each user without sacrificing security or functionality. As no adaptations of the source are required to add users, the network can readily be scaled to a large number of clients, whereby no trust in the provider of the quantum source is required. Unlike previous attempts at multi-user networks, which have been based on active components, and thus limited to some duty cycle, our implementation is fully passive and thus provides the potential for unprecedented quantum communication speeds. We experimentally demonstrate the feasibility of our approach using a single source of bi-partite polarization entanglement which is multiplexed into 12 wavelength channels to distribute 6 states between 4 users in a fully connected graph using only 1 fiber and polarization analysis module per user.

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arXiv2017

Cosmic Bell Test: Measurement Settings from Milky Way Stars

Bell's theorem states that some predictions of quantum mechanics cannot be reproduced by a local-realist theory. That conflict is expressed by Bell's inequality, which is usually derived under the assumption that there are no statistical correlations between the choices of measurement settings and anything else that can causally affect the measurement outcomes. In previous experiments, this "freedom of choice" was addressed by ensuring that selection of measurement settings via conventional "quantum random number generators" was space-like separated from the entangled particle creation. This, however, left open the possibility that an unknown cause affected both the setting choices and measurement outcomes as recently as mere microseconds before each experimental trial. Here we report on a new experimental test of Bell's inequality that, for the first time, uses distant astronomical sources as "cosmic setting generators." In our tests with polarization-entangled photons, measurement settings were chosen using real-time observations of Milky Way stars while simultaneously ensuring locality. Assuming fair sampling for all detected photons, and that each stellar photon's color was set at emission, we observe statistically significant $\gtrsim 7.31 σ$ and $\gtrsim 11.93 σ$ violations of Bell's inequality with estimated $p$-values of $ \lesssim 1.8 \times 10^{-13}$ and $\lesssim 4.0 \times 10^{-33}$, respectively, thereby pushing back by $\sim$600 years the most recent time by which any local-realist influences could have engineered the observed Bell violation.

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arXiv2015

Significant-loophole-free test of Bell's theorem with entangled photons

Local realism is the worldview in which physical properties of objects exist independently of measurement and where physical influences cannot travel faster than the speed of light. Bell's theorem states that this worldview is incompatible with the predictions of quantum mechanics, as is expressed in Bell's inequalities. Previous experiments convincingly supported the quantum predictions. Yet, every experiment requires assumptions that provide loopholes for a local realist explanation. Here we report a Bell test that closes the most significant of these loopholes simultaneously. Using a well-optimized source of entangled photons, rapid setting generation, and highly efficient superconducting detectors, we observe a violation of a Bell inequality with high statistical significance. The purely statistical probability of our results to occur under local realism does not exceed $3.74 \times 10^{-31}$, corresponding to an 11.5 standard deviation effect.

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