Multipartite entangled states are an essential building block for advanced quantum networking applications. Realizing such tasks in practice puts stringent requirements on the characteristics of the states in terms of fidelity and generation rate, along with a desired compatibility with telecommunication network deployment. Here, we demonstrate a photonic platform design capable of producing high-fidelity Greenberger-Horne-Zeilinger (GHZ) states, at telecom wavelength and in a compact and scalable configuration. Our source relies on spontaneous parametric down-conversion in a layered Sagnac interferometer, which only requires a single nonlinear crystal. This enables the generation of highly indistinguishable photon pairs, followed by entanglement fusion to create four-qubit polarization-entangled GHZ states with fidelity up to (94.73±0.21)% with respect to the ideal state, at a rate of 1.7 Hz. We provide a complete characterization of our source and highlight its suitability for practical quantum network applications.

Quantum transmission links are central elements in essentially all protocols involving the exchange of quantum messages. Emerging progress in quantum technologies involving such links needs to be accompanied by appropriate certification tools. In adversarial scenarios, a certification method can be vulnerable to attacks if too much trust is placed on the underlying system. Here, we propose a protocol in a device-independent framework, which allows for the certification of practical quantum transmission links in scenarios in which minimal assumptions are made about the functioning of the certification setup. In particular, we take unavoidable transmission losses into account by modeling the link as a completely positive trace-decreasing map. We also, crucially, remove the assumption of independent identically distributed samples, which is known to be incompatible with adversarial settings. Particular emphasis is put on a one-sided device-independent scenario, in which the sender possesses trusted resources. Finally, in view of the use of the certified transmitted states for follow-up applications, our protocol moves beyond certification of the channel to allow us to estimate the quality of the transmitted quantum message itself. To illustrate the practical relevance and the feasibility of our protocol with currently available technology, we provide an experimental implementation in the one-sided device-independent setting, based on a state-of-the-art polarization-entangled photon-pair source in a Sagnac configuration, and analyze its robustness for realistic losses and errors.

We present the implementation of a new simulation-secure quantum oblivious transfer protocol based on one-way functions. The protocol allows an efficient and noise-tolerant experimental realization, surpassing prior works' performances in terms of required quantum and classical resources. We provide the complete integration of a software and an experimental source, achieving a black-box implementation of the quantum oblivious transfer primitive, to be leveraged in the future for secure multiparty computation.

Authentication of quantum resources is a critical tool in the development of quantum information processing protocols. In particular, the verification of quantum states is often used as a building block for communication tasks, determining whether the communicating parties can trust the resources at hand to exchange information or whether the protocol should be aborted. Self-testing methods have been used to tackle such verification tasks in a device-independent (DI) scenario. However, these approaches commonly consider the limit of large, identically and independently distributed (IID) samples, which weakens the DI claim and poses serious challenges to their experimental implementation. To address these issues, Gocanin et al. [1] developed a protocol to certify quantum states in the few-copies and non-IID regime. In this work, we adopt their protocol to experimentally demonstrate the device-independent verification of a four-photon GHZ state, produced with our compact and high-fidelity multipartite entangled photon source.

A pair of time-bin entangled states are generated using the spontaneous parametric down-conversion process in two PPLN crystals. One photon from either source interferes on an optical switch. A four-qubit GHZ-state is generated by post-selecting the photons at all the four output ports.

We present a new simulation-secure quantum oblivious transfer (QOT) protocol based on one-way functions in the plain model. With a focus on practical implementation, our protocol surpasses prior works in efficiency, promising feasible experimental realization. We address potential experimental errors and their correction, offering analytical expressions to facilitate the analysis of the required quantum resources. Technically, we achieve simulation security for QOT through an equivocal and relaxed-extractable quantum bit commitment.

With the rapid development and early industrialization of quantum technologies, it is of great interest to analyze their overall energy consumption before planning for their wide-scale deployments. The evaluation of the total energy requirements of quantum networks is a challenging task: different networks require very disparate techniques to create, distribute, manipulate, detect, and process quantum signals. This paper aims to lay the foundations of a framework to model the energy requirements of different quantum technologies and protocols applied to near-term quantum networks. Different figures of merit are discussed and a benchmark on the energy consumption of bipartite and multipartite network protocols is presented. An open-source software to estimate the energy consumption of photonic setups is also provided.

We provide the first proof-of-principle experimental implementation of certified qubit transmission through an untrusted quantum channel, using one part of a maximally-entangled photon as a probe. The theoretical framework includes self-testing in a non-IID and semi device-independent setting, combined with new results on lossy quantum channels.

Quantum transmission links are central elements in essentially all implementations of quantum information protocols. Emerging progress in quantum technologies involving such links needs to be accompanied by appropriate certification tools. In adversarial scenarios, a certification method can be vulnerable to attacks if too much trust is placed on the underlying system. Here, we propose a protocol in a device independent framework, which allows for the certification of practical quantum transmission links in scenarios where minimal assumptions are made about the functioning of the certification setup. We take in particular unavoidable transmission losses into account by modeling the link as a completely-positive trace-decreasing map. We also crucially remove the assumption of independent and identically distributed samples, which is known to be incompatible with adversarial settings. Finally, in view of the use of the certified transmitted states for follow-up applications, our protocol allows to estimate the quality of the state and does not certify the channel only. To illustrate the practical relevance and the feasibility of our protocol with currently available technology we provide an experimental implementation based on a state-of-the-art polarization entangled photon pair source in a Sagnac configuration and analyse its robustness for realistic losses and errors.

All physical systems employed for quantum information tasks must act as unbiased carriers of encoded quantum states. Ensuring such indistinguishability of information carriers is a major challenge in many quantum information applications, including advanced quantum communication protocols. For photons, the workhorses of quantum communication networks, it is difficult to obtain and maintain their indistinguishability because of environment-induced transformations and loss imparted by communication channels, especially in noisy scenarios. Conventional strategies to mitigate these transformations often require hardware or software overhead that is restrictive (e.g., adding noise), infeasible (e.g., on a satellite), or time-consuming for deployed networks. Here we propose and develop resource-efficient Bayesian optimization techniques to rapidly and adaptively calibrate the indistinguishability of individual photons for quantum networks using only information derived from their measurement. To experimentally validate our approach, we demonstrate the optimization of Hong-Ou-Mandel interference between two photons-a central task in quantum networking- finding rapid, efficient, and reliable convergence towards maximal photon indistinguishability in the presence of high loss and shot noise. We expect our resource-optimized and experimentally friendly methodology will allow fast and reliable calibration of indistinguishable quanta, a necessary task in distributed quantum computing, communications, and sensing, as well as for fundamental investigations.

Cristian L. Cortes, Pascal Lefebvre, Nikolai Lauk, Michael J. Davis, Neil Sinclair, 5 Stephen K. Gray, and Daniel Oblak Center for Nanoscale Materials, Argonne National Laboratory, Lemont, Illinois 60439, USA Institute for Quantum Science and Technology, and Department of Physics and Astronomy, University of Calgary, 2500 University Drive NW, Calgary, Alberta T2N 1N4, Canada Division of Physics, Mathematics and Astronomy, and Alliance for Quantum Technologies (AQT), California Institute of Technology, 1200 E. California Boulevard, Pasadena, California 91125, USA Chemical Sciences and Engineering Division, Argonne National Laboratory, Lemont, Illinois 60439, United States John A. Paulson School of Engineering and Applied Sciences, Harvard University, 29 Oxford Street, Cambridge, Massachusetts 02138, USA

Entangled photon pair sources are essential for applications such as quantum communication and metrology. Here we present a compact energy–time entangled photon pair source at telecom wavelengths realized through cascaded second harmonic generation and spontaneous parametric down conversion in a single periodically poled lithium niobate waveguide. We introduce and characterize methods to diminish the effects of Raman scattering, the principal being quasi-CW pumping. The quality of energy–time entanglement produced by the compact source is analyzed using two-photon interference and Franson interference, and visibilities as high as 93.9 % ± 0.4 % and 90.5 % ± 0.6 % are achieved, respectively.

We design and implement an atomic frequency comb quantum memory for 793-nm wavelength photons using a monolithic cavity based on a thulium- (Tm-) doped ${\mathrm{Y}}_{3}{\mathrm{Al}}_{5}{\mathrm{O}}_{12}$ crystal. Approximate impedance matching results in the absorption of 90% of input photons and a memory efficiency of $(27.5\ifmmode\pm\else\textpm\fi{}2.7)%$ over a 500-MHz bandwidth. The cavity enhancement leads to a significant improvement over the previous efficiency in Tm-doped crystals using a quantum memory protocol. In turn, this allows us to store and recall quantum states of light in such a memory. Our results demonstrate progress toward efficient and faithful storage of single-photon qubits with a large time-bandwidth product and multimode capacity for quantum networking.

There are both fundamental and practical motivations for studying whether quantum entanglement can exist in macroscopic systems. However, multiparty entanglement is generally fragile and difficult to quantify. Dicke states are multiparty entangled states where a single excitation is delocalized over many systems. Building on previous work on quantum memories for photons, we create a Dicke state in a solid by storing a single photon in a crystal that contains many large atomic ensembles with distinct resonance frequencies. The photon is re-emitted at a well-defined time due to an interference effect analogous to multi-slit diffraction. We derive a lower bound for the number of entangled ensembles based on the contrast of the interference and the single-photon character of the input, and we experimentally demonstrate entanglement between over two hundred ensembles, each containing a billion atoms. We also illustrate the fact that each individual ensemble contains further entanglement.