Entanglement-based Quantum Key Distribution Research Articles

High-rate multiplexed entanglement source based on time-bin qubits for advanced quantum networks

Entanglement distribution based on time-bin qubits is an attractive option for emerging quantum networks. We demonstrate a 4.09-GHz repetition rate source of photon pairs entangled across early and late time bins separated by 80 ps. Simultaneous high rates and high visibilities are achieved through frequency multiplexing the spontaneous parametric down conversion output into eight time-bin entangled channel pairs. We demonstrate entanglement visibilities as high as 99.4%, total entanglement rates up to 3.55×106 coincidences/s, and predict a straightforward path towards achieving up to an order of magnitude improvement in rates without compromising visibility. Finally, we resolve the density matrices of the entangled states for each multiplexed channel and express distillable entanglement rates in ebit/s, thereby quantifying the trade-off between visibility and coincidence rates that contributes to useful entanglement distribution. This source is a fundamental building block for high-rate entanglement-based quantum key distribution systems or advanced quantum networks.

Transformation of Bell states using linear optics

Bell states form a complete set of four maximally polarization entangled two-qubit quantum state. Being a key ingredient of many quantum applications such as entanglement based quantum key distribution protocols, superdense coding, quantum teleportation, entanglement swapping etc, Bell states have to be prepared and measured. Spontaneous parametric down-conversion (SPDC) is the easiest way of preparing Bell states and a desired Bell state can be prepared from any entangled photon pair through single-qubit logic gates. In this paper, we present the generation of complete set of Bell states, only by applying unitary transformations of half-wave plate (HWP) on the initial Bell state. The initial Bell state is prepared using a combination of a nonlinear crystal and a beam-splitter (BS) and the rest of the Bell states are created by applying single-qubit logic gates on the entangled photon pairs using HWPs. Our results can be useful in many quantum applications, especially in superdense coding where control over basis of maximally entangled state is required.

Robustness of entanglement-based discrete- and continuous-variable quantum key distribution against channel noise

Discrete-variable (DV) and continuous-variable (CV) schemes constitute the two major families of quantum key distribution (QKD) protocols. Unfortunately, since the setup elements required by these schemes are quite different, making a fair comparison of their potential performance in particular applications is often troublesome, limiting the experimenters’ capability to choose an optimal solution. In this work we perform a general comparison of the major entanglement-based DV and CV QKD protocols in terms of their resistance to the channel noise, with the otherwise perfect setup, showing the definite superiority of the DV family. We analytically derive fundamental bounds on the tolerable channel noise and attenuation for entanglement-based CV QKD protocols. We also investigate the influence of DV QKD setup imperfections on the obtained results in order to determine benchmarks for the parameters of realistic photon sources and detectors, allowing the realistic DV protocols to outperform even the ideal CV QKD analogs. Our results indicate the realistic advantage of DV entanglement-based schemes over their CV counterparts and suggests the practical efforts for maximizing this advantage.

High-dimensional quantum key distribution using energy-time entanglement over 242 km partially deployed fiber

Entanglement-based quantum key distribution (QKD) is an essential ingredient in quantum communication, owing to the property of source-independent security and the potential on constructing large-scale quantum communication networks. However, implementation of entanglement-based QKD over long-distance optical fiber links is still challenging, especially over deployed fibers. In this work, we report an experimental QKD using energy-time entangled photon pairs that transmit over optical fibers of 242 km (including a section of 19 km deployed fibers). The QKD is realized through the protocol of dispersive-optics QKD (DO-QKD) with high-dimensional encoding to utilize coincidence counts more efficiently. A reliable, high-accuracy time synchronization technology for long-distance entanglement-based QKD is developed based on the distribution of optical pulses in quantum channels. Our system operates continuously for more than 7 d without active polarization or phase calibration. We ultimately generate secure keys with secure key rates of 0.22 bps and 0.06 bps in the asymptotic and finite-size regimes, respectively. It shows that entanglement-based DO-QKD is reliable for long-distance realization in the field if its high requirement on time synchronization is satisfied.

Operational entanglement-based quantum key distribution over 50 km of field-deployed optical fibers

Operational entanglement-based quantum key distribution over 50 km of field-deployed optical fibers

Flexible reconfigurable entanglement-based quantum key distribution network

AbstractRecently, we implemented a basic star-shaped entanglement-based quantum key distribution network without trusted nodes. It enables simultaneous pairwise exchange of quantum keys between multiple users. In this paper, we demonstrate its flexibility by focusing on several aspects. We show continuous system operation over a partially deployed 100 km fiber link for multiple days as well as dynamic reconfiguration of the communication partners. The photon pair source enables type-II or type-0 photon pair generation by SPDC as well as various demultiplexing strategies and repetition rates.

Photonic-reconfigurable entanglement distribution network based on silicon quantum photonics

The entanglement distribution network connects remote users by sharing entanglement resources, which is essential for realizing quantum internet. We propose a photonic-reconfigurable entanglement distribution network (PR-EDN) based on a silicon quantum photonic chip. The entanglement resources are generated by a quantum light source array based on spontaneous four-wave mixing in silicon waveguides and distributed to different users through time-reversed Hong–Ou–Mandel interference by on-chip Mach–Zehnder interferometers with thermo-optic phase shifters (TOPSs). A chip sample is designed and fabricated, supporting a PR-EDN with 3 subnets and 24 users. The network topology of the PR-EDN could be reconfigured in three network states by controlling the quantum interference through the TOPSs, which is demonstrated experimentally. Furthermore, a reconfigurable entanglement-based quantum key distribution network is realized as an application of the PR-EDN. The reconfigurable network topology makes the PR-EDN suitable for future quantum networks requiring complicated network control and management. Moreover, it is also shown that silicon quantum photonic chips have great potential for large-scale PR-EDN, thanks to their capacities for generating and manipulating plenty of entanglement resources.

Collective excitation of spatio-spectrally distinct quantum dots enabled by chirped pulses

Nanoscale bright sources that produce high-purity single photons and high-fidelity entangled photon pairs are the building blocks to realize high security quantum communication devices. To achieve high communication rates, it is desirable to have an ensemble of quantum emitters that can be collectively excited, despite their spectral variability. In case of semiconductor quantum dots, Rabi rotations are the most popular method for resonant excitation. However, these cannot assure a universal, highly efficient excited state preparation, due to the sensitivity to excitation parameters. In contrast, adiabatic rapid passage (ARP), relying on chirped optical pulses, is immune to quantum dot spectral inhomogeneity. Here, we show that the robustness of ARP holds true for the simultaneous excitation of the biexciton states in multiple, spatially separated and spectrally different quantum dots. For positive chirps, we also find a regime where the influence of phonons relax the sensitivity to spectral detunings and lower the needed excitation power. Being able to generate high-purity photons from spatially multiplexed quantum dot sources using the biexciton to ground state cascade is a big step towards the implementation of high photon rate, entanglement-based quantum key distribution protocols.

Simulating quantum key distribution in fiber-based quantum networks

Quantum networks exploit the unique properties of quantum mechanics to enable communication and networking tasks unavailable to existing distributed classical systems. Recently, the research community has focused considerable effort on the simulation of large-scale quantum networks with the ultimate goal of understanding their general properties, developing technical standards, and estimating their expected performance. However, comparatively little effort has been spent considering how quantum networks may impact tactical scenarios of military relevance where both quantum and classical resources may be severely constrained. Here, we develop a custom framework, called QuanTACT, for quantum network simulation explicitly designed for future integration into existing tactical simulation tools. In particular, our framework extends the existing quantum networking tool, SQUANCH, to include channel models required to simulate deployed fiber environments. Furthermore, we implement the additional subroutines needed to simulate entanglement-based quantum key distribution (QKD) and use published results from various field-deployed QKD experiments to benchmark the performance of our framework.

Active basis choice for quantum key distribution with entangled states

Entangled quantum key distribution (QKD) is a promising way to generate pairs of unconditionally secret keys. In this paper we review possible realizations of entanglement-based QKD and assess their feasibility in terms of implementation complexity and provided security. We also propose a novel active basis choice approach that enables to use only one single-photon detector per user. The paper provides all necessary details including the required electro-optic crystal configurations to implement such a scheme experimentally.

Experimental full-domain mapping of quantum correlation in Clauser-Horne-Shimony-Holt scenarios.

Quantum correlation between two parties serves as an important resource in the surging applications of quantum information. The Bell nonlocality and quantum steering have been proposed to describe non-classical correlations against local-hidden-variable and local-hidden-state theories, respectively. To characterize the two types of non-classical correlations, various nonlocality and steering inequalities have been established, and the amount of inequality violation serves as an important indicator for many entanglement-based tasks. Quantum state tomography has been employed for measuring quantum states, while the method requires intensive computation and does not directly verify either nonlocality or steering over the full domain independent of established theories. Here, we experimentally map the full-domain correlation with bipartite states for nonlocality and quantum steering in CHSH scenarios. The measurement of the maps automatically accounts for detection imperfections. Furthermore, we demonstrate the application of the correlation maps in the entanglement-based quantum key distribution protocol with arbitrary bipartite states. The correlation maps show direct measurements and simple interpretations that can answer fundamental questions about nonlocality and quantum steering as well as contribute to quantum information applications in a straightforward manner.

Photonic-chip-based dense entanglement distribution

The dense quantum entanglement distribution is the basis for practical quantum communication, quantum networks and distributed quantum computation. To make entanglement distribution processes stable enough for practical and large-scale applications, it is necessary to perform them with the integrated pattern. Here, we first integrate a dense wavelength-division demultiplexing system and unbalanced Mach-Zehnder interferometers on one large-scale photonic chip and demonstrate the multi-channel wavelength multiplexing entanglement distribution among distributed photonic chips. Specifically, we use one chip as a sender to produce high-performance and wideband quantum photon pairs, which are then sent to two receiver chips through 1-km standard optical fibers. The receiver chip includes a dense wavelength-division demultiplexing system and unbalanced Mach-Zehnder interferometers and realizes multi-wavelength-channel energy-time entanglement generation and analysis. High quantum interference visibilities prove the effectiveness of the multi-chip system. Our work paves the way for practical entanglement-based quantum key distribution and quantum networks.

Direct generation of spatially entangled qudits using quantum nonlinear optical holography

Nonlinear holography shapes the amplitude and phase of generated new harmonics using nonlinear processes. Classical nonlinear holography influenced many fields in optics, from information storage, demultiplexing of spatial information, and all-optical control of accelerating beams. Here, we extend the concept of nonlinear holography to the quantum regime. We directly shape the spatial quantum correlations of entangled photon pairs in two-dimensional patterned nonlinear photonic crystals using spontaneous parametric down conversion, without any pump shaping. The generated signal-idler pair obeys a parity conservation law that is governed by the nonlinear crystal. Furthermore, the quantum states exhibit quantum correlations and violate the Clauser-Horne-Shimony-Holt inequality, thus enabling entanglement-based quantum key distribution. Our demonstration paves the way for controllable on-chip quantum optics schemes using the high-dimensional spatial degree of freedom.

Device-independent quantum key distribution using random quantum states

We Haar uniformly generate random states of various ranks and study their performance in an entanglement-based quantum key distribution (QKD) task. In particular, we analyze the efficacy of random two-qubit states in realizing device-independent (DI) QKD. We first find the normalized distribution of entanglement and Bell-nonlocality which are the key resource for DI-QKD for random states ranging from rank-1 to rank-4. The number of entangled as well as Bell-nonlocal states decreases as rank increases. We observe that decrease of the secure key rate is more pronounced in comparison to that of the quantum resource with increase in rank. We find that the pure state and Werner state provide the upper and lower bound, respectively, on the minimum secure key rate of all mixed two-qubit states possessing the same magnitude of entanglement under general as well as optimal collective attack strategies.

Daylight entanglement-based quantum key distribution with a quantum dot source

Entanglement-based quantum key distribution can enable secure communication in trusted node-free networks and over long distances. Although implementations exist both in fiber and in free space, the latter approach is often considered challenging due to environmental factors. Here, we implement a quantum communication protocol during daytime for the first time using a quantum dot source. This technology presents advantages in terms of narrower spectral bandwidth—beneficial for filtering out sunlight—and negligible multiphoton emission at peak brightness. We demonstrate continuous operation over the course of three days, across an urban 270 m-long free-space optical link, under different light and weather conditions.

Flow of quantum correlations in noisy two-mode squeezed microwave states

The authors experimentally investigate the behavior of quantum discord and entanglement in two-mode squeezed states generated by superconducting Josephson parametric amplifiers. They show how correlations flow between the two modes and the noisy environment, and relate their findings to the security of entanglement-based quantum key distribution.

Realistic and general model for quantum key distribution with entangled-photon sources

Entanglement-based quantum key distribution (EB-QKD) has several advantages over prepare-and-measure protocols, such as eliminating the need for decoy states and active modulators. At present, entanglement sources based on the spontaneous parametric down-conversion process have been widely used in EB-QKD experiments. Most of the existing models regard the photon-number distribution of pulse-pumped entanglement sources as thermal. However, doing so ignores any effects of spectral modes, which will lead to the overestimation of the secure key rate. In this paper, we analyze the correlation between the photon-number distribution and the joint spectral distribution and rewrite the quantum state of the photon pairs. Then we apply the results to EB-QKD and present a general model which can be used to calculate the secure key rate not only for any pulse-pumped systems but even for cw-pumped systems.

Distributing Polarization-Entangled Photon Pairs with High Rate Over Long Distances through Standard Telecommunication Fiber

Entanglement distribution over long distances is essential for many quantum communication schemes like quantum teleportation, some variants of quantum key distribution, or implementations of a quantum internet. Distributing entanglement through standard telecommunication fiber is particularly important for quantum key distribution protocols with low vulnerability over metropolitan distances. However, entanglement distribution over long distance through optical fiber so far could only be accomplished with moderate photon pair rates. In this work, we present entanglement distribution over 50km of standard telecommunication fiber with pair rate more than 10,000 s$^{-1}$ using a bright non-degenerate photon pair source. Signal and idler wavelengths of this source are optimized for low dispersion in optical fiber and high efficiency for single-photon avalanche diode detectors, respectively. The resulting modest hardware requirement and high rate of detected entangled photon pairs could significantly enhance practical entanglement-based quantum key distribution in existing metropolitan fiber networks.

Hyperentangled Time-Bin and Polarization Quantum Key Distribution

Fiber-based quantum communication networks are currently limited without quantum repeaters. Satellite-based quantum links have been proposed to extend the network domain. We have developed a quantum communication system, suitable for realistic satellite-to-ground communication. With this system, we have executed an entanglement-based quantum key distribution (QKD) protocol developed by Bennett, Brassard, and Mermin in 1992 (BBM92), achieving quantum bit error rates (QBER) below 2$\%$ in all bases. More importantly, we demonstrate low QBER execution of a higher dimensional hyperentanglement-based QKD protocol, using photons simultaneously entangled in polarization and time-bin, leading to significantly higher secure key rates, at the cost of increased technical complexity and system size. We show that our protocol is suitable for a space-to-ground link, after incorporating Doppler shift compensation, and verify its security using a rigorous finite-key analysis. Additionally, We discuss system engineering considerations relevant to those and other quantum communication protocols, and their dependence on what photonic degrees of freedom are utilized.

BBM92 quantum key distribution over a free space dusty channel of 200 meters

Free-space quantum communication assumes importance as it is a precursor for satellite-based quantum communication needed for secure key distribution over longer distances. Prepare and measure quantum key distribution (QKD) protocols like BB84 consider the satellite as a trusted device, which is fraught with security threat looking at the current trend for satellite-based optical communication. Therefore, entanglement-based protocols must be preferred, so that one can consider the satellite as an untrusted device. The current work reports the effect of atmospheric aerosols on the key rate obtained with BBM92 QKD protocol, an entanglement-based QKD protocol over 200 m distance, using an indigenous facility developed at Physical Research Laboratory (PRL), Ahmedabad, India. Our results show that concentration and extinction coefficient of atmospheric aerosols play a major role in influencing the observed sift key rate, and eventually, the secure key rate. Such experiments are important to validate the models to account for the atmospheric effects on the key rates achieved through satellite-based QKD.