We present a feasible approach to testing Bell nonlocality and implementing device-independent quantum key distribution (DI) between distant atomic states in cavity-based architectures, mediated by hybrid atom-light entanglement. We develop a full theoretical model that incorporates realistic sources of noise—such as transmission loss, limited light-matter coupling efficiency, and imperfect detection. Our analysis shows that strong Bell-Clauser-Horne-Shimony-Holt violations and secure key generation over tens of kilometers are within reach using current or near-term technology. These results position cavity-based platforms with coherent-state encodings as a promising foundation for future DI quantum communication networks.

The original version of this article unfortunately contained error in corresponding authorship and in Eqs. (9) and (12). The corresponding author of this article is Vikash Mittal (vikashmittal.iiser@gmail.com) but instead published incorrectly as Jaskaran Singh. For completeness and transparency, both the incorrect and correct equations of Eqs. (9) and (12) displayed below. Incorrect Eq. (9): (Formula presented.) Correct Eq. (9): (Formula presented.) Incorrect Eq. (12): (Formula presented.) Correct Eq. (12): (Formula presented.) The original article has been corrected.

In recent times, hybrid entanglement (HE) between a qubit and a coherent state has demonstrated superior performance in various quantum information processing tasks, particularly in quantum key distribution. Despite its theoretical advantages, efficient generation of such states in the laboratory has been a challenge. Here, we introduce a deterministic and efficient approach for generating HE states using quantum walks. Our method achieves a remarkable fidelity of 99.9% with just 20 time steps in a one-dimensional split-step quantum walk. This represents a significant improvement over prior approaches for probabilistic generation of HE states with fidelity as low as 80%. Our scheme not only provides a robust solution to the generation of HE states but also highlights a unique advantage of quantum walks, thereby contributing to the advancement of this burgeoning field. Moreover, our scheme is experimentally feasible with the current technology.

We introduce a feasible scheme to produce high-rate long-distance entanglement that uses hybrid entanglement between continuous variables (CVs) and discrete variables (DVs). We show that hybrid entanglement can effectively remove the experimental limitations of existing CV and DV systems to produce long-range entanglement. We benchmark the resulting DV entangled states using an entanglement-based-quantum-key-distribution protocol. We show, using hybrid entangled states, that entanglement-based quantum key distribution is possible with standard telecommunication fibers for 300 km. The key idea is the use of the CV part, which can be adjusted to be robust regarding photon losses, for increasing the transmission distance, and the use of the DV part for achieving high secure key rates. Our results show that hybrid entangled states provide a clear advantage for practical generation of long-distance and high-rate entanglement that may lead to further applications in quantum information processing.

In this paper we analyze entanglement-based (EB) continuous-variable (CV) quantum-key distribution (QKD) with bright multimode non-Gaussian light. Our analysis is centered around the role of non-Gaussianity in mitigating the excess noise arising due to the mismatch between the signal modes and the local oscillators used for measurements. To be specific, we consider the non-Gaussian resources generated by single-photon subtraction and zero-photon catalysis applied on a two-mode squeezed vacuum (TMSV) state. We show that, at a given strength of the mode-mismatch noise, zero-photon catalysis leads to the maximum transmission distance, compared to the TMSV. However, considering the unavoidable issue of photon loss in the linear optical scheme for implementing zero-photon catalysis, our results hints at the single-photon-subtracted TMSV being the optimal choice for maximizing the transmission distance in EB CV QKD.

We compare single-photon qubits and hybrid qubits as information carriers through quantum teleportation using a Gaussian continuous-variable channel. A hybrid qubit in our study is in the form of entanglement between a coherent state and a single photon. We find that hybrid qubits outperform photonic qubits when coherent amplitudes of the hybrid qubits are as low as α≲1, while single-photon qubits yield better results for larger amplitudes. We analyze further the effect of photon losses and observe that the overall character of teleportation for different qubits remains the same although the teleportation fidelities are degraded by photon losses. Our work provides a comparative look at practical quantum information processing with different types of qubits.

Non-Gaussian operations in continuous variable (CV) quantum key distribution (QKD) have been limited to photon subtraction on squeezed vacuum states only. This is mainly due to the ease of calculating the covariance matrix representation of such states. In this paper we study the effects of general non-Gaussian operations corresponding to photon addition, catalysis, and subtraction on squeezed coherent states on CV measurement-device-independent (MDI) QKD. We find that non-Gaussianity coupled with coherence can yield significantly longer transmission distances than without. Particularly we observe that zero photon catalysis on the two-mode squeezed coherent state (TMSC) is an optimal choice for CV MDI QKD, while single photon subtraction is also a good candidate; both of them offer nearly 70 km of transmission distances. We also derive a single generalized covariance matrix for the aforementioned states which will be useful in several other aspects of CV quantum information processing.

Quantum teleportation (QT) lies at the heart of modern day quantum information science and technology. Despite extensive studies over past two decades, obtaining the necessary and/or sufficient criterion for QT with continuous-variable (CV) resources, besides entanglement, still remains an open concern. In this backdrop, here we analyze the role of a purely quantum optical (QO) attribute, known as quadrature squeezing, in CV teleportation. We first provide an analytic proof that for Gaussian resources quadrature squeezing is necessary for QT. However, for non-Gaussian resources we show a clear distinction between the pure and the mix states. For the pure states, quadrature squeezing appears to be necessary for QT, in the sense that there is no QT without quadrature squeezing. However, in the case of mix states we observe otherwise, i.e., QT could be achieved even without quadrature squeezing. Our results present the exotic character of the QO attributes of the CV resources and necessitate a deeper search for the necessary and/or sufficient criterion for CV QT.

Recent theoretical and experimental advances have demonstrated advantages of using non-Gaussian optical resources compared to the Gaussian ones in the context of quantum teleportation (QT), an important quantum information processing task. From both theoretical and experimental points of view the question of which attributes of the resources, besides entanglement, render them useful for QT is an important one. In this paper, we examine the question of whether two well-studied attributes of optical resources, viz., squeezed vacuum affinity (SVA) and Einstein-Podolsky-Rosen (EPR) correlation are necessary and/or sufficient for QT. The specific class of non-Gaussian resources that we have considered for this purpose are the two-mode entangled states generated by mixing nonclassical inputs with vacuum at the beam splitter (BS). Our analytical results show that SVA is not always nonzero and hence it cannot be considered to be a genuine attribute. Our numerical results show that there exist some BS-generated entangled states that do not give QT in spite of being EPR correlated, implying that EPR correlation is not sufficient for QT. In conjunction with the earlier observation in the literature to the effect that EPR correlation is not necessary for QT, our results lead to the conclusion that in general, EPR correlation is neither necessary nor sufficient for QT. Our results leave the question open as to what attributes, in general, may be necessary and/or sufficient for QT.

Quantum teleportation (QT) plays a central role in state-of-the-art information science and technology that necessitates proper characterization of the resources. While entanglement is known to be necessary, condition of sufficiency for QT still remains an open question. Here, we partially answer this question in light of Einstein-Podolsky-Rosen (EPR) correlation. In the case of input coherent state, we provide an analytic proof that with a general two-mode Gaussian entangled resources EPR correlation is sufficient for QT. For a relatively restricted set of Gaussian states, however bigger than that of the symmetric states, we further show that EPR correlation is both necessary and sufficient. On the other hand, in the case of Gaussian pure input, our numerical results hint that EPR correlation mostly appears to be a necessary condition only. However, the necessary and/or sufficient condition for QT of a Gaussian mixed input state, apart from the entanglement, still remains an open question.

Heterogeneity in dynamics in the form of non-Gaussian molecular displacement distributions appears ubiquitously in soft matter. We address the quantification of such heterogeneity using an information-theoretic measure of the distance between the actual displacement distribution and its nearest Gaussian estimation. We explore the usefulness of this measure in two generic scenarios of random walkers in heterogeneous media. We show that our proposed measure leads to a better quantification of non-Gaussianity than the conventional ones based on moment ratios.

Non-Gaussian operations on two-mode squeezed vacuum states in continuous-variable (CV) measurement-device-independent (MDI) quantum key distribution (QKD) protocols have been shown to effectively increase the total transmission distances drastically. In this paper we show that photon subtraction on a two-mode squeezed coherent (PSTMSC) state can further improve the transmission distances remarkably. To that end we also provide a generalized covariance matrix corresponding to the PSTMSC state. We show that coherence, defined as the amount of displacement of the vacuum state, along with non-Gaussianity can help improve the performance of prevalent CV MDI QKD protocols. Furthermore, since we use realistic parameters, our technique is experimentally feasible and can be readily implemented.

Nonclassical states of a quantized light are described in terms of Glauber-Sudarshan P distribution, which is not a genuine classical probability distribution. Despite several attempts, defining a uniform measure of nonclassicality (NC) for the single-mode quantum states of light remains an open task. In our previous work Bose and Sanjay Kumar (2017 Phys. Rev. A 95 012330), we have shown that the existing well-known measures fail to quantify the NC of single-mode states that are generated under multiple NC-inducing operations. In this paper, we propose a new quantification of NC for the single-mode quantum states as the difference between the total Wehrl entropy of the state and the maximum Wehrl entropy arising due to its classical characteristics. The classical reference state is chosen at the same randomness, given by the von-Neumann entropy, as the state itself. We further suggest operational interpretation of the proposed measure in terms of phase-space sampling entropy, as well as potential to generate entanglement in the case of pure states. We obtain analytic expressions of NC for pure states and Gaussian mixed states. We show that, along with the states generated under single NC-inducing operations as well as the broader class of states that are generated under multiple NC-inducing operations, our quantification enumerates the NC consistently. However, the evaluation of NC for the non-Gaussian mixed states is subject to extensive numerical computation that lies beyond the scope of the current work. We finally conclude with certain open questions.

Continuous-variable beam-splitter (BS)-generated entanglement from single-mode optical states generated by a single nonclassicality (NC)-inducing operation has been found to be immensely important in several information processing tasks. There exists a broader class of optical states, generated from successive action of multiple different NC-inducing operations, which show many intriguing nonclassical properties; however, the BS conversion of the NC for such states remains unexplored. In this work we have critically analyzed the BS-generated entanglement from such nonclassical optical states at input. Here we present a scenario where BS output entanglement becomes nonmonotonic with the input NC parameters, accessible experimentally (e.g., number of photon excitation and squeezing strength), in contrast to the previous results with states comprising a single NC-inducing operation. We explain this counterintuitive feature in terms of the competition between these two NC-inducing operations as manifest in the contours of the Q functions associated with these states.