Open research questions in Quantum Information and Cryptography

What the literature leaves open

• Future research could investigate the implementation of this scheme in dynamic transmission environments, such as drone-based links, where time-varying path delays might broaden the coincidence peak [31].

  Sequential Sliced Time Synchronization for Practical Entanglement Distribution · 2026 · DOI

• In the field of quantum information science, quantum steering (QS) and nonlocal advantage of quantum coherence (NAQC) are the core quantum resources, while their practical applications are often limited by the decoherence effect caused by environmental noise in open quantum systems.

  Quantum steering and nonlocal advantage of quantum coherence in non-Markovian environment · 2026 · DOI

• While our current focus has been on creating GHZ states, exploring the distribution of other graph states, such as cluster states, would be rele- vant for distributed and measurement-based quantum computing. Additionally, analyzing in detail the robustness of our approach against link and node failures is of particular interest, especially in light of classical and quantum research on network resilience, as demonstrated by Refs. 77–80. Intuitively, by creating local, small-scale GHZ states independently, our protocol is less susceptible to the failure of any single GHZ state creation, which might result from the failure of nearest- neighbor entanglement generation in networks of lossy optical fiber channels. In cases of failure, the other GHZ states can be stored in memory, potentially reducing overall waiting times in the presence of link and node failures. Studying the waiting time of our protocol would generalize studies on the waiting time of linear repeater chains for bipartite entanglement, e.g., Refs. 81,82 to the multipartite setting. Currently, our protocol requires full, global knowledge of the network. Therefore, developing variants of our protocol that rely primarily on local knowledge and require only a few rounds of classical communication to establish global awareness, leveraging the ideas of, e.g., Ref. 83 would be beneficial. npj Quantum Information | (2026) 12:101 Beyond theoretical studies, implementing our algorithm on various experimental platforms is also of great interest. Another intriguing approach is to add an optimization layer to our protocol to enhance the way the stars are selected and merged, particularly in the context of lossy Bell pairs and lossy memory stations. We anticipate that this can be accomplished using reinforcement learning, by expressing our problem as a Markov decision process (MDP) and leveraging the tools from Refs. 82,84–88. Moreover, employing further tools such as (probability) generating functions, the protocols could be optimized in the presence of noise 89,90.

  A resource- and computationally-efficient protocol for multipartite entanglement distribution in Bell-pair networks · 2026 · DOI

• We also present the skewed SIC family of measurements, and use them to partially solve an open problem: we prove that $2 \log d$ bits of randomness, the maximal amount, can be generated device-dependently (or source-device-independently) in any dimension in which there exists a SIC measurement.

  Quantum randomness beyond projective measurements · 2026

• This architecture can be extended in several directions. The versatility of digital circuits incorporating non-local connectivity can be used for optimization of sensors such as programmable optical clocks [20, 25], featuring error correction in state preparation, state read out (signal “decoding”), or even during sensor operation, where all stages besides sensing occur in the robust nuclear qubit while sensing arises after mapping to the optical qubit [16, 22, 23, 60]. Such circuits could also be combined with stroboscopic shelving of optical coherence into the nuclear qubit for mid-circuit measurements on an atomic clock, to enable adaptive feedback. At the same time, the mapping of Rydberg qubits to computational qubits allows for coherent atom rearrangement — otherwise precluded by the Rydberg lifetime — enabling direct access to entanglement witnesses of quantum criticality and topological order [61, 62]. Meanwhile, this mapping allows analog quantum simulations to be leveraged for generating resource states for quantum computation [63, 64] and quantum metrol- 7 ogy [21, 65]. Conversely, mapping the computational qubit to the Rydberg qubit allows programmable entangled state preparation for subsequent many-body evolution. The high-fidelity decay detection enabled by our approach opens significant opportunities for both quantum simulation and quantum computing. When the Rydberg decay rate is larger than the characteristic energy scales of the Hamiltonian, the capability to isolate no-decay events can reveal exotic phenomena such as entanglement propagation beyond the Lieb-Robinson bound [66, 67]. On the other hand, when the decay rate is smaller, we numerically find that the system behaves indistinguishably from the unitary evolution after post-selection. The achieved effective Rydberg lifetime could enable Rydberg based quantum simulations with unprecedented evolution time. Moreover, high-fidelity control of the Rydberg transition via loss-detected error mitigation is particularly intriguing, as it eases the requirements for optical power and may contribute to scalable two-qubit gate implementation for neutral-atom quantum computers. Finally, the use of GRAPE in many-body systems has enabled the discovery of a robust, experiment-tailored protocol that departs significantly from conventional monotonic adiabatic ramps. These results highlight the potential of optimal control techniques to uncover efficient strategies for steering strongly interacting systems and are expected to drive further advances in analog quantum simulation of many-body systems. For example, studies of quantum spin liquids might be advanced by combining robust sweep protocols with the error detection schemes reported here. M. Saffman, Quantum computing with atomic qubits and Rydberg interactions: Progress and challenges, J. Phys. B 49, 202001 (2016). D. Bluvstein, S. J. Evered, A. A. Geim, S. H. Li, H.

  High-fidelity entanglement and coherent multi-qubit mapping in an atom array · 2026 · DOI

• In this paper, we have compared the performance of three different protocols in the presence of state preparation errors and operational errors. The three protocols are: non-catalytic EC, catalytic EC and distillation. To do this, we have extended Accepted in Quantum 2026-05-04, click title to verify. Published under CC-BY 4.0. 7 mauxmm(a)(b) help with the HPC and Nina Codreanu for her help with Adobe Illustrator. JH acknowledges funding from the Dutch Research Council (NWO) through a Veni grant (grant No.VI.Veni.222.331). JH and HS acknowledge funding from the Quan- tum Software Consortium (NWO Zwaartekracht Grant No.024.003.037). JB acknowledges sup- port from The AWS Quantum Discovery Fund at the Harvard Quantum Initiative. Part of this work was performed while HS was on a research visit at Harvard University funded by a Quantum Delta NL travel grant.

  Catalytic entanglement transformations with noisy hardware · 2026 · DOI

• The paper introduced the concept of a hybrid quantum-classical authentication scheme that combines lattice-based post-quantum digital signatures with quantum authentication and de- coy qubit verification that are assisted by entanglement. The need to develop such an au- thentication scheme stemmed from the need to ensure long-term cryptographic trust in the presence of quantum computational attacks, while at the same time ensuring the capability to identify interception during the transmission of information. The use of the lattice-based digital signatures ensured the capability of the scheme to exhibit excellent existential unforgeability under standard lattice assumptions, while at the same time ensuring the scheme’s compatibility with traditional public-key infrastructures. The use of the quantum authentication scheme ensured the security of the communication by leveraging the disturbance of quantum measurement, thereby ensuring the capability to identify interception with high probability. The security evaluation of the scheme indicated that the probability of failing to identify interception decreases exponentially with the number of decoy qubits used, while the performance evaluation of the scheme indicated that the additional quantum communication overhead is controllable. IJDDT, Volume 16 Issue 38s, 2026 Page 33 A Hybrid Quantum–Classical Signature Scheme Based on Lattice Cryptography with Entanglement and Decoy-Qubit Authentication for Eavesdropping Detection The future research directions include the extension of the proposed scheme to more re- alistic quantum channels by considering the effects of depolarizing noise, photon loss, and measurement imperfections. Another important research direction is the optimization of the strategies of entanglement distribution and the minimization of the quantum communication overhead while ensuring the capability to identify interception with high probability. The extension of the proposed scheme to other emerging applications such as quantum internet routing, blockchain validation, and IoT communication is another important research direc- tion that has the potential to be realized in the future. The implementation of the proposed scheme on experimental quantum communication testbeds and the evaluation of the scheme’s performance relative to other quantum digital signature schemes are other important research directions that need to be considered in the future. P. W.

  A Hybrid Quantum–Classical Signature Scheme Based on Lattice Cryptography with Entanglement and Decoy-Qubit Authentication for Eavesdropping Detection · 2026 · DOI

• Summarizing, building on the discrete model presented in for implementing a quantum perceptron, we modified the unitary gate (25) to explore how variations in the oscillation period affect the system’s behavior. Then, using the replica method, we analytically computed the storage capacity as a function of the oscillation frequency λ. In the limit of vanishing frequency, the classical value of 2 is recovered. However, as the frequency increases, we observe an indefinite enhancement in the quantum storage capacity beyond the classical threshold. Looking ahead, it 9 01234560246801234560.00.51.01.52.0 would be worthwhile to investigate 1) the presence of overfitting induced by high-frequencies, via the study of the generalization error in a teacher-student setting and 2) how the results would be affected by a replica symmetry breaking, for example withing the one-step replica symmetric ansatz. The observed improvement of the storage capacity beyond the classical threshold of αc = 2 arises solely from the particular form of the activation function. This suggests that, in principle, similar enhancements could be replicated within a wholly classical framework. Consequently, a genuine quantum advantage appears unlikely for a simple perceptron. Nevertheless, such an advantage may instead emerge in a quantum neural network, namely in an interconnected ensemble of quantum simple quantum perceptrons, where quantum interference effects could amplify computational capabilities, including those related to the storage of information. A Computation of the quantum storage capacity (34) We now compute the expression of the storage capacity αc(λ) of Subsection 3.2. We consider a dataset of the form {xµ, ξµ}p µ=1, where xµ ∈ {−1, 1}N, while ξµ = ±1 is a binary label. Using the parity of sin(z), the Gardner volume (28) can be rewritten as N ({xµ, ξµ}p V λ µ=1) = (cid:90) p (cid:89) (cid:18) (cid:18) Θ sin λ dµ(w) µ=1 (cid:19)(cid:19), w · rµ √ N rµ = ξµxµ. (40) The quantity of interest is the expectation value of ln V w.r.t. the distribution of patterns and the labels. Note that the distribution (8) on {xµ, ξµ}p µ=1 induces the distribution P (rµ j = 1) = P (rµ j = −1) = 1 2. (41) In the following we will denote with ⟨·⟩ expectations with respect to the distribution (41). The computation of ⟨ln V λ N ⟩ is performed by the replica trick (10), which leads us to consider the following expectation value ⟨(V λ N)n⟩ = (cid:90) n (cid:89) p (cid:89) (cid:28) n (cid:89) (cid:18) (cid:18) Θ sin λ dµ(wγ) γ=1 µ=1 γ=1 wγ · rµ √ N (cid:19)(cid:19) (cid:29).

  Pseudo quantum advantages in perceptron storage capacity · 2026 · DOI

• A Computation of the quantum storage capacity (34) A.1 Replica symmetric ansatz.................................................................

  Pseudo quantum advantages in perceptron storage capacity · 2026 · DOI

• No experimental validation or concrete instantiations of the hash function families H1, H2, F are provided to demonstrate practical applicability of the protocol.

  QCCC commitment from quantum inaccessible entropy generator · 2026 · DOI

• Finally, we discuss the open challenges as well as the future trends of QSDC networks, emphasizing again that QSDC is not a pure quantum key distribution (QKD) protocol, but a fully-fledged secure communication scheme.

  The Evolution of Quantum Secure Direct Communication: On the Road to the Qinternet · 2024 · DOI

• Given recent developments in quantum technologies and the longstanding paradigm in classical computing of delegating computationally intensive tasks to shared systems, the emerging interest in delegated quantum computation is both understandable and timely. While the progress discussed in this review illustrates the potential of BQC and related protocols, the field is still in its infancy, with new results coming on a regular basis but with many open questions still remaining. Perhaps the most prominent open question is that of whether or not blind or verifiable computation is possible with a single server and a completely classical client. In this setting, even when multiple nonentangled non-communicating servers are allowed, the existence of secure protocols for blind and verifiable computation remains an open question. the precise relationship between blindness and verification is currently unresolved. In the context of homomorphic encryption, the existence of fully homomorphic quantum encryption under plausible computational assumptions remains open, despite the promising progress of Dulek et al.63 In the context of verification, the most significant challenges facing the field include the necessity to drastically reduce overhead and Indeed, the variety of sensitivity to noise of current device-independent verification protocols, and the development of methods to verify analogue quantum simulators and other special purpose devices. While some progress has been made on the question of verifying nonuniversal devices,93–96 much more progress in this direction is necessary to fully unlock the potential of such devices. Lastly it should be noted that progress to date has only scratched the future quantum surface of networks may unlock. Recent developments, in terms of multiuser blind computation97 and publicly verifiable quantum computation,98 together with established results on secure multi-party quantum computation28, 99, 100 give some indication of the potential for new secure quantum computing protocols beyond the two party setting. Given these open questions, there is the potential for significant theoretical advance in the coming years. Harnessing the latest advances in experimental capabilities to go beyond the current generation of proof-of-principle experiments is also likely to be an important future direction.

  Private quantum computation: an introduction to blind quantum computing and related protocols · 2017 · DOI

• A second open problem is the analytical study of optimal decoders for CFT-based quantum codes consid- ered in this work.

  Probing mixed-state phases on a quantum computer via Renyi correlators and variational decoding · 2026 · DOI

• The directionality of the total power flow is limited by the phase independent contributions of the reflected drive and of spontaneous emission, which sets a routing efficiency that we measure as a function of the input power.

  Observation of a power transfer controlled by the phase of a quantum superposition · 2026

• Future work will investigate error-tolerant variants, potentially using quantum error correction or fault-tolerant techniques to maintain security under realistic conditions. One limitation of our current protocol is its assumption of noise- free quantum systems and perfect quantum gates.

  Verifier-initiated quantum message-authentication via quantum zero-knowledge proofs · 2026 · DOI

• Our finding showed that by increasing the cutoff photons in the system, can transitions the system from a decoherence-limited to a sustained coherence regime.

  Effects of intrinsic decoherence in multipartite system subjected to Kerr effect and parametric amplification with quantum correlations and estimation · 2026 · DOI

• However, probing quantum chaos on integrated photonic platforms remains largely unexplored because a clear connection between programmable photonic dynamics and established chaos diagnostics is still lacking.

  Boson Sampling as a Probe of Chaotic and Integrable Quantum Dynamics · 2026

• A major open problem in quantum communication complexity is whether quantum protocols can be exponentially more efficient than classical protocols for computing total Boolean functions; the prevailing conjecture is that they cannot be so.

  Quantum-Classical Equivalence for AND-Functions · 2026

• However, it remained an open question whether encrypted cloning could be generalised to arbitrary dimensions, and the broader relationship between the two schemes had not been formally established.

  Encrypted Cloning, Absolute Maximal Entanglement and Quantum Secret Sharing · 2026

• The analysis presented concerns solely the P(α, β*) function and its behavior under coordinate transformations, with the question of whether the change of variables affects the classical limit remaining largely unaddressed beyond this specific treatment.

  Attosecond quantum optical interferometry · 2026 · DOI

• Previous work kept the squeezing parameter ξ fixed, whereas this work considers ξ(ϕ) varying with ϕ to ensure consistent squeezing of the 2ω field regardless of phase difference.

  Attosecond quantum optical interferometry · 2026 · DOI

• The paper assumes the receiver remains honest despite holding a quantum state, but does not explore scenarios with dishonest quantum receivers or discuss limitations of this assumption.

  QCCC commitment from quantum inaccessible entropy generator · 2026 · DOI

• The paper does not discuss computational complexity or practical efficiency considerations for implementing the commitment scheme with large values of m (number of blocks).

  QCCC commitment from quantum inaccessible entropy generator · 2026 · DOI

• The proof of Theorem 5 closely resembles the classical case and details are omitted, leaving a gap in the complete technical justification for the quantum honest-receiver hiding property.

  QCCC commitment from quantum inaccessible entropy generator · 2026 · DOI

Most-cited papers in Quantum Information and Cryptography

• Entanglement of nanophotonic quantum memory nodes in a telecom network · Nature · 2024 · 269 citations
• Private quantum computation: an introduction to blind quantum computing and related protocols · npj Quantum Information · 2017 · 233 citations
• <i>Colloquium</i>: Quantum batteries · Reviews of Modern Physics · 2024 · 211 citations
• Continuous-variable quantum key distribution system: Past, present, and future · Applied Physics Reviews · 2024 · 172 citations
• The Evolution of Quantum Secure Direct Communication: On the Road to the Qinternet · IEEE Communications Surveys & Tutorials · 2024 · 158 citations
• Advances in the quantum internet · Communications of the ACM · 2022 · 146 citations
• Logical states for fault-tolerant quantum computation with propagating light · Science · 2024 · 140 citations
• Creation of memory–memory entanglement in a metropolitan quantum network · Nature · 2024 · 127 citations
• Long-distance continuous-variable quantum key distribution over 100-km fiber with local local oscillator · Science Advances · 2024 · 119 citations
• Metropolitan-scale heralded entanglement of solid-state qubits · Science Advances · 2024 · 89 citations

Most recent work

• Attosecond quantum optical interferometry · Reports on Progress in Physics · 2026
• Scalable entangling gates on ion qubits via structured light addressing · Science Advances · 2026
• Security analysis of orthogonal state attack on a high-speed quantum key distribution system · npj Quantum Information · 2026
• Maximizing the Nondemolition Nature of a Quantum Measurement Via an Adaptive Readout Protocol · PRX Quantum · 2026
• Fast Native Three-Qubit Gates and Fault-Tolerant Quantum Error Correction with Trapped Rydberg Ions · Quantum Science and Technology · 2026
• Imaging Walk-Off–Driven Distortions in EPR Photon-Pair Correlations · Optics Express · 2026
• Quantum metrology based on superconducting qubits · npj Quantum Information · 2026
• Teaching entanglement: overcoming common misconceptions · Physics Education · 2026
• QCCC commitment from quantum inaccessible entropy generator · Cybersecurity · 2026
• Preferential path attachment model for quantum key distribution networks · Scientific Reports · 2026

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