Classical Communication Research Articles

Optimized Compiler for Distributed Quantum Computing

Practical distributed quantum computing requires the development of efficient compilers, able to make quantum circuits compatible with some given hardware constraints. This problem is known to be tough, even for local computing. Here, we address it on distributed architectures. As generally assumed in this scenario, telegates represent the fundamental remote (inter-processor) operations. Each telegate consists of several tasks: (i) entanglement generation and distribution, (ii) local operations, and (iii) classical communications. Entanglement generations and distribution is an expensive resource, as it is time-consuming. To mitigate its impact, we model an optimization problem that combines running-time minimization with the usage of distributed entangled states. Specifically, we formulated the distributed compilation problem as a dynamic network flow. To enhance the solution space, we extend the formulation, by introducing a predicate that manipulates the circuit given in input and parallelizes telegate tasks. To evaluate our framework, we split the problem into three sub-problems, and solve it by means of an approximation routine. Experiments demonstrate that the run-time is resistant to the problem size scaling. Moreover, we apply the proposed algorithm to compile circuits under different topologies, showing that topologies with a higher ratio between edges and nodes give rise to shallower circuits.

Quantum Contextuality Provides Communication Complexity Advantage

Despite the conceptual importance of contextuality in quantum mechanics, there is a hitherto limited number of applications requiring contextuality but not entanglement. Here, we show that for any quantum state and observables of sufficiently small dimensions producing contextuality, there exists a communication task with quantum advantage. Conversely, any quantum advantage in this task admits a proof of contextuality whenever an additional condition holds. We further show that given any set of observables allowing for quantum state-independent contextuality, there exists a class of communication tasks wherein the difference between classical and quantum communication complexities increases as the number of inputs grows. Finally, we show how to convert each of these communication tasks into a semi-device-independent protocol for quantum key distribution.

Performance analysis of quantum repeaters enabled by deterministically generated photonic graph states

By encoding logical qubits into specific types of photonic graph states, one can realize quantum repeaters that enable fast entanglement distribution rates approaching classical communication. However, the generation of these photonic graph states requires a formidable resource overhead using traditional approaches based on linear optics. Overcoming this challenge, a number of new schemes have been proposed that employ quantum emitters to deterministically generate photonic graph states. Although these schemes have the potential to significantly reduce the resource cost, a systematic comparison of the repeater performance among different encodings and different generation schemes is lacking. Here, we quantitatively analyze the performance of quantum repeaters based on two different graph states, i.e. the tree graph states and the repeater graph states. For both states, we compare the performance between two generation schemes, one based on a single quantum emitter coupled to ancillary matter qubits, and one based on a single quantum emitter coupled to a delayed feedback. We identify the numerically optimal scheme at different system parameters. Our analysis provides a clear guideline on the selection of the generation scheme for graph-state-based quantum repeaters, and lays out the parameter requirements for future experimental realizations of different schemes.

Quantum superposition of causal structures as a universal resource for local implementation of nonlocal quantum operations

Spatial separation restricts the set of locally implementable quantum operations on distributed multipartite quantum systems. We propose that indefinite causal structure arising due to quantum superposition of different space-time geometries can be used as an independent universal resource for local implementation of any quantum operation on spatially distributed quantum systems. Consequently, all such quantum tasks that are not accomplishable by local operations and classical communication (LOCC) only also become locally accomplishable. We show that exploiting indefinite causal structure as the sole resource, it is possible to perfectly teleport the state of one agent's subsystem to the other distant laboratory in such a way that the agent at the distant laboratory can have access to the whole initially shared state in his or her laboratory and can perform any global quantum operation on the joint state locally. We further find that, after the teleportation process, the resource -- indefinite causal structure of the space-time does not get consumed. Hence, after implementing the desired quantum operation the state of the first agent's subsystem can be teleported back to its previous laboratory using the same resource. We show that this two-way teleportation is not always necessary for locally executing all nonlocal quantum tasks that are not realisable by LOCC only. Without invoking any kind of teleportation, we present a protocol for perfect local discrimination of the set of four Bell states that exploits indefinite causal structure as the sole resource. As immediate upshots, we present some more examples of such nonlocal tasks as local discrimination of the set of states exhibiting ``quantum nonlocality without entanglement" and activation of bound entangled states that are also achievable by our proposed protocol incorporating indefinite causal structure as a resource.

Quantum Machine Learning for Distributed Quantum Protocols with Local Operations and Noisy Classical Communications

Distributed quantum information processing protocols such as quantum entanglement distillation and quantum state discrimination rely on local operations and classical communications (LOCC). Existing LOCC-based protocols typically assume the availability of ideal, noiseless, communication channels. In this paper, we study the case in which classical communication takes place over noisy channels, and we propose to address the design of LOCC protocols in this setting via the use of quantum machine learning tools. We specifically focus on the important tasks of quantum entanglement distillation and quantum state discrimination, and implement local processing through parameterized quantum circuits (PQCs) that are optimized to maximize the average fidelity and average success probability in the respective tasks, while accounting for communication errors. The introduced approach, Noise Aware-LOCCNet (NA-LOCCNet), is shown to have significant advantages over existing protocols designed for noiseless communications.

Multi-hop quantum teleportation of an arbitrary two-qubit state based on hierarchical simultaneous entanglement swapping

Combined with hierarchy simultaneous entanglement swapping, the scheme of the multi-hop quantum teleportation (QT) is proposed by using multi-pair Bell states. The scheme consists of two levels based on simultaneous entanglement swapping, namely Level-1 and Level-2. The former realizes simultaneous entanglement swapping of the inner segment, while the latter completes simultaneous entanglement swapping of the inter segment. Besides, the effect of four different types of noise (phase-damping, amplitude-damping, phase-flip, and bit-flip noise) on our scheme is analyzed by calculating the fidelity and concurrence. The results indicate that the fidelity and concurrence are related to the amplitude parameter, decoherence rate and number of nodes. Furthermore, compared with the scheme of the multi-hop QT adopting simultaneous entanglement swapping, our scheme can greatly reduce classical communication cost, and shows that the optimal classical communication cost can be obtained by selecting different segmentation methods.

Remote creation of quantum coherence via indefinite causal order

Quantum coherence is a prime resource in quantum computing and quantum communication. Quantum coherence of an arbitrary qubit state can be created at a remote location using maximally entangled state, local operation and classical communication. However, if there is a noisy channel acting on one side of the shared resource, then, it is not possible to create perfect quantum coherence remotely. Here, we present a method for the creation of quantum coherence at a remote location via the use of entangled state and indefinite causal order. We show this specifically for the superposition of two completely depolarizing channels, two partially depolarizing channels and one completely depolarizing channel along with a unitary operator. We find that when the indefinite causal order of channels act on one-half of the entangled pair, then the shared state looses entanglement, but can retain non-zero quantum discord. This finding may have some interesting applications on its own where discord can be consumed as a resource. Our results suggest that the indefinite causal order along with a tiny amount of quantum discord can act as a resource in creating non-zero quantum coherence in the absence of entanglement.

Guest Editorial: Integrated Sensing and Communications for 6G

6G networks have been envisioned as key enablers for numerous emerging applications, including smart cities and homes, intelligently connected vechiles, smart manufactoring, and industrial Internet-of-Things (IoT). These applications require both extreme wireless connectivity as well as highly accurate and reliable sensing capability. Indeed, among many 6G visions, a common theme is that sensing will play a more significant role than ever before. By equipping wireless systems with the sensing functionality, 6G networks will go beyond classical communication and provide ubiquitous sensing services to measure, or to image, surrounding environments. This sensing functionality and the corresponding ability of the network to collect sensory data from the environment are seen as the foundation for building intelligence in the future smart world. Toward that end, there is a strong need to jointly design sensing and communication operations in 6G networks, which motivates the recent research of Integrated Sensing and Communications (ISAC).

Active Tunable Pulse Shaping Using MoS2‐Assisted All‐Dielectric Metasurface

While optical pulse shapers have important applications in classical and quantum communication regimes and laser resonant cavities, engineering of group delay dispersion (GDD) remains one of their greatest challenges. Herein, by taking advantage of the electrically tunable optical properties of 2D material and the low‐loss nature of dielectric material. This paper demonstrates how an active tunable all‐dielectric metasurface assisted by 2D material can be leveraged to shape the temporal profile of a pulse. The proposed metasurface consists of an array of nanobars covered by a 2D sheet and positioned on a distributed Bragg reflector (DBR) as a perfect mirror to design a phase‐only modulator. Upon introducing in‐plane asymmetries, the quasi‐bound state in the continuum (QBIC) resonance emerges under normal incidence, which subsequently leads to achieving both significant GDD and the two regimes of pulse stretching and compressing via boosting the effect of the permittivity variation of molybdenum disulfide (MoS2). The monolayer MoS2 proves to be an excellent substitute for other tunable materials with inherent dissipative loss in the visible frequency range. Following such an active tunable geometrically fixed configuration, various pulse‐shaping operations are achieved, including compression (peak intensity up to 350%), expansion (peak intensity from 60%), splitting, and higher‐order distortion.

Quantifying the performance of approximate teleportation and quantum error correction via symmetric 2-PPT-extendible channels

The ideal realization of quantum teleportation relies on having access to a maximally entangled state; however, in practice, such an ideal state is typically not available and one can instead only realize an approximate teleportation. With this in mind, we present a method to quantify the performance of approximate teleportation when using an arbitrary resource state. More specifically, after framing the task of approximate teleportation as an optimization of a simulation error over one-way local operations and classical communication (LOCC) channels, we establish a semi-definite relaxation of this optimization task by instead optimizing over the larger set of two-PPT-extendible channels. The main analytical calculations in our paper consist of exploiting the unitary covariance symmetry of the identity channel to establish a significant reduction of the computational cost of this latter optimization. Next, by exploiting known connections between approximate teleportation and quantum error correction, we also apply these concepts to establish bounds on the performance of approximate quantum error correction over a given quantum channel. Finally, we evaluate our bounds for various examples of resource states and channels.

Multi-rate and multi-protocol continuous-variable quantum key distribution.

Quantum key distribution (QKD) is an effective solution to ensure the secure transmission of information. However, for the large-scale application of QKD, the interoperability and flexibility of the transmitter and receiver are urgent issues to be solved. Here, for the first time, to the best of our knowledge, we experimentally verify the feasibility of one continuous-variable (CV) QKD system to achieve multiple protocols and rates. The flexibility of the system comes from the modulator realizing multiple protocols and a broadband coherent detector realizing multiple symbol rates. The results show that this system can switch between different rates and protocols to generate the secure key, and reveal its similarity to classical optical communication. Therefore, It can be adjusted according to user needs and provides a system-level solution for building a flexible quantum network.

Gravity entanglement, quantum reference systems, degrees of freedom

Gravity mediated entanglement (GME) has been proposed as the first experimentally testable signature of quantum gravity. However, to what extent the effect is due to quantum gravity is under debate. In this note, we argue in several ways that the observation of GME does indeed tell us something new about gravity compared to previous experiments. In particular, we consider a quantum reference frame treatment of the experiment that allows us to pinpoint the single degree of freedom responsible for the effect. We also discuss the relevance of the Newtonian limit, the longitudinal/transverse decomposition of the field, and the local operations and classical communication theorem. Our conclusion is that experiments trying to detect GME would be interesting because (a) either positive or negative results would be able to falsify several theories (b) observation of GME would represent something truly novel.

Application of Ramsey theory to localization of set of product states via multicopies

It is well known that any N orthogonal pure states can always be perfectly distinguished under local operation and classical communications (LOCC) if \((N-1)\) copies of the state are available (Walgate et al. Phys. Rev. Lett. 85, 4972 (2000)). It is important to reduce the number of quantum state copies that ensures the LOCC distinguishability in terms of resource saving and nonlocality strength characterization. Denote \(f_r(N)\) the least number of copies needed to LOCC distinguish any N orthogonal r-partite product states. This work will be devoted to the estimation of the upper bound of \(f_r(N)\). In fact, we first relate this problem with Ramsey theory, a branch of combinatorics dedicated to studying the conditions under which orders must appear. Subsequently, we prove \(f_2(N)\le \lceil \frac{N}{6}\rceil +2\), which is better than \(f_2(N)\le \lceil \frac{N}{4}\rceil\) obtained in Shu (Eur. Phys. J. Plus 136, 1172 (2021)) when \(N>32\). We further exhibit that for arbitrary \(\epsilon >0\), \(f_r(N)\le \lceil \epsilon N\rceil\) always holds for sufficiently large N.

A mind-reading puzzle: Autistic people are more efficient at a theory-of-mind task

Theory of Mind (ToM) is essential to adapt in social situations; however, a ToM deficit might be involved in autism. To better understand how ToM reasoning affects problem solving in autistic and non-autistic individuals, we compared autistic and non-autistic children and adults in a series of problems presented in social and non-social framings, using an adapted version of a classical referential communication task. In the social framing, participants were asked to anticipate the behavior of an agent who might ignore some components of the scene. In the non-social framing, the task required participants to consider and ignore similar features of the scene, but an agent was not involved. Simply framing the task as a social one increased the difficulty, particularly for non-autistic participants. Interestingly, the framing had less of an impact on autistic participants, who showed better performance in the social task relative to non-autistics and maintained similar performance across framings. We propose that autistic participants might have translated the social instructions into a general rule that proved more efficient in this situation. Our findings suggest a critical distinction between ToM understanding and the continuous use of a ToM strategy in repeated situations.

Non-local polarization alignment and control in fibers using feedback from correlated measurements of entangled photons.

Quantum measurements that use the entangled photons' polarization to encode quantum information require calibration and alignment of the measurement bases between spatially separate observers. Because of the changing birefringence in optical fibers arising from temperature fluctuations or external mechanical vibrations, the polarization state at the end of a fiber channel is unpredictable and time-varying. Polarization tracking and stabilization methods originally developed for classical optical communications cannot be applied to polarization-entangled photons, where the separately detected photons are statistically unpolarized, yet quantum mechanically correlated. We report here a fast method for automatic alignment and dynamic tracking of the polarization measurement bases between spatially separated detectors. The system uses the Nelder-Mead simplex method to minimize the observed coincidence rate between non-locally measured entangled photon pairs, without relying on classical wavelength-multiplexed pilot tones or temporally interleaved polarized photons. Alignment and control is demonstrated in a 7.1 km deployed fiber loop as well as in a controlled drifting scenario.

Entanglement measures based on the complete information of reduced states

Quantum entanglement has been verified experimentally and applied in quantum computing, quantum sensing, and quantum networks. It is of great significance to find measures to characterize the quantum entanglement faithfully. In this work, by exploiting the Schmidt decomposition of bipartite states, we first establish a one-to-one correspondence between the characteristic polynomial of the reduced state of a bipartite pure state and the trace of the reduced state. We introduce a family of entanglement measures based on the eigenvalues of the reduced density matrices. Specific measures called informationally complete entanglement measures (ICEMs) are presented. It is demonstrated that such ICEMs can characterize better the entanglement than the existing well-known entanglement measures. The ICEMs also give rise to criteria of state transformations under local operation and classical communication. Moreover, it is shown that the ICEMs can be efficiently estimated on a quantum computer. The full separability, entanglement, and genuine multipartite entanglement can be detected faithfully on quantum devices.

Incoherent Gaussian equivalence of m -mode Gaussian states

Necessary and sufficient conditions for arbitrary multimode (pure or mixed) Gaussian states to be equivalent under incoherent Gaussian operations are derived. We show that two Gaussian states are incoherent equivalence if and only if they are related by incoherent unitaries. This builds the counterpart of the celebrated result that two pure entangled states are equivalent under local operations and classical communication (LOCC) if and only if they are related by local unitaries. Furthermore, incoherent equivalence of Gaussian states is equivalent to frozen coherence [Phys. Rev. Lett. 114, 210401 (2015)]. Basing this as foundation, we find all measures of coherence are frozen for an initial Gaussian state under strongly incoherent Gaussian operations if and only if the relative entropy measure of coherence is frozen for the state. This gives an entropy-based dynamical condition in which the coherence of an open quantum system is totally unaffected by noise.

Necessary criterion for extracting thermodynamical work from qudit-entangled state

A novel criterion of extracting thermodynamical work from a bipartite pure qudit-entangled state using local operation and classical communication (LOCC) has been presented. We have shown that non-vanishing G-concurrence is a necessary condition for extracting work from a higher-dimensional entangled state in LOCC paradigm.

Catalytic transformations in coherence theory

In 2016, Winter and Yang [A. Winter and D. Yang, Phys. Rev. Lett. 116, 120404 (2016)] provided an operational meaning to relative entropy of coherence and coherence of formation by introducing coherence distillation and dilation protocols in an asymptotic setup. Here we have provided relative entropy of coherence [via incoherent operations (IO)] and coherence of formation (via IO) and quantum incoherent relative entropy [via local quantum incoherent operations with classical communications (LQICC)] as a measure using the concept of a catalyst. We have proved an existential correspondence between asymptotic and catalytic state transformation using IO, local incoherent operations with classical communications, and LQICC. We have also discussed two very important protocols, assisted distillation and quantum incoherent state merging using a catalyst. Monotone properties of relative entropy of coherence, coherence of formation, and quantum incoherent relative entropy under the catalytic transformation are also discussed here.

Local approximation for perfect discrimination of quantum states

Quantum state discrimination involves identifying a given state out of a set of possible states. When the states are mutually orthogonal, perfect state discrimination is alway possible using a global measurement. In the case of multipartite systems when the parties are constrained to use multiple rounds of local operations and classical communications (LOCC), perfect state discrimination is often impossible even with the use of \emph{asymptotic LOCC}, wherein an error is allowed but must vanish in the limit of an infinite number of rounds. Utilizing our recent results on asymptotic LOCC, we derive a lower bound on the error probability for LOCC discrimination of any given set of mutually orthogonal pure states. We use this lower bound to prove necessary conditions for perfect state discrimination by asymptotic LOCC. We then illustrate by example the power of these necessary conditions in significantly simplifying the determination of whether perfect discrimination of a given set of states can be accomplished arbitrarily closely using LOCC. The latter examples include a proof that perfect discrimination by asymptotic LOCC is impossible for any \emph{minimal} unextendible product basis (UPB), where minimal means that for the given multipartite system, no UPB with a smaller number of states can exist. We also give a simple proof that what has been called \emph{strong nonlocality without entanglement} is considerably stronger than had previously been demonstrated.