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In this paper, we define the operational and denotational semantics of a special class of parallel quantum programs, namely disjoint parallel quantum programs. Based on them, a proof system for reasoning about disjoint parallel quantum programs is developed, which is (relatively) complete even when entanglement between different processes appears in the preconditions and postconditions.

Quantum spin liquids, exotic phases of matter with topological order, have been a major focus of explorations in physical science for the past several decades. Such phases feature long-range quantum entanglement that can potentially be exploited to realize robust quantum computation. We use a 219-atom programmable quantum simulator to probe quantum spin liquid states. In our approach, arrays of atoms are placed on the links of a kagome lattice and evolution under Rydberg blockade creates frustrated quantum states with no local order. The onset of a quantum spin liquid phase of the paradigmatic toric code type is detected by evaluating topological string operators that provide direct signatures of topological order and quantum correlations. Its properties are further revealed by using an atom array with nontrivial topology, representing a first step towards topological encoding. Our observations enable the controlled experimental exploration of topological quantum matter and protected quantum information processing.

In a quantum universe with a strong arrow of time, we postulate a low-entropy boundary condition (the Past Hypothesis) to account for the temporal asymmetry. In this paper, I show that the Past Hypothesis also contains enough information to simplify the quantum ontology and define a natural initial condition. First, I introduce Density Matrix Realism, the thesis that the quantum state of the universe is objective and impure. This stands in sharp contrast to Wave Function Realism, the thesis that the quantum state of the universe is objective and pure. Second, I suggest that the Past Hypothesis is sufficient to determine a natural density matrix, which is simple and unique. This is achieved by what I call the Initial Projection Hypothesis: the initial density matrix of the universe is the (normalized) projection onto the Past Hypothesis subspace (in the Hilbert space). Third, because the initial quantum state is unique and simple, we have a strong case for the Nomological Thesis: the initial quantum state of the universe is on a par with laws of nature. This new package of ideas has several interesting implications, including on the harmony between statistical mechanics and quantum mechanics, theoretical unity of the universe and the subsystems, and the alleged conflict between Humean supervenience and quantum entanglement., Minor changes to conform to the published version

Quantum computers are at the forefront of computing; however, few people understand how they work and their capabilities. We present two versions of an interactive activity designed for high school students (ages 13 to 18) that introduce a core quantum concept—entanglement. The first version illustrates a simple connection between two particles, and the second explores different ways that two particles could be entangled. This activity works well for entry-level quantum computing learning and requires minimal materials.

A quantum antenna consisting of a set of two-level atoms situated on a straight line and prepared in different initial states is investigated. It is shown that radiation in defined directions can be totally suppressed if the initial state of the quantum antenna is a symmetric Dicke state. The probability of simultaneous detection of photons in these directions is equal to zero. This result is impossible for the pseudo-classical symmetric initial state of the antenna that does not have quantum entanglement. The probability of detecting a photon at any angle is always nonzero for such a state. The qualitative difference of the interference mechanism for the pseudo-classical and quantum antenna states is described.

By exploiting the subblock structure of a tensor product code, we provide an upper bound on the maximum correctable burst length of tensor product codes used with interleavers designed based on only the minimum distance and the maximum correctable burst length of its component codes, without taking the specific code geometry and soft decoding into consideration. We provide necessary and sufficient conditions for an interleaver to achieve this upper bound. Further, for two cases of the tensor product codes, we design optimal intracodeword interleavers that satisfy the proposed conditions.

The enigmatic nonlocal quantum correlation that was famously derided by Einstein as "spooky action at a distance" has now been experimentally demonstrated to be authentic. The quantum entanglement and nonlocal correlations emerged as inevitable consequences of John Bell's epochal paper on Bell's inequality. However, in spite of some extraordinary applications as well as attempts to explain the reason for quantum nonlocality, a satisfactory account of how Nature accomplishes this astounding phenomenon is yet to emerge. A cogent mechanism for the occurrence of this incredible event is presented in terms of a plausible quantum mechanical Einstein-Rosen bridge., Comment: 7 pages

Objective: Four laboratory studies and an online experiment explored psychophysical (mind-matter) interactions with quantum entangled photons. Method: Entanglement correlation strength measured in real-time was presented via a graph or dynamic images displayed on a computer monitor or web browser. Participants were tasked with mentally influencing that metric. Results: A statistically significant increase in entanglement strength was obtained in experimental conditions in the four lab studies (p < 0.02), with particularly strong results observed in three studies conducted at the Institute of Noetic Sciences (p < 0.0002). Modest results (p < 0.05) were observed in a high-quality subset of entanglement samples in an online experiment. Control experiments using the same equipment and protocols, but without observers present, showed results consistent with chance expectation in both the lab and online studies. Conclusion: These outcomes suggest that the fidelity of entangled states and the nonlocal resource they entail may be mutable in systems that include conscious awareness. This is potentially of interest for quantum information technologies such as quantum computation, encryption, key distribution, and teleportation. The results are also relevant for interpretations of quantum theory, especially if future studies show that entanglement strength can be mentally modulated above the Tsirelson Bound – the upper limit predicted by quantum theory. Such an outcome would suggest that quantum theory in its present form does not hold when physical systems interact with certain mental states. The results of these exploratory experiments justify continued investigation of entangled photons as targets of mind-matter interaction.

A coherent ensemble of spins interfaced with a proxy qubit is an attractive platform to create many-body coherences and probe the regime of collective excitations. An electron spin qubit in a semiconductor quantum dot can act as such an interface to the dense nuclear spin ensemble within the quantum dot consisting of multiple high-spin atomic species. Earlier work has shown that the electron can relay properties of its nuclear environment through the statistics of its mean-field interaction with the total nuclear polarization, namely its mean and variance. Here, we demonstrate a method to probe the spin state of a nuclear ensemble that exploits its response to collective spin excitations, enabling a species-selective reconstruction beyond the mean field. For the accessible range of optically prepared mean fields, the reconstructed populations indicate that the ensemble is in a non-thermal, correlated nuclear state. The sum over reconstructed species-resolved polarizations exceeds the classical prediction threefold. This stark deviation follows from a spin ensemble that contains inter-particle coherences, and serves as an entanglement witness that confirms the formation of a dark many-body state. Atoms in a semiconductor can have non-zero nuclear spins, creating a large ensemble with many quantum degrees of freedom. An electron spin coupled to the nuclei of a semiconductor quantum dot can witness the creation of entanglement within the ensemble.

Quantum computing is an emerging paradigm with the potential to offer significant computational advantage over conventional classical computing by exploiting quantum-mechanical principles such as entanglement and superposition. It is anticipated that this computational advantage of quantum computing will help to solve many complex and computationally intractable problems in several areas such as drug design, data science, clean energy, finance, industrial chemical development, secure communications, and quantum chemistry. In recent years, tremendous progress in both quantum hardware development and quantum software/algorithm have brought quantum computing much closer to reality. Indeed, the demonstration of quantum supremacy marks a significant milestone in the Noisy Intermediate Scale Quantum (NISQ) era - the next logical step being the quantum advantage whereby quantum computers solve a real-world problem much more efficiently than classical computing. As the quantum devices are expected to steadily scale up in the next few years, quantum decoherence and qubit interconnectivity are two of the major challenges to achieve quantum advantage in the NISQ era. Quantum computing is a highly topical and fast-moving field of research with significant ongoing progress in all facets. This article presents a comprehensive review of quantum computing literature, and taxonomy of quantum computing. Further, the proposed taxonomy is used to map various related studies to identify the research gaps. A detailed overview of quantum software tools and technologies, post-quantum cryptography and quantum computer hardware development to document the current state-of-the-art in the respective areas. We finish the article by highlighting various open challenges and promising future directions for research.

Quantum teleportation, the faithful transfer of an unknown input state onto a remote quantum system1, is a key component in long-distance quantum communication protocols2 and distributed quantum computing3,4. At the same time, high-frequency nano-optomechanical systems5 hold great promise as nodes in a future quantum network6, operating on-chip at low-loss optical telecom wavelengths with long mechanical lifetimes. Recent demonstrations include entanglement between two resonators7, a quantum memory8 and microwave-to-optics transduction9–11. Despite these successes, quantum teleportation of an optical input state onto a long-lived optomechanical memory is an outstanding challenge. Here we demonstrate quantum teleportation of a polarization-encoded optical input state onto the joint state of a pair of nanomechanical resonators. Our protocol also allows to store and retrieve an arbitrary qubit state onto a dual-rail encoded optomechanical quantum memory. This work demonstrates the full functionality of a single quantum repeater node and presents a key milestone towards applications of optomechanical systems as quantum network nodes. Quantum teleportation of a photonic qubit into mechanical modes of two silicon photonic crystal nanobeams is demonstrated. It allows to store and retrieve an arbitrary qubit state onto a dual-rail encoded long-lived optomechanical quantum memory.

In this work, we investigate the Schrödinger dynamics of photon excitation numbers and entanglement in a system composed by two non-resonant time-dependent coupled oscillators. By considering π periodically pumped parameters (oscillator frequencies and coupling) and using suitable transformations, we show that the quantum dynamics can be determined by two classical Meissner oscillators. We then study analytically the stability of these differential equations and the dynamics of photon excitations and entanglement in the quantum system numerically. Our analysis shows two interesting results, which can be summarized as follows: (i) Classical instability of classical analog of quantum oscillators and photon excitation numbers (expectations Nj) are strongly correlated, and (ii) photon excitations and entanglement are connected to each other. These results can be used to shed light on the link between quantum systems and their classical counterparts and provide a nice complement to the existing works studying the dynamics of coupled quantum oscillators.

A relational bipartite communication problem is presented that has an efficient quantum simultaneous-messages protocol, but no efficient classical two-way protocol.

We look into multipartite quantum states on which quantum cryptographic protocols including quantum key distribution and quantum secret sharing can be perfectly performed, and define the quantum cryptographic resource distillable rate as the asymptotic rate at which such multipartite state can be distilled from a given multipartite state. Investigating several relations between entanglement and the rate, we show that there exists a multipartite bound entangled state whose quantum cryptographic resource distillable rate is strictly positive, that is, there exists a multipartite entangled state which is not distillable, but can be useful for quantum cryptography such as quantum key distribution and quantum secret sharing., 9 pages, 2 figures

Image processing with any potential quantum computing hardware requires a quantum color model capable of capturing and manipulating color information in images. In this study, a quantum hue, saturation, and lightness (QHSL) model is proposed as a first attempt to encode perceptually relevant triplet color components using the properties of quantum mechanics (i.e., entanglement and parallelism). The proposed color model was used to define a representation of two-dimensional QHSL images for storage and transformation with fewer computing resources. The configuration of color-assignment attributes within this QHSL representation offer useful applications for image analysis. Specifically, a pseudocolor technique with flexible gray depth divisions is presented for highlighting fine visual details.

In this paper, by using entanglement swapping of Bell states, an efficient quantum private comparison(QPC) protocol with a semi-honest party is proposed. The semi-honest third party (TP) is required to help two participants perform the comparison. She can record intermediate results and do some calculations in the whole process of the protocol execution, but she is not allowed to conspire with any participants. Moreover, TP cannot get two participants’ privacy information except the comparison results. The security analysis shows that the proposed protocol can resist both outsider attacks and insider attacks. Compared with other similar QPC protocols, the proposed one needs neither unitary operations nor the entanglement swapping of multiparticle quantum states, and it only needs the entanglement swapping of Bell states, which makes it more practical. Since three-bit classical information could be compared in each comparison, the proposed protocol has a good performance in its efficiency.

Combining the ideas of quantum state sharing and multi-party quantum remote control, a novel four-participant quantum rotation operation sharing protocol was proposed, which utilizes pre-shared entanglement and local operations. Concretely, using 4-qubit entanglement of Greenberger-Home-Zeilinger (GHZ) type state as quantum channel, the two senders of the protocol can remotely and independently implement various rotation operations on the target state of any site, which requires mutual cooperation between two agents. Moreover, by utilizing (M + N)-qubit GHZ type state as the entanglement channel, the above four-participant situation can be easily generalized to the scenario with M senders and N agents.

Indeterminacy associated with probing of a quantum state is commonly expressed through spectral distances (metric) featured in the outcomes of repeated experiments. Here we express it as an effective amount (measure) of distinct outcomes instead. The resulting $\mu$-uncertainties are described by the effective number theory [1] whose central result, the existence of a minimal amount, leads to a well-defined notion of intrinsic irremovable uncertainty. We derive $\mu$-uncertainty formulas for arbitrary set of commuting operators, including the cases with continuous spectra. The associated entropy-like characteristics, the $\mu$-entropies, convey how many degrees of freedom are effectively involved in a given measurement process. In order to construct quantum $\mu$-entropies, we are led to quantum effective numbers designed to count independent, mutually orthogonal states effectively comprising a density matrix. This concept is basis-independent and leads to a measure-based characterization of entanglement., Comment: 14 pages; v2: typos fixed, minor improvements; v3: 18 pages, appendices added, published version

The most important problem of spectroscopic chiral analysis is the enantioselective effects of the light-molecule interactions are inherently weak and severely reduced by the environment noises. Enormous efforts had been spent to overcome this problem by enhancing the symmetry break in the light-molecule interactions or reducing the environment noises. Here, we propose an alternative way to solve this problem by using frequency-entangled two-photon pairs as probe signals and detecting them in coincidence, i.e., using quantum chiral spectroscopy. For this purpose, we develop the theory of entanglement-assisted quantum chiral spectroscopy. Our results show that the quantum spectra of the left- and right-handed molecules are always distinguishable by suitably configuring the frequency-entangled two-photon pairs. In contrast, the classical spectra of the two enantiomers, where the broadband signal photon is frequency-uncorrelated with the idle one, become indistinguishable in the strong dissipation region. This offers our quantum chiral spectroscopy a great advantage over the classical chiral spectroscopy. Our work opens up an exciting area that exploring profound advantages of the quantum spectroscopy in chiral analysis.

Quantum computing will change the way we tackle certain problems. It promises to dramatically speed-up many chemical, financial, and machine-learning applications. However, to capitalize on those promises, complex design flows composed of steps such as compilation, decomposition, or mapping need to be employed before being able to execute a conceptual quantum algorithm on an actual device. This results in descriptions at various levels of abstraction which may significantly differ from each other. The complexity of the underlying design problems necessitates to not only provide efficient solutions for the single steps, but also to verify that the originally intended functionality is preserved throughout all levels of abstraction. This motivates methods for equivalence checking of quantum circuits. However, most existing methods are inspired by the classical realm and have merely been extended to support quantum circuits (i.e., circuits which do not only rely on 0's and 1's, but also employ superposition and entanglement). In this work, we propose an advanced methodology which takes the different paradigms of quantum circuits not only as a burden, but as an opportunity. In fact, the proposed methodology explicitly utilizes characteristics unique to quantum computing in order to overcome the shortcomings of existing approaches. We show that, by exploiting the reversibility of quantum circuits, complexity can be kept feasible in many cases. Moreover, we show that, in contrast to the classical realm, simulation is very powerful in verifying quantum circuits. Experimental evaluations confirm that the resulting methodology allows one to conduct equivalence checking dramatically faster than ever before--in many cases just a single simulation run is sufficient. An implementation of the proposed methodology is publicly available at https://iic.jku.at/eda/research/quantum_verification/., 14 pages, 10 figures

We consider a parametric oscillation driven by coherent light beams and coupled to a two-mode thermal reservoir. Applying the solutions of the quantum Langevin equations, we investigate the effect of decoherence and the driving coherent light beams on the squeezing and entanglement as well as statistical properties of the two-mode cavity radiation. We show that the two-mode thermal noise and the driving coherent light beams lead to an increase in the intensity of the cavity radiation. However, the driving coherent light beams do not have any effect on the squeezing and entanglement properties of the two-mode cavity radiation while the two-mode thermal noise leads to a decrease in the degree of squeezing and entanglement of the two-mode cavity radiation.

The interaction between two modes quantized field and asymmetric two atoms represents one of the important quantum models to study the nonclassical quantized phenomena. The two modes multiphoton processes and interacting asymmetric two two-level atoms are introduced in the proposed quantum system. The time dependent solution of the proposed quantum system is computed analytically for asymmetric and symmetric cases. This analytical solution depends on the eigenvalues and the eigenvectors of the coefficient matrix of the Schrodinger equation. The nonclassical features of the proposed quantum system are evaluated as the entanglement and the atomic populations. The entanglement between atom and field is measured via the von Neumann entropy while the entanglement between two atoms is measured by the concurrence. These quantized features are analyzed for variety of the detuning parameter, the atom-atom coupling constant, and the numbers of two photon modes. The most important observations are that: If the numbers of two photon modes are identical, the von Neumann entropy is strong most of the time, the periodic time of the entropy and atomic populations is double comparing with other cases, and the concurrence is non-existent for asymmetric case. In the presence the detuning parameter and the atom-atom coupling constant, the von Neumann entropy increases with time while the amplitude of the concurrence decreases with time for symmetric case. The fluctuations of the von Neumann entropy for symmetric case are stable and uniform more than its for asymmetric case. The concurrence is irregular for asymmetric case. Furthermore, there are other quantized effects of the proposed model which are discussed for symmetric and asymmetric cases.

Quantum computers offer huge potential performance, and biological computers can revolutionize pharmaceuticals. However, simple “engineering” descriptions and standardized approaches are needed. We provide “layperson” descriptions of quantum and biological computer architectures, comparisons with digital computers, and discussions of industry-standard models.

Quantum private information retrieval (QPIR) is a protocol in which a user retrieves one of multiple files from $ \mathsf {n}$ non-communicating servers by downloading quantum systems without revealing which file is retrieved. As variants of QPIR with stronger security requirements, symmetric QPIR is a protocol in which no other files than the target file are leaked to the user, and $ \mathsf {t}$ -private QPIR is a protocol in which the identity of the target file is kept secret even if at most $ \mathsf {t}$ servers may collude to reveal the identity. The QPIR capacity is the maximum ratio of the file size to the size of downloaded quantum systems, and we prove that the symmetric $ \mathsf {t}$ -private QPIR capacity is $\min \{1,2( \mathsf {n}- \mathsf {t})/ \mathsf {n}\}$ for any $1\leq \mathsf {t} . We construct a capacity-achieving QPIR protocol by the stabilizer formalism and prove the optimality of our protocol. The proposed capacity is greater than the classical counterpart.

Quantum certainty mechanics is a theory for measuring the position and momentum of a particle. Mathematically proven certainty principle from uncertainty principle, which is basically one of the most important formulas of quantum certainty mechanics theory. The principle of uncertainty can be proved by the principle of certainty and why uncertainty comes can also be proved. The principle of certainty can be proved from the theory of relativity and in the uncertainty principle equation, the principle of certainty can be proved by fulfilling the conditions of the principle of uncertainty by multiplying the uncertain constant with the certain values of momentum-position and energy-time. The principle of certainty proves that the calculation of θ ≥π/2 between the particle and the wave involved in the particle leads to uncertainty. But calculating with θ=0 does not bring uncertainty. Again, if the total energy E of the particle is measured accurately in the laboratory, the momentum and position can be measured with certainty. Quantum certainty mechanics has been established by combining Newtonian mechanics, relativity theory and quantum mechanics. Quantum entanglement can be explained by protecting the conservation law of energy.

We study two disjoint universes in an entangled pure state. When only one universe contains gravity, the path integral for the $n^{\text{th}}$ R\'enyi entropy includes a wormhole between the $n$ copies of the gravitating universe, leading to a standard "island formula" for entanglement entropy consistent with unitarity of quantum information. When both universes contain gravity, gravitational corrections to this configuration lead to a violation of unitarity. However, the path integral is now dominated by a novel wormhole with $2n$ boundaries connecting replica copies of both universes. The analytic continuation of this contribution involves a quotient by $\mathbb{Z}_n$ replica symmetry, giving a cylinder connecting the two universes. When entanglement is large, this configuration has an effective description as a "swap wormhole", a geometry in which the boundaries of the two universes are glued together by a "swaperator". This description allows precise computation of a generalized entropy-like formula for entanglement entropy. The quantum extremal surface computing the entropy lives on the Lorentzian continuation of the cylinder/swap wormhole, which has a connected Cauchy slice stretching between the universes -- a realization of the ER=EPR idea. The new wormhole restores unitarity of quantum information., Comment: 39 pages, 11 figures

Holistic Health can be understood as the coherence of the human being as a whole including his organism, mind, and relationship to his environment. Quantum entanglement of subsystems on all levels of the human being is responsible for the wholeness of the living entity. The sum total of entanglements is defined here as an individual Information Field which corresponds to the ancient vedic concept of akasha. Entangled frequencies might be able to resonate with and influence human bioenergetic processes and systems with the aim of creating coherence of the bioenergetic system, both within the individual and between the individual and the surrounding fields. Here a bioenergetic process is proposed that uses entangled frequencies to increase the coherence of living systems selected through biofeedback of a Quantum Noise Generator, which seems to be correlated to the ancient vedic concept of prana. It has been tested in a controlled observation study for its effect on the sense of coherence of 3545 participants. The promising results of this study are additionally presented here.

Quantum entanglement is one of the key methods in quantum information processing, but it is difficult to prepare quantum entanglement. Quantum walk is widely used in quantum computation and quantum simulation, it can be applied to the preparation of quantum entangled states. In this paper, a controllable quantum network coding scheme based on quantum walk is proposed. With the help of quantum walk, the scheme preliminary realized the entanglement distribution of butterfly network, reduced entanglement resources and enhanced scalability. According to the existing technology, it is feasible to implement the quantum network coding scheme proposed in this paper.

Quantum computing can be realized with numerous different hardware platforms and computational protocols. A highly promising, and potentially scalable, idea is to combine a photonic platform with measurement-induced quantum information processing. In this approach, gate operations can be implemented through optical measurements on a multipartite entangled quantum state—a so-called cluster state. Previously, a few quantum gates on non-universal or non-scalable cluster states have been performed, but a full set of gates for universal scalable quantum computing has not been realized. Here we propose and demonstrate the deterministic implementation of a multi-mode set of measurement-induced quantum gates in a large two-dimensional optical cluster state using phase-controlled continuous-variable quadrature measurements. Each gate is programmed into the phases of high-efficiency quadrature measurements, which execute the transformations by teleportation through the cluster state. We further execute a small quantum circuit consisting of 10 single-mode gates and 2 two-mode gates on a three-mode input state. Fault-tolerant universal quantum computing is possible with this platform if the cluster-state entanglement is improved and a supply of states with Gottesman–Kitaev–Preskill encoding is available. Measurement-based quantum computing performs quantum gates on entangled states without difficult multi-qubit coherent dynamics. A set of gates sufficient for universal quantum computing has now been implemented on a programmable optical platform.

By harnessing unique quantum mechanical phenomena, such as superposition and entanglement, quantum computers offer the possibility to drastically outperform classical computers for certain classes of problems. The realization of this potential, however, presents a substantial challenge, because noise and imperfections associated with the materials used to fabricate devices can obscure the delicate quantum mechanical effects that enable quantum computing. Hence, progress in synthesis, characterization, and modeling of materials for quantum computing have driven many exciting advances in recent years and will become increasingly important in the years to come. As progressively more complex, multi-qubit systems come online, and as significant government and industrial investment drives research forward, new challenges and opportunities for materials science continue to emerge. The articles in this issue survey the current state of materials science progress and obstacles for some leading quantum computing platforms; opportunities for deeper involvement by materials scientists abound. Ultimate realization of the full potential of quantum computers will require a multidisciplinary effort spanning many traditional areas of expertise.

Nuclear spins in semiconductors are leading candidates for future quantum technologies, including quantum computation, communication, and sensing. Nuclear spins in diamond are particularly attractive due to their long coherence time. With the nitrogen-vacancy (NV) centre, such nuclear qubits benefit from an auxiliary electronic qubit, which, at cryogenic temperatures, enables probabilistic entanglement mediated optically by photonic links. Here, we demonstrate a concept of a microelectronic quantum device at ambient conditions using diamond as wide bandgap semiconductor. The basic quantum processor unit – a single 14N nuclear spin coupled to the NV electron – is read photoelectrically and thus operates in a manner compatible with nanoscale electronics. The underlying theory provides the key ingredients for photoelectric quantum gate operations and readout of nuclear qubit registers. This demonstration is, therefore, a step towards diamond quantum devices with a readout area limited by inter-electrode distance rather than by the diffraction limit. Such scalability could enable the development of electronic quantum processors based on the dipolar interaction of spin-qubits placed at nanoscopic proximity., Nuclear spins in diamond are promising for applications in quantum technologies due to their long coherence times. Here, the authors demonstrate a scalable electrical readout of individual intrinsic 14N nuclear spins in diamond, mediated by hyperfine coupling to electron spin of the NV center, as a step towards room-temperature nanoscale diamond quantum devices.

We have studied carefully the behaviors of entangled qubits on the IBM Rochester with various connectivities and under a “noisy” environment. A phase trajectory analysis based on our measurements of the GHZ-like states is performed. Our results point to an important fact that entangled qubits are “protected” against environmental noise by a scaling property that impacts only the weighting of their amplitudes. The reproducibility of most measurements has been confirmed within a reasonably short gate operation time. But there still are a few combinations of qubits that show significant entanglement evolution in the form of transitions between quantum states. The phase trajectory of an entangled evolution, and the impact of the sudden death of GHZ-like states and the revival of newly excited states are analyzed in details. All observed trajectories of entangled qubits arise under the influences of the newly excited states in a “noisy” intermediate-scale quantum (NISQ) computer.

Bell’s inequality is investigated in parity-time (PT) symmetric quantum mechanics, using a recently developed form of the inequality by Maccone, with two PT-qubits in the unbroken phase with real energy spectrum. It is shown that the inequality produces a bound that is consistent with the standard quantum mechanics even after using Hilbert space equipped with CPT inner product and therefore, the entanglement has identical structure with standard quantum mechanics. Consequently, the no-signaling principle for a two-qubit system in PT-symmetric quantum theory is preserved.

The world of information technology is buzzing around the possibilities that quantum computers might open up for organizations. But what are quantum computers and what could be their business potential, eventually? Part of why quantum computers are steeped in mystery is because they rely on some very strange properties of the microcosm. From quantum interference to quantum entanglement, not to mention some of the underlying philosophical quantum implications like the possibilities for multiple universes and the foundations of quantum physics that revolve around peculiar mathematical formulations, quantum computers have been under development for decades, with IBM, D-Wave, and Google being amongst a handful of companies investing in this space.

New capabilities enabled by quantum networks, e.g. quantum key distribution, rely on the concurrent quantum entanglements via multiple untrusted repeaters between the source-destination pairs, which is known as quantum routing. Existing quantum routing algorithms pre-assign the free qubits to specific paths, in order to avoid resource contention in the process of entanglement swapping. Since not all pre-assigned qubits can be utilized, the resource fragmentation problem caused can limit the network throughout. In this paper, a new fragmentation-aware entanglement routing (FER) algorithm is proposed. With FER, the entangled qubits are shared by all the paths, and the resource contention problem is solved by a priority connection mechanism. Simulation results show that FER consistently outperforms existing algorithms under different network settings.

Entangled photon pairs have been used for molecular spectroscopy in the form of entangled two-photon absorption and in quantum interferometry for precise measurements of light source properties and time delays. We present an experiment that combines molecular spectroscopy and quantum interferometry by utilizing the correlations of entangled photons in a Hong-Ou-Mandel (HOM) interferometer to study molecular properties. We find that the HOM signal is sensitive to the presence of a resonant organic sample placed in one arm of the interferometer, and the resulting signal contains information pertaining to the light-matter interaction. We can extract the dephasing time of the coherent response induced by the excitation on a femtosecond time scale. A dephasing time of 102 fs is obtained, which is relatively short compared to times found with similar methods and considering line width broadening and the instrument entanglement time As the measurement is done with coincidence counts as opposed to simply intensity, it is unaffected by even-order dispersion effects, and because interactions with the molecular state affect the photon correlation, the observed measurement contains only these effects and no other classical losses. The experiments are accompanied by theory that predicts the observed temporal shift and captures the entangled photon joint spectral amplitude and the molecule's transmission in the coincidence counting rate. Thus, we present a proof-of-concept experimental method based of entangled photon interferometry that can be used to characterize optical properties in organic molecules and can in the future be expanded on for more complex spectroscopic studies of nonlinear optical properties.

Research related to the free-space quantum information transfer is very important now. Entanglement of quantum bits ensures absolute security of data transmission. We study the ability of an atmospheric quantum channel to preserve the entanglement during the transmission of information encoded by the modes of the Gaussian beam. The operator approach is used. Namely, we estimate the quality of the entanglement transmission by finding the distance between the transformation matrix of the quantum channel and the subspace of matrices being tensor products of matrices corresponding to disentangled qubits. The technique is related to singular expansions for matrices. Teleportation algorithm is considered.

Billions of dollars have poured into research on quantum computers and sensors, but many experts say the devices will flourish only when they are yoked to each other over long distances in a "quantum internet." By taking advantage of a strange quantum mechanical property called entanglement, once derided by Albert Einstein as a "spooky distant effect," researchers can create intimate, instantaneous connections across long distances. A quantum internet could weld telescopes into arrays with ultrahigh resolution, precisely synchronize clocks, yield hypersecure communication networks for finance and elections, and make it possible to do quantum computing from anywhere. The first strands are already being woven, though a global network will depend on "quantum repeaters," still to be developed, that can keep photons entangled across thousands of kilometers of optical fiber.

We give a capacity formula for the classical communication over a noisy quantum channel, when local operations and global permutations allowed in the encoding and bipartite states preshared between the sender and the receiver. The two endpoints of this formula are the Holevo capacity (without entanglement assistance) and the entanglement-assisted capacity (with unlimited entanglement assistance). What's more, we show that the capacity satisfies the strong converse property and thus the formula serves as a sharp dividing line between achievable and unachievable rates of communication. We prove that the difference between the assisted capacity and the Holevo capacity is upper bounded by the discord of formation of the preshared state. As examples, we derive analytically the classical capacity of various quantum channels of interests. Our result witnesses the power of random permutation in classical communication, whenever entanglement assistance is available., 27 pages, 4 figures. Comments are welcome

We investigate the system-environment information flow from the point of view ofcomplete complementarity relations. We consider some commonly used noisy quantum channels:Amplitude damping, phase damping, bit flip, bit-phase flip, phase flip, depolarizing, and correlatedamplitude damping. By starting with an entangled bipartite pure quantum state, with the linearentropy being the quantifier of entanglement, we study how entanglement is redistributed and turnedinto general correlations between the degrees of freedom of the whole system. For instance, it ispossible to express the entanglement entropy in terms of the multipartite quantum coherence or interms of the correlated quantum coherence of the different partitions of the system. In addition,we notice that for the depolarizing and bit-phase flip channels the wave and particle aspects candecrease or increase together. Besides, by considering the environment as part of a pure quantumsystem, the linear entropy is shown to be not just a measure of mixedness of a particular subsystem,but a correlation measure of the subsystem with rest of the world., Comment: 19 pages

A clear relationship between the quantum version of Fisher information and the bipartite system entanglement has been explored recently. Known existing results for some quantum systems having strongly nonlocal correlations are still limited. In this paper, based on the interaction between a two-qubit and squeezed field, we investigate the correlation between the dynamical behavior of qubit-qubit entanglement and the quantum Fisher information (QFI). We focus on the effect of a number of photon multiplicity, low and high squeezing regime of the field on the dynamics of the QFI and entanglement quantifier. We show how the QFI and entanglement quantifier can be affected by the number of photon multiplicity and squeeze parameter during the time evolution of cases of absence and presence of dissipation effect. Our results exhibit the correlation between the entanglement quantifiers and the QFI with respect to the appearance of collapse and revival phenomena of the atomic inversion.