Epitaxial superconductor-semiconductor heterostructures combine superconductivity with strong spin-orbit interaction resulting in synthetic Rashba superconductors. The theoretical description of such superconductors involves Lifshitz invariants that are predicted to feature numerous exotic effects with so far sparse experimental evidence. Using a new observable—vortex inductance—we investigate the pinning properties of epitaxial Al=InAs-based heterostructures. We find a pronounced decrease of the vortex inductance with increasing in-plane field which corresponds to a counterintuitive increase of the pinning force. When rotating the in-plane component of the field with respect to the current direction, the pinning interaction turns out to be highly anisotropic. We analytically demonstrate that both the pinning enhancement and its anisotropy are consequences of the presence of Lifshitz invariant terms in the Ginzburg-Landau free energy. Hence, our experiment provides access to a fundamental property of Rashba superconductors and offers an entirely new approach to vortex manipulation.

Nobelovu cenu za fyziku v tomto roku získali Francúz Alain Aspect, Američan John Clauser a Rakúšan Anton Zeilinger za experimenty s kvantovo previazanými fotónmi. Príbeh kvantového previazania začal fyzik, ktorý dostal Nobelovu cenu pred 101 rokmi. Viedli ho k tomu dlhoročné diskusie s fyzikom, ktorý získal Nobelovu cenu pred 100 rokmi. Ale k tomu sa ešte dostaneme ...

We investigate robustness of probabilistic storage and retrieval device optimized for phase gates to noise. We use noisy input composed of convex combination of unitary channel with either depolarizing or dephasing channel. We find out that the resistance to dephasing noise is higher than to depolarization. Interestingly, for the depolarization the retrieval reduces the degree of noise. We also examine the possible realizations showing that their performance is different when the noise is present.

The necessary and sufficient criteria for violating the Mermin and Svetlichny inequalities by arbitrary three-qubit states are presented. Several attempts have been made, earlier, to find such criteria, however, those extant criteria are neither tight for most of the instances, nor fully general. We generalize the existing criteria for Mermin and Svetlichny inequalities which are valid for the local projective measurement observables as well as for the arbitrary ones. We obtain the maximal achievable bounds of the Mermin and Svetlichny operators with unbiased measurement observables for arbitrary three-qubit states and with arbitrary observables for three-qubit states having maximally mixed marginals. We find that for certain ranges of measurement strengths, it is possible to violate Mermin and Svetlichny inequalities only by biased measurement observables. The necessary and sufficient criteria of violating any one of the six possible Mermin and Svetlichny inequalities are also derived.

Quantum uncertainty relations are typically analyzed for a pair of incompatible observables, however, the concept per se naturally extends to situations of more than two observables. In this work, we obtain tripartite quantum-memory-assisted entropic uncertainty relations for multiple measurements and show that the lower bounds of these relations have three terms that depend on the complementarity of the observables, the conditional von-Neumann entropies, the Holevo quantities, and the mutual information. The saturation of these inequalities is analyzed.

The uncertainty principle bounds the measurement precision of two complementary variables and the bipartite scenario of quantum-memory-assisted entropic uncertainty relation (QMA-EUR) has been extensively studied. We investigate the tight uncertainty bound (UB) of a tripartite scenario of QMA-EUR in a system consisting of three qubits coupled either to their independent random telegraph noises (RTNs) or to a common source of RTN. For both the initial GHZ- and W -type states, the results show that the UB always increases monotonically with time in the Markovian regime and oscillates in the non-Markovian regime. Moreover, the UB can be significantly reduced due to the non-Markovian effects in the finite-time region and approaches the same asymptotic values in the infinite-time limit for both Markovian and non-Markovian cases.

Decoherence of quantum systems is described by quantum channels. However, a complete understanding of such channels, especially in the multiparticle setting, is still an ongoing difficult task. We propose the family of quantum maps that preserve or completely erase the components of a multiqubit system in the basis of Pauli strings, which we call Pauli component erasing maps. For the corresponding channels, it is shown that the preserved components can be interpreted as a finite vector subspace, from which we derive several properties and complete the characterization. Moreover, we show that the obtained family of channels forms a semigroup and derive its generators. We use this simple structure to determine physical implementations and connect the obtained family of channels with Markovian processes.

Povieme si, aké sú pravidlá, ktoré sa používajú, keď hovoríme o kvantovom počítaní a ako súvisia s fyzikálnymi vlastnosťami kvantových systémov.

Povieme si, ako teoreticky vieme kvantové pravidlá použiť na počítanie. O kvantových výpočtoch hovoríme, keď namiesto núl a jednotiek používame kvantové bity a namiesto logických hradiel hradlá kvantové. Povieme si ako funguje kvantový výpočet akejkoľvek funkcie a ukážeme si, že niektoré z týchto výpočtov vieme spraviť na kvantových počítačoch výrazne rýchlejšie. Popritom sa dozvieme, že existujú elementárne hradlá, z ktorých vieme poskladať akýkoľvek výpočet, a preto môžeme hovoriť o univerálnom kvantovom počítači, že vymyslieť kvantový algoritmus vôbec nie je jednoduché, ale niektoré algoritmy si ukážeme, a že chyby pri kvantovom počítaní vieme opraviť.

Povieme si ako sú na tom súčasné kvantové procesory po praktickej a technickej stránke, a ako to zapadá do oblasti vysokovýkonného počítania. Existuje množstvo platforiem, z ktorých každá má svoje prednosti a svoje nedostatky. Avšak zatiaľ všetky oplývajú vysokou chybovosťou a malým množstvom použiteľných qubitov. V dnešnej dobe je tak potrebné hľadať cestu, ako s týmito obmedzeniami pracovať. To sa deje jednak optimalizáciou na úrovni samotných výpočtov a jednak na algoritmickej úrovni, kde hľadáme (nenáročné) výpočty, pri ktorých dané obmedzenia nie sú zničujúce, ale zároveň pri ktorých môžeme očakávať aspoň čiastočné výhody (napr. v urýchlení výpočtov).

Povieme si, kedy a kde má zmysel kvantové výpočty používať, a ktoré problémy a systémy bude ťažké počítať aj na počítačoch kvantových.

The ability to know and verifiably demonstrate the origins of messages can often be as important as encrypting the message itself. Here we present an experimental demonstration of an unconditionally secure digital signature (USS) protocol implemented for the first time, to the best of our knowledge, on a fully connected quantum network without trusted nodes. We choose a USS protocol which is secure against forging, repudiation and messages are transferrable. We show the feasibility of unconditionally secure signatures using only bi-partite entangled states distributed throughout the network and experimentally evaluate the performance of the protocol in real world scenarios with varying message lengths.

Second order quantum phase transitions, with well-known features such as long-range entanglement, symmetry breaking, and gap closing, exhibit quantum enhancement for sensing at criticality. However, it is unclear which of these features are responsible for this enhancement. To address this issue, we investigate phase transitions in free-fermionic topological systems that exhibit neither symmetry-breaking nor long-range entanglement. We analytically demonstrate that quantum enhanced sensing is possible using topological edge states near the phase boundary. Remarkably, such enhancement also endures for ground states of such models that are accessible in solid state experiments. We illustrate the results with 1D Su-Schrieffer-Heeger chain and a 2D Chern insulator which are both experimentally accessible. While neither symmetry-breaking nor long-range entanglement are essential, gap closing remains as the major candidate for the ultimate source of quantum enhanced sensing. In addition, we also provide a fixed and simple measurement strategy that achieves near-optimal precision for sensing using generic edge states irrespective of the parameter value. This paves the way for development of topological quantum sensors which are expected to also be robust against local perturbations.

This Technical Review collects values of selected performance characteristics of semiconductor spin qubits defined in electrically controlled nanostructures. The characteristics are envisaged to serve as a community source for the values of figures of merit with agreed definitions allowing the comparison of different spin-qubit platforms. We include characteristics on the qubit coherence, speed, fidelity and qubit size of multi-qubit devices. The focus is on collecting and curating the values of these characteristics as reported in the literature, rather than on their motivation or significance.

Semiconductor quantum dots in which electrons or holes are isolated via electrostatic potentials generated by surface gates are promising building blocks for semiconductor-based quantum technology. Here, we investigate double-quantum-dot (DQD) charge qubits in GaAs capacitively coupled to high-impedance superconducting quantum interference device array and Josephson-junction array resonators. We tune the strength of the electric-dipole interaction between the qubit and the resonator in situ using surface gates. We characterize the qubit-resonator coupling strength, the qubit decoherence, and the detuning noise affecting the charge qubit for different electrostatic DQD configurations. We find all quantities to be systematically tunable over more than one order of magnitude, resulting in reproducible decoherence rates &Gamma₂=2π < 5 MHz in the limit of high interdot capacitance. In the opposite limit, by reducing the interdot capacitance, we increase the DQD electric-dipole strength and, therefore, its coupling to the resonator. Employing a Josephson-junction array resonator with an impedance of approximately 4 kΩ and a resonance frequency of ωr =2π ∼ 5.6 GHz, we observe a coupling strength of g=2π ∼ 630 MHz, demonstrating the possibility to operate electrons hosted in a semiconductor DQD in the ultrastrong-coupling regime (USC). The presented results are essential for further increasing the coherence of quantum-dot-based qubits and investigating USC physics in semiconducting QDs.

The isotropic Heisenberg two-spin-1/2 model in the XXX configuration under an external transverse nonuniform magnetic field is considered at thermal equilibrium. In the context of the Heitler–London approach, the variation of the spin–spin exchange coupling strength in terms of the position is adopted. The effects of the inter-spin relative coupling distance r and nonuniform magnetic field on the thermal evolution of quantum correlations are studied in detail. By tuning the coupling distance r , temperature T and nonuniform magnetic field B, quantum correlations can be scaled in the bipartite system. Astonishingly, we find the long sustainable behavior of geometric quantum discord in comparison with entanglement over the coupling distance r. Moreover, we show the existence of separable quantum states with nonzero quantum correlations in terms of trace discord. Besides, the quantum correlations shared between the considered bipartite system parts are only the entanglement type for a fixed temperature and suitable strong nonuniform magnetic field values. An entangled-unentangled phase transition at T = 0.1 with threshold relative distance rf can be detected by the entanglement behavior in terms of B and r. A kind of correspondence between thermal entanglement and thermal non-classical correlations for a strong value of B can be observed. It is our hope that this research may open a new path to consider the role of Heitler–London approach in non-classical correlations preservation.

Shaping single-mode operation in high-power fibers requires a precise knowledge of the gain-medium optical properties. This requires precise measurements of the refractive index differences (Δn) between the core and the cladding of the fiber. We exploit a quantum optical method based on low-coherence Hong-Ou-Mandel interferometry to perform practical measurements of the refractive index difference using broadband energy-time entangled photons. The precision enhancement reached with this method is benchmarked with a classical method based on single photon interferometry. We show in classical regime an improvement by an order of magnitude of the precision compared to already reported classical methods. Strikingly, in the quantum regime, we demonstrate an extra factor of 4 on the precision enhancement, exhibiting a state-of-the-art Δn precision of 6×10⁻⁷ . This work sets the quantum photonics metrology as a powerful characterization tool that should enable a faster and reliable design of materials dedicated to light amplification.

We study classical Ising spin-1/2 models on a two-dimensional (2D) square lattice with ferromagnetic or antiferromagnetic nearest-neighbor interactions, under the effect of a pure imaginary magnetic field. The complex Boltzmann weights of spin configurations cannot be interpreted as a probability distribution, which prevents application of standard statistical algorithms. In this work, the mapping of the Ising spin models under consideration onto symmetric vertex models leads to real (positive or negative) Boltzmann weights. This enables us to apply accurate numerical methods based on the renormalization of the density matrix, namely, the corner transfer matrix renormalization group and the higher-order tensor renormalization group. For the 2D antiferromagnet, varying the imaginary magnetic field, we calculate with high accuracy the curve of critical points related to the symmetry breaking of magnetizations on the interwoven sublattices. The critical exponent β and the anomaly number C are shown to be constant along the critical line, equal to their values β=1/8 and C=1/2 for the 2D Ising model in a zero magnetic field. The 2D ferromagnets behave in analogy with their 1D counterparts defined on a chain of sites, namely, there exists a transient temperature which splits the temperature range into its high-temperature and low-temperature parts. The free energy and the magnetization are well defined in the high-temperature region. In the low-temperature region, the free energy exhibits singularities at the Yang-Lee zeros of the partition function and the magnetization is also ill-defined: It varies chaotically with the size of the system. The transient temperature is determined as a function of the imaginary magnetic field by using the fact that from the high-temperature side both the first derivative of the free energy with respect to the temperature and the magnetization diverge at this temperature.

Bilayer graphene has two nonequivalent sublattices and, therefore, the same adatom impurity can manifest in spectrally distinct ways—sharp versus broad resonances near the charge neutrality—depending on the sublattice it adsorbs at. Employing Green's function analytical methods and the numerical kwant package, we investigate the spectral and transport interplay between the resonances and superconducting coherence induced in bilayer graphene by proximity to an s-wave superconductor. Analyzing doping and temperature dependencies of quasiparticle spin-relaxation rates, energies of Yu-Shiba-Rusinov states, Andreev spectra, and the supercurrent characteristics of Josephson junctions, we find unique superconducting signatures discriminating between resonant and off-resonant regimes. Our findings are in certain aspects going beyond the superconducting bilayer graphene and hold for generic s-wave superconductors functionalized by the resonant magnetic impurities.

Random access codes (RACs) are an intriguing class of communication tasks that reveal an operational and quantitative difference between classical and quantum information processing. We formulate a natural generalization of RACs and call them random access tests (RATs), defined for any finite collection of measurements in an arbitrary finite dimensional general probabilistic theory. These tests can be used to examine collective properties of collections of measurements. We show that the violation of a classical bound in a RAT is a signature of either measurement incompatibility or super information storability. The polygon theories are exhaustively analysed and a critical difference between even and odd polygon theories is revealed.

Quantum theory sets a bound on the minimum time required to transform from an initial state to a target state. The bound is known as quantum speed limit time. Quantum speed limit time can be used to determine the rate of quantum evolution for closed and open quantum systems. In the real world, we are dealing with open quantum systems. So, the study of quantum speed limit time for open quantum systems has particular importance. In this work, we consider the topological qubit realized by two Majorana modes. We consider the case in which the topological qubit is influenced by the fermionic and bosonic environment. Fermionic and bosonic environments are assumed to have Ohmic-like spectral density. The quantum speed limit time is investigated for the various environments with different Ohmic parameters. It is observed that for the super-Ohmic environment with increasing Ohmic parameter the quantum speed limit time gradually reaches a constant value and so the speed of evolution reaches a uniform value. It is also observed that the quantum speed limit time reaches zero value by increasing initial time parameter for small value of Ohmic parameter while it reaches constant value for larger Ohmic parameter. The effects of the external magnetic field on the quantum speed limit time are also studied. It is observed that with increasing magnitude of the magnetic field, the quantum speed limit time decreases.

Quantum discord and quantum uncertainty are two important features of the quantum world. In this work, the relation between entropic uncertainty relation and the shareability of quantum discord is studied. By using tripartite quantum‑memory‑assisted entropic uncertainty relation, an upper bound for the shareability of quantum discord among different parties of a composite system is obtained. It is also shown that, for a specific class of tripartite states, the obtained relation could be expressed as monogamy of quantum discord. Moreover, it is illustrated that the relation could be generalized and an upper bound for the shareability of quantum discord for multipartite states is derived.

Anonymity in networked communication is vital for many privacy-preserving tasks. Secure key distribution alone is insufficient for high-security communications. Often, knowing who transmits a message to whom and when must also be kept hidden from an adversary. Here, we experimentally demonstrate five information-theoretically secure anonymity protocols on an eight user city-wide quantum network using polarisation entangled photon pairs. At the heart of these protocols is anonymous broadcasting, which is a cryptographic primitive that allows one user to reveal one bit of information while keeping their identity anonymous. For a network of n users, the protocols retain anonymity for the sender, given that no more than n − 2 users are colluding. This is an implementation of genuine multi-user cryptographic protocols beyond standard QKD. Our anonymous protocols enhance the functionality of any fully-connected Quantum Key Distribution network without trusted nodes.

We generalize a tensor-network algorithm to study the thermodynamic properties of self-similar spin lattices constructed on a square-lattice frame with two types of couplings, J1 and J2 , chosen to transform a regular square lattice (J1 = J2 ) onto a fractal lattice if decreasing J2 to zero (the fractal fully reconstructs when J2 = 0). We modified the higher-order tensor renormalization group (HOTRG) algorithm for this purpose. Single-site measurements are performed by means of so-called impurity tensors. So far, only a single local tensor and uniform extension-contraction relations have been considered in HOTRG. We introduce 10 independent local tensors, each being extended and contracted by 15 different recursion relations. We applied the Ising model to the J1 − J2 planar fractal whose Hausdorff dimension at J2 = 0 is d(H ) = ln 12/ ln 4 ≈ 1.792. The generalized tensor-network algorithm is applicable to a wide range of fractal patterns and is suitable for models without translational invariance.

A variant of energy scale deformation is considered for the S = 1/2 antiferromagnetic Heisenberg model on polyhedra. The deformation is induced by the perturbations to the uniform Hamiltonian, whose coefficients are determined by the bond coordinates. On the tetrahedral, octahedral, and cubic clusters, the perturbative terms do not affect the ground state of the uniform Hamiltonian when they are sufficiently small. On the other hand, for the icosahedral and dodecahedral clusters, it is numerically confirmed that the ground state of the uniform Hamiltonian is almost insensitive to the perturbations unless they lead to a discontinuous change in the ground state. The obtained results suggest the existence of a generalization of sine-square deformation in higher dimensions.

Quantum measurement not only can destroy coherence but also can create it. Here, we estimate the maximum amount of coherence, one can create under a complete non-selective measurement process. For our analysis, we consider projective as well as POVM measurements. Based on our observations, we characterize the measurement processes into two categories, namely, the measurements with the ability to induce coherence and the ones without this ability. Our findings also suggest that the more POVM elements present in a measurement that acts on the quantum system, the less will be its coherence creating ability. We also introduce the notion of raw coherence in the POVMs that helps to create quantum coherence. Finally, we find a trade-off relation between the coherence creation, entanglement generation between system and apparatus, and the mixedness of the system in a general measurement setup.

The uncertainty principle imposes a limitation on the measurement accuracy of two incompatible observables and has potential applications in quantum information science. We explore the bipartite entropic uncertainty relation and dense coding for two qubits coupled via dipole–dipole interaction and subject to a bosonic reservoir. It is shown that there exists a trade-off between the uncertainty bound and the dense coding capacity which results in their opposite dynamical behaviors. Moreover, the behaviors of the measurement uncertainty and the dense coding capacity rely crucially on the initial system–reservoir correlation, the strength of the dipole–dipole interaction, and the degree of non-Markovianity, and one can reduce the measurement uncertainty and enhance the dense coding capacity by tuning these parameters to appropriate values.