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The protection of quantum coherence is essential for building a practical quantum computer able to manipulate, store and read quantum information with a high degree of fidelity. Recently, it has been proposed to increase the operation time of a qubit by means of strong pulses to achieve a dynamical decoupling of the qubit from its environment. We propose and demonstrate a simple and highly efficient alternative route based on Floquet modes, which increases the Rabi decay time ( T R ) in a number of materials with different spin Hamiltonians and environments. We demonstrate the regime T R ≈ T 1 with T 1 the relaxation time, thus providing a route for spin qubits and spin ensembles to be used in quantum information processing and storage. [ABSTRACT FROM AUTHOR]

During service hot forging dies are exposed to a combination of cyclic thermo-mechanical, tribological and chemical loads. Besides abrasive and adhesive wear on the die surface, fatigue crack initiation with subsequent fracture is one of the most frequent cause of failure. Fatigue cracks in forging dies are caused by local elasto-plastic strains due to cyclic mechanical and thermal loads. Aiming at a reduction of tool and production costs of forgings, tool life extension is necessary. In order to reach this goal, process design and process optimisation are currently done with the help of the finite element analysis (FEA). An integral part of this procedure is the tool design based upon fatigue life maximisation of forging dies in the Low-Cycle-Fatigue (LCF) region. Within the framework of this research the material fatigue damage of a forging die due to cyclic thermo-mechanical loads is analysed based on FEA process simulations. Besides this a material model which considers cyclic elasto-plastic material behaviour was used to describe the constitutive behaviour of the forging dies. The results of a fatigue life damage analysis applied to an industrial forging process in which the lower die shows a clear fatigue crack initiation. The crack initiation sites predicted with the FEA process simulations agree with the ones observed on the real die component. [ABSTRACT FROM AUTHOR]

Motivated by recent experimental observations of size quantization of electron energy levels in graphene quantum dots [7] we investigate the level statistics in the simplest tight-binding model for different dot shapes by computer simulation. The results are in a reasonable agreement with the experiment which confirms qualitatively interpretation of observed level statistics in terms of “Dirac billiards” without taking into account many-body effects. It is shown that edge effects are in general sufficient to produce the observed level distribution and that even strong bulk disorder does not change the results drastically. [ABSTRACT FROM AUTHOR]

The general conclusion of Seevinck and Larsson is that our model exploits the so-called coincidence-time loophole and produces sinusoidal (quantum-like) correlations but does not model the singlet state because it does not violate the relevant Bell inequality derived by Larsson and Gill, since in order to obtain the sinusoidal correlations the probability of coincidences in our model goes to zero. In this reply, we refute their arguments that lead to this conclusion and demonstrate that our model can reproduce results of photon and ion-trap experiments with frequencies of coincidences that are not in conflict with the observations. [ABSTRACT FROM AUTHOR]

Decoherence in a system of two antiferromagnetically coupled spins that interact with a spin bath environment is studied. Systems are considered that range from the rotationally invariant to highly anisotropic spin models and have different topologies and values of parameters that are fixed or are allowed to fluctuate randomly. We explore the conditions under which the two-spin system clearly shows an evolution from the initial spin-up-spin-down state towards the maximally entangled singlet state. We demonstrate that frustration and, especially, glassiness of the spin environment strongly enhances the decoherence of the two-spin system. [ABSTRACT FROM AUTHOR]

Can quantum correlations of the singlet state be produced by two separate subsystems which have interacted in the past but do not communicate? We show that, using a locally causal realist model of the Einstein-Podolsky-Rosen-Bohm experiment, the answer is affirmative if coincidence in time is used to decide which detection events are stemming from a single two-particle system, the criterion employed in all experimental realizations of the Einstein-Podolsky-Rosen-Bohm gedanken experiment. [ABSTRACT FROM AUTHOR]

We study effects of the physical realization of quantum computers on their logical operation. Through simulation of physical models of quantum computer hardware, we analyze the difficulties that are encountered in programming physical realizations of quantum computers. Examples of logically identical implementations of the controlled-NOT operation and Grover's database search algorithm are used to demonstrate that the results of a quantum computation are unstable with respect to the physical realization of the quantum computer. We discuss the origin of these instabilities and discuss possibilities to overcome this, for practical purposes, fundamental limitation of quantum computers. [ABSTRACT FROM AUTHOR]

In the Results and discussion section, under the subheading "Qubit dynamics", "The general condition is HT ht showing a comensurate motion of the qubit and HT ht on the Bloch sphere." now reads: "The general condition is HT ht showing a comensurate motion of the qubit and HT ht on the Bloch sphere." Correction to: I Scientific Reports i https://doi.org/10.1038/s41598-020-77047-5, published online 10 December 2020 The original version of this Article contained an error. [Extracted from the article]