The isolation of qubits from noise sources, such as surrounding nuclear spins and spin-electric susceptibility , has enabled extensions of quantum coherence times in recent pivotal advances towards the concrete implementation of spin-based quantum computation. In fact, the possibility of achieving enhanced quantum coherence has been substantially doubted for nanostructures due to the characteristic high degree of background charge fluctuations . Still, a sizeable spin-electric coupling will be needed in realistic multiple-qubit systems to address single-spin and spin-spin manipulations . Here, we realize a single-electron spin qubit with an isotopically enriched phase coherence time (20 μs) and fast electrical control speed (up to 30 MHz) mediated by extrinsic spin-electric coupling. Using rapid spin rotations, we reveal that the free-evolution dephasing is caused by charge noise-rather than conventional magnetic noise-as highlighted by a 1/f spectrum extended over seven decades of frequency. The qubit exhibits superior performance with single-qubit gate fidelities exceeding 99.9% on average, offering a promising route to large-scale spin-qubit systems with fault-tolerant controllability.

The spectral density of various ensembles of sparse symmetric random matrices is analyzed using the cavity method. We consider two cases: matrices whose associated graphs are locally treelike, and sparse covariance matrices. We derive a closed set of equations from which the density of eigenvalues can be efficiently calculated. Within this approach, the Wigner semicircle law for Gaussian matrices and the Marcenko-Pastur law for covariance matrices are recovered easily. Our results are compared with numerical diagonalization, finding excellent agreement.

We analyze the internal quantum efficiency (IQE) of whole-composition-range AlGaN multiquantum wells (MQWs) on AlGaN with various dislocation densities (DDs) by excitation-density-dependent photoluminescence measurement. IQEs of deep ultraviolet/ultraviolet (DUV/UV) MQWs are strongly dependent on the DD. IQE with an excess carrier density of 1×1018 cm-3 changes from 4 to 64% when the DD changes from 6×109 to 2×108 cm-2. This trend is almost the same for DUV/UV MQWs with emission wavelength ranging from 230 to 350 nm. Thus, the reduction of the DD is very important for the realization of a high-IQE DUV/UV active layer.

This is the first experimental demonstration of a fault-tolerant spin qubit in industry-compatible isotopically natural silicon.

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Abstract. -An approach to analyze the performance of the code division multiple access (CDMA) scheme, which is a core technology used in modern wireless communication systems, is provided. The approach characterizes the objective system by the eigenvalue spectrum of a cross-correlation matrix composed of signature sequences used in CDMA communication, which enable us to handle a wider class of CDMA systems beyond the basic model reported by Tanaka in Europhys. Lett., 54 (2001) 540. The utility of the scheme is shown by analyzing a system in which the generation of signature sequences is designed for enhancing the orthogonality.Introduction. -Over the last decade, the scope of statistical mechanics has rapidly expanded beyond its original goal of analyzing many-body problems that arise when dealing with material objects. Information theory is a major source of problems, and research activity aimed at solving these problems is becoming popular. Identifying information bits with Ising spins, many problems in information theory, such as error correcting/compression codes [1-10] and cryptosystems [11,12], can be formulated as virtual many-body systems that are subject to disordered interactions. Analysis of the formulated problems using techniques of statistical mechanics has provided various nontrivial results that have not been obtained by conventional methods of information theory [13,14].Code division multiple access (CDMA), which is a core technology used in modern wireless communication, is an example of the successful application of such a statistical mechanical approach. This technology realizes simultaneous communication between multiple users and a single base station by modulating each user's bit signal (symbol) into a sequence of random pattern, termed the signature sequence [15]. CDMA has already been employed in thirdgeneration mobile phone systems and wireless LANs.Tanaka (2001) showed that the replica method of statistical mechanics enables the accurate assessment of the communication performance of a basic CDMA model in which users' sequences are generated independently of each other in a large system limit [16,17]. This research

We demonstrate a new method for projective single-shot measurement of two electron spin states (singlet versus triplet) in an array of gate-defined lateral quantum dots in GaAs. The measurement has very high fidelity and is robust with respect to electric and magnetic fluctuations in the environment. It exploits a long-lived metastable charge state, which increases both the contrast and the duration of the charge signal distinguishing the two measurement outcomes. This method allows us to evaluate the charge measurement error and the spin-to-charge conversion error separately. We specify conditions under which this method can be used, and project its general applicability to scalable quantum dot arrays in GaAs or silicon.

We study the four-dimensional Z 2 random-plaquette lattice gauge theory as a model of topological quantum memory, the toric code in particular. In this model, the procedure of quantum error correction works properly in the ordered (Higgs) phase, and phase boundary between the ordered (Higgs) and disordered (confinement) phases gives the accuracy threshold of error correction. Using self-duality of the model in conjunction with the replica method, we show that this model has exactly the same mathematical structure as that of the two-dimensional random-bond Ising model, which has been studied very extensively. This observation enables us to derive a conjecture on the exact location of the multicritical point (accuracy threshold) of the model, p c = 0.889972 . . ., and leads to several nontrivial results including bounds on the accuracy threshold in three dimensions. *