Yuuki Tokunaga

Generation of high-fidelity four-photon cluster state and quantum-domain demonstration of one-way quantum computing.

We experimentally demonstrate a simple scheme for generating a four-photon entangled cluster state with fidelity over 0.860+/-0.015. We show that the fidelity is high enough to guarantee that the produced state is distinguished from Greenberger-Horne-Zeilinger, W, and Dicke types of genuine four-qubit entanglement. We also demonstrate basic operations of one-way quantum computing using the produced state and show that the output state fidelities surpass classical bounds, which indicates that the entanglement in the produced state essentially contributes to the quantum operation.

Error and loss tolerances of surface codes with general lattice structures

We propose a family of surface codes with general lattice structures, where the error-tolerances against bit and phase errors can be controlled asymmetrically by changing the underlying lattice geometries. The surface codes on various lattices are found to be efficient in the sense that their threshold values universally approach the quantum Gilbert-Varshamov bound. We find that the error-tolerance of surface codes depends on the connectivity of underlying lattices; the error chains on a lattice of lower connectivity are easier to correct. On the other hand, the loss-tolerance of surface codes exhibits an opposite behavior; the logical information on a lattice of higher connectivity has more robustness against qubit loss. As a result, we come upon a fundamental trade-off between error- and loss-tolerances in the family of the surface codes with different lattice geometries.

Fault-tolerant topological one-way quantum computation with probabilistic two-qubit gates.

We propose a scalable way to construct a 3D cluster state for fault-tolerant topological one-way computation (TOWC) even if the entangling two-qubit gates succeed with a small probability. It is shown that fault-tolerant TOWC can be performed with the success probability of the two-qubit gate such as 0.5 (0.1) provided that the unheralded error probability of the two-qubit gate is less than 0.040% (0.016%). Furthermore, the resource usage is considerably suppressed compared to the conventional fault-tolerant schemes with probabilistic two-qubit gates.

Coexistence of ferromagnetism and superconductivity in magnetic superconductor RuSr2YCu2O8 revealed by 63Cu-NMR

Abstract 63 Cu -NMR measurement has been performed to investigate the coexistence of superconductivity and ferromagnetism in a hybrid ruthenium (Ru)–copper (Cu) oxide superconductor RuSr2YCu2O8 (RuY1212). Broadening of the Cu-NMR signal below TM demonstrates that Cu nuclei in CuO2 planes feel a dipolar field from ordered Ru moments in RuO2 planes. On the other hand, the opening of the SC gap is confirmed from the observation of a decrease in the nuclear spin–lattice relaxation rate, 63 1/T 1 , below Tconset=45 K. These NMR results provide evidence that the superconductivity coexists with the (field induced) ferromagnetism at a microscopic scale. We also found that the T dependence and the magnitude of 1/T1T are quite similar to those in YBa2Cu3O7 (Y1237), indicative of the same spin-fluctuation character in both compounds. This suggests that the hole content in RuY1212 is comparable to that in Y1237.

High-fidelity cluster state generation for ultracold atoms in an optical lattice.

We propose a method for generating high-fidelity multipartite spin entanglement of ultracold atoms in an optical lattice in a short operation time with a scalable manner, which is suitable for measurement-based quantum computation. To perform the desired operations based on the perturbative spin-spin interactions, we propose to actively utilize the extra degrees of freedom (DOFs) usually neglected in the perturbative treatment but included in the Hubbard Hamiltonian of atoms, such as, (pseudo-)charge and orbital DOFs. Our method simultaneously achieves high fidelity, short operation time, and scalability by overcoming the following fundamental problem: enhancing the interaction strength for shortening the operation time breaks the perturbative condition of the interaction and inevitably induces unwanted correlations among the spin and extra DOFs.

Discrete Fourier-based correlations for entanglement detection.

We introduce two forms of correlations on two d-level (qudit) systems for entanglement detection. The correlations can be measured via experimentally tractable two local measurement settings and their separable bounds are determined by discrete Fourier-based uncertainty relations. They are useful to estimate lower bounds of the Schmidt number in order to clarify generation of a genuine qudit entanglement. We also present inseparable conditions for multiqudit systems associated with the qudit stabilizer formalism as another role of the correlations on the inseparability problem.

Microscopic coexistence of antiferromagnetism and superconductivity in HgBa2Ca4Cu5Oy:Cu-NMR study

Abstract We report a coexistence of superconductivity and antiferromagnetism in HgBa 2 Ca 4 Cu 5 O y with T c =108 K trough Cu-NMR measurements. This compound is composed of two kinds of CuO 2 planes in a unit cell, three inner planes (IP) and two outer planes (OP). The OP is optimally-doped, undergoing a superconducting transition at T c =108 K, whereas the IP is strongly underdoped, doing an antiferromagnetic transition below T N ∼60 K.

Cu NQR study of spin fluctuation in Ni substituted YBa2Cu3O7

Abstract From Cu-NQR measurements in YBa2(Cu1−x)3O7, all the 63(1/T1) are found to be on a universal curve, when 1/T1 is plotted against t = T/Tc(x) above Tc. This result indicates that Tc is intimately correlated with the Cu antiferromagnetic spin fluctuation (AFSF) that governs 63(1/T1 to Tc is a direct evidence for the pairing mechanism to be magnetic in origin. The reduction in Tc by the Ni substitution is considered to be due to the decrease in the characteristic energy, ΓQ of AFSF around Q = (π/a, π/a).

Schmidt-number benchmark for genuine quantum memories and gates

We propose to apply the notion of the Schmidt number in order to show that a quantum memory or gate process is capable of maintaining a genuine multi-level quantum coherence. We present a simple criterion in terms of an average gate fidelity with respect to the input states of two mutually unbiased bases and demonstrate the coherence of the gate operation in several existing experiments. The Schmidt-number benchmark naturally includes the quantum benchmark as a specific case.

NMR of highTc superconductors

Results of our NMR measurements of highTc superconductors are reviewed. From systematic measurements of NQR frequency, Knight shift, nuclear spin lattice relaxation rate and nuclear transverse relaxation rate over a wide hole concentration range from light to heavy doped compounds together with the impurity and pressure effect, following results are concluded. The numbers of the holes at the respective sites of Cu and O site in the CuO2 plane govern the several physical properties of the system. There are optimum values of these hole numbers to produce highestTc. The superconducting properties are well explained consistently by thed-wave pairing model.Tc is found to be correlated intimately with the antiferromagnetic spin fluctuation parameters that governs the nuclear relaxation. The mechanism for the superconductivity is magnetic in origin.