arXiv subjects

Katsuji Yamamoto

Publications and source records attributed to Katsuji Yamamoto.

At least 19 recordsLinked to original sources

arXiv2014

Optical interference in view of the probability distribution of photon detection

We investigate interference of optical fields by examining the probability distribution of photon detection. The usual description of interference patterns in terms of superposition of classical mean fields with definite phases is elucidated in quantum fashion. Especially, for interference of two independent mixtures of number states with Poissonian or sub-Poissonian statistics, despite lack of intrinsic phases, it is found that the joint probability has a distinct peak manifold in the multi-dimensional space of the detector outcomes, which is along the trajectory of the mean-field values as the relative phase varies on the unit circle. Then, an interference pattern should mostly appear in each shot of measurement as a point in the peak manifold with a randomly chosen relative phase. On the other hand, for super-Poissonian sources the mean-field description is likely invalidated with rather broad probability distributions.

quant-ph↗

arXiv2011

Role of particle-number statistics in interference of independent Bose fields

We elucidate generally the interference of independent Bose fields in view of the conditional probability for the particle number measurements, and clarify its relation to the source number statistics. Despite lack of intrinsic phases, the interference phase can be inferred from the particle number registered at one detector by using the classical mean fields. If the conditional number distributions for the other detectors, given the outcome of the first detector, exhibit sufficiently narrow peaks around the values specified by the estimated phases, the mean field description is valid in a single run of interference. The widths in the conditional distribution are determined by the number statistics of the sources, among which notable scaling behavior is found depending on the detector configurations with the boundary at the Poissonian. The mean field description is found to be applicable to Poissonian and sub-Poissonian sources, whereas for super-Poissonian sources it is likely invalidated with the rather broad conditional distribution.

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arXiv2011

Robust and Scalable Scheme to Generate Large-Scale Entanglement Webs

We propose a robust and scalable scheme to generate an $N$-qubit $W$ state among separated quantum nodes (cavity-QED systems) by using linear optics and postselections. The present scheme inherits the robustness of the Barrett-Kok scheme [Phys. Rev. A {\bf 71}, 060310(R) (2005)]. The scalability is also ensured in the sense that an arbitrarily large $N$-qubit $W$ state can be generated with a quasi-polynomial overhead $\sim 2^{O[(\log_2 N)^2]}$. The process to breed the $W$ states, which we introduce to achieve the scalability, is quite simple and efficient, and can be applied for other physical systems.

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arXiv2010

Topological One-Way Quantum Computation on Verified Logical Cluster States

We present a scheme to improve the noise threshold for the fault-tolerant topological one-way computation with a constant overhead. Certain cluster states of finite size, say star clusters, are constructed with logical qubits through an efficient verification process to achieve high fidelity. Then, the star clusters are connected near-deterministically with verification to form a three-dimensional cluster state to implement the topological one-way computation. The necessary postselection for verification is localized within the star clusters, ensuring the salability of computation. This scheme works with a high error rate $ \sim 1 % $ and reasonable resources comparable to or less than those for the other fault-tolerant schemes, suggesting potentially a noise threshold higher than 5%.

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arXiv2010

Photon creation in a resonant cavity with a nonstationary plasma mirror and its detection with Rydberg atoms

We investigate the dynamical Casimir effect and its detection with Rydberg atoms. The photons are created in a resonant cavity with a plasma mirror of a semiconductor slab which is irradiated by periodic laser pulses. The canonical Hamiltonian is derived for the creation and annihilation operators showing the explicit time-variation in the couplings, which originates from the external configuration such as a nonstationary plasma mirror. The number of created photons is evaluated as squeezing from the Heisenberg equations with the Hamiltonian. Then, the detection of the photons as the atomic excitations is examined through the atom-field interaction. Some consideration is made for a feasible experimental realization with a semiconductor plasma mirror.

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arXiv2010

Anti-Zeno Effect for Quantum Transport in Disordered Systems

We demonstrate that repeated measurements in disordered systems can induce quantum anti-Zeno effect under certain condition to enhance quantum transport. The enhancement of energy transfer is really exhibited with a simple model under repeated measurements. The optimal measurement interval for the anti-Zeno effect and the maximal efficiency of energy transfer are specified in terms of the relevant physical parameters. Since the environment acts as frequent measurements on the system, the decoherence-induced energy transfer, which has been discussed recently for photosynthetic complexes, may be interpreted in terms of the anti-Zeno effect. We further find an interesting phenomenon, where local decoherence or repeated measurements may even promote entanglement generation between the non-local sites.

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arXiv2010

Optical homodyne detection in view of joint probability distribution

Optical homodyne detection is examined in view of joint probability distribution. It is usually discussed that the relative phase between independent laser fields are localized by photon-number measurements in interference experiments such as homodyne detection. This provides reasoning to use operationally coherent states for laser fields in the description of homodyne detection and optical quantum-state tomography. Here, we elucidate these situations by considering the joint probability distribution and the invariance of homodyne detection under the phase transformation of optical fields.

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arXiv2009

Cluster-based architecture for fault-tolerant quantum computation

We present a detailed description of an architecture for fault-tolerant quantum computation, which is based on the cluster model of encoded qubits. In this cluster-based architecture, concatenated computation is implemented in a quite different way from the usual circuit-based architecture where physical gates are recursively replaced by logical gates with error-correction gadgets. Instead, some relevant cluster states, say fundamental clusters, are recursively constructed through verification and postselection in advance for the higher-level one-way computation, which namely provides error-precorrection of gate operations. A suitable code such as the Steane seven-qubit code is adopted for transversal operations. This concatenated construction of verified fundamental clusters has a simple transversal structure of logical errors, and achieves a high noise threshold ~ 3 % for computation by using appropriate verification procedures. Since the postselection is localized within each fundamental cluster with the help of deterministic bare controlled-Z gates without verification, divergence of resources is restrained, which reconciles postselection with scalability.

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arXiv2009

Flavor-changing interactions with singlet quarks and their implications for the LHC

We investigate the flavor-changing interactions in an extension of the standard model with singlet quarks and singlet Higgs, which are induced by the mixing between the ordinary quarks and the singlet quarks ($ q $-$ Q $ mixing). We consider the effects of the gauge and scalar interactions in the $ ΔF = 2 $ mixings of $ K^0 $, $ B_d $, $ B_s $ and $ D^0 $ mesons to show the currently allowed range of the $ q $-$ Q $ mixing. Then, we investigate the new physics around the electroweak scale to the TeV scale, which is accessible to the Large Hadron Collider. Especially, the scalar coupling mediated by the singlet Higgs may provide distinct signatures for the decays of the singlet quarks and Higgs particles, which should be compared with the conventionally expected ones via the gauge and standard Higgs couplings. Observations of the singlet quarks and Higgs particles will present us important insights on the $ q $-$ Q $ mixing and Higgs mixing.

hep-ph↗

arXiv2008

Entanglement Purification with Double Selection

We investigate efficient entanglement purification through double selection process. This method works with higher noise thresholds for the communication channels and local operations, and achieves higher fidelity of purified states. Furthermore it provides a yield comparable to the usual protocol with single selection. It is shown by general considerations that the double selection is optimal to remove the first-order errors, achieving the upper bound on the fidelity of purified states in the low noise regime. The double selection is also applied to purification of multi-partite entanglement such as two-colorable graph states.

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arXiv2008

A standard Hamiltonian formulation for the dynamical Casimir effect

We present a quantum description of photon creation via dynamical Casimir effect based on the standard Hamiltonian formulation. The particle representation is constructed in the expansion of field operators fixed with the initial modes. The Hamiltonian is presented in terms of the creation and annihilation operators with the time-varying couplings which originate from the external properties such as an oscillating boundary or a plasma mirror of a semiconductor slab. Some consideration is also made for the experimental realization with a semiconductor plasma mirror.

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arXiv2008

Fault-tolerant quantum computation in concatenation of verified cluster states

A novel scheme is presented for fault-tolerant quantum computation based on the cluster model. Some relevant logical cluster states are constructed in concatenation by post-selection through verification, without necessity of recovery operation, where a suitable code such as the Steane's 7-qubit code is adopted for transversal operations. This simple concatenated construction of verified cluster states achieves a high noise threshold ~1%, and restrains the divergence of resources.

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arXiv2007

Efficient atomic quantum memory for photonic qubits in cavity QED

We investigate a scheme of atomic quantum memory to store photonic qubits of polarization in cavity QED. It is observed that the quantum-state swapping between a single-photon pulse and a $ Λ$-type atom can be made via scattering in an optical cavity [T. W. Chen, C. K. Law, P. T. Leung, Phys. Rev. A {\bf 69} (2004) 063810]. This swapping operates limitedly in the strong coupling regime for $ Λ$-type atoms with equal dipole couplings. We extend this scheme in cavity QED to present a more feasible and efficient method for quantum memory combined with projective measurement. This method works without requiring such a condition on the dipole couplings. The fidelity is significantly higher than that of the swapping, and even in the moderate coupling regime it reaches almost unity by narrowing sufficiently the photon-pulse spectrum. This high performance is rather unaffected by the atomic loss, cavity leakage or detunings, while a trade-off is paid in the success probability for projective measurement.

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arXiv2006

Fault-tolerant Quantum Computation with Highly Verified Logical Cluster States

We investigate a scheme of fault-tolerant quantum computation based on the cluster model. Logical qubits are encoded by a suitable code such as the Steane's 7-qubit code. Cluster states of logical qubits are prepared by post-selection through verification at high fidelity level, where the unsuccessful ones are discarded without recovery operation. Then, gate operations are implemented by transversal measurements on the prepared logical cluster states. The noise threshold is improved significantly by making the high fidelity preparation and transversal measurement. It is estimated to be about 3% by a numerical simulation.

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arXiv2006

Universality of strength for Yukawa couplings with extra down-type quark singlets

We investigate the quark masses and mixings by including vector-like down-type quark singlets in universality of strength for Yukawa couplings (USY). In contrast with the standard model with USY, the sufficient $ CP $ violation is obtained for the Cabibbo-Kobayashi-Maskawa matrix through the mixing between the ordinary quarks and quark singlets. The top-bottom mass hierarchy $ m_t \gg m_b $ also appears naturally in the USY scheme with the down-type quark singlets.

hep-ph↗

arXiv2005

Leptogenesis with supersymmetric Higgs triplets in TeV region

The leptogenesis with supersymmetric Higgs triplets is studied in the light of experimental verification in the TeV region. The lepton number asymmetry appears just after the inflation via multiscalar coherent evolution of Higgs triplets and antislepton on a flat manifold. If the Higgs triplet mass terms dominate over the negative thermal-log term for the Hubble parameter H comparable to the Higgs triplet mass M_Δ, the asymmetry is fixed readily to some significant value by the redshift and rotation of these scalar fields, providing the sufficient lepton-to-entropy ratio n_L / s \sim 10^-10. This can be the case even with M_Δ\sim 1 TeV for the reheating temperature T_R \sim 10^6 GeV and the mass parameter M / λ\sim 10^22 GeV of the nonrenormalizable superpotential terms relevant for leptogenesis.

hep-ph↗

arXiv2005

Atomic Quantum Memory for Photonic Qubits via Scattering in Cavity QED

We investigate a scheme of atomic quantum memory to store photonic qubits in cavity QED. This is motivated on the recent observation that the quantum-state swapping between a single-photon pulse and a Lambda-type atom trapped in a cavity is ideally realized via scattering for some specific case in the strong coupling cavity regime [T. W. Chen, C. K. Law, and P. T. Leung, Phys. Rev. A 69, 063810 (2004)]. We derive a simple formula for calculating the fidelity of this atom-photon swapping for quantum memory. We further propose a feasible method which implements conditionally the quantum memory operation with the fidelity of almost unity even if the atom-photon coupling is not so strong. This method can also be applied to store a photonic entanglement in spatially separated atomic quantum memories.

quant-ph↗

arXiv2004

Analysis for practical realization of number-state manipulation by number-sum Bell measurement with linear optics

We analyze the linear optical realization of number-sum Bell measurement and number-state manipulation by taking into account the realistic experimental situation, specifically imperfectness of single-photon detector. The present scheme for number-state manipulation is based on the number-sum Bell measurement, which is implemented with linear optical elements, i.e., beam splitters, phase shifters and zero-one-photon detectors. Squeezed vacuum states and coherent states are used as optical sources. The linear optical Bell state detector is formulated quantum theoretically with a probability operator measure. Then, the fidelity of manipulation and preparation of number-states, particularly for qubits and qutrits, is evaluated in terms of the quantum efficiency and dark count of single-photon detector. It is found that a high fidelity is achievable with small enough squeezing parameters and coherent state amplitudes.

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