Kouichi Ichimura

A simple frequency-domain quantum computer with ions in a crystal coupled to a cavity mode

A quantum computer where quantum bits (qubits) are defined in frequency domain and interaction between qubits is mediated by a single cavity mode is proposed. In this quantum computer, qubits can be individually addressed regardless of their positions. Therefore, randomly distributed systems in space can be directly employed as qubits. An application of nuclear spins in rare-earth ions in a crystal for the quantum computer is quantitatively analyzed.

Population transfer via stimulated Raman adiabatic passage in a solid

We report experimental demonstration of population transfer via stimulated Raman adiabatic passage with a rare-earth-ion-doped crystal (Pr{sup 3+}:Y{sub 2}SiO{sub 5}). About 90% of population of a ground level is transferred to another ground level with two pulses whose width is shorter than the lifetime of the excited state. The dependence of transferred population on the delay time of the pulse sequence is also experimentally investigated. The delay-time dependence observed in our experiment is similar to that obtained by numerical simulation.

Experimental determination of intracavity losses of monolithic Fabry-Perot cavities made of Pr 3+ :Y 2 SiO 5

We propose an experimental method with which all the following quantities can be determined separately: the intracavity loss and individual cavity-mirror transmittances of a monolithic Fabry-Perot cavity and furthermore the coupling efficiency between the cavity mode and the incident light. It is notable that the modified version of this method can also be applied to whispering-gallery-mode cavities. Using this method, we measured the intracavity losses of monolithic Fabry-Perot cavities made of Pr3+:Y2SiO5 at room temperature. The knowledge of the intracavity losses is very important for applications of such cavities, e.g., to quantum information technologies. It turns out that fairly high losses (about 0.1%) exist even for a sample with extremely low dopant concentration (2×10(-5) at. %). The experimental results also indicate that the loss may be mainly due to the bulk loss of Y2SiO5 crystal. The bulk loss is estimated to be 7×10(-4) cm(-1) (0.003 dB/cm) or lower.

Condition for fault-tolerant quantum computation with a cavity-QED scheme

A condition for fault-tolerant quantum computation (FTQC) with cavity schemes is discussed. It is shown that the condition is very hard if the standard error threshold of FTQC is simply applied. To relax the condition, we propose to combine the cavity-quantum-electrodynamics (QED) scheme proposed by Duan et al. [Phys. Rev. A 72, 032333 (2005)] and Xiao et al. [Phys. Rev. A 70, 042314 (2004)] with the recently proposed FTQC scheme with probabilistic two-qubit gates [Goto and Ichimura, Phys. Rev. A 80, 040303(R) (2009)]. It is shown that the condition for FTQC is dramatically relaxed compared to the case of the standard threshold. The optimization of the cavity-QED scheme is also discussed.

Cavity-enhanced spectroscopy of a rare-earth-ion-doped crystal: Observation of a power law for inhomogeneous broadening

We report an erratum regarding the polarization of the probe in the experiment reported in our paper [Opt. Express21, 24332-24343 (2013)]. Although we wrote in the paper that the polarization of the probe is set to be parallel to the D2 axis of YSO crystal, we found that the polarization was parallel to the D1 axis of YSO crystal. However, this fact does not affect the two main results of the work: observation of very small absorption and the power law for the inhomogeneous broadening.