Akihisa Tomita

Mode demultiplexer using angularly multiplexed volume holograms

This study proposes a volume holographic demultiplexer (VHDM) for extracting the spatial modes excited in a multimode fiber. A unique feature of the demultiplexer is that it can separate a number of multiplexed modes output from a fiber in different directions by using multi-recorded holograms without beam splitters, which results in a simple configuration as compared with that using phase plates instead of holograms. In this study, an experiment is conducted to demonstrate the basic operations for three LP mode groups to confirm the performance of the proposed VHDM and to estimate the signal-to-crosstalk noise ratio (SNR). As a result, an SNR of greater than 20 dB is obtained.

Spatial cross modulation method using a random diffuser and phase-only spatial light modulator for constructing arbitrary complex fields

We propose a spatial cross modulation method using a random diffuser and a phase-only spatial light modulator (SLM), by which arbitrary complex-amplitude fields can be generated with higher spatial resolution and diffraction efficiency than off-axis and double-phase computer-generated holograms. Our method encodes the original complex object as a phase-only diffusion image by scattering the complex object using a random diffuser. In addition, all incoming light to the SLM is consumed for a single diffraction order, making a diffraction efficiency of more than 90% possible. This method can be applied for holographic data storage, three-dimensional displays, and other such applications.

Two-channel algorithm for single-shot, high-resolution measurement of optical wavefronts using two image sensors

We propose a two-channel holographic diversity interferometer (2ch-HDI) system for single-shot and highly accurate measurements of complex amplitude fields with a simple optical setup. In this method, two phase-shifted interference patterns are generated, without requiring a phase-shifting device, by entering a circularly polarized reference beam into a polarizing beam splitter, and the resulting patterns are captured simultaneously using two image sensors. However, differences in the intensity distributions of the two image sensors may lead to serious measurement errors. Thus, we also develop a two-channel algorithm optimized for the 2ch-HDI to compensate for these differences. Simulation results show that this algorithm can compensate for such differences in the intensity distributions in the two image sensors. Experimental results confirm that the combination of the 2ch-HDI and the calculation algorithm significantly enhances measurement accuracy.

Generation of phase-squeezed optical pulses with large coherent amplitudes by post-selection of single photon and weak cross-Kerr non-linearity

Phase-squeezed light can enhance the precision of optical phase estimation. The larger the photon numbers are and the stronger the squeezing is, the better the precision will be. We propose an experimental scheme for generating phase-squeezed light pulses with large coherent amplitudes. In our scheme, one arm of a single-photon Mach–Zehnder interferometer interacts with coherent light via a non-linear optical Kerr medium to generate a coherent superposition state. Post-selecting the single photon by properly tuning a variable beam splitter in the interferometer yields a phase-squeezed output.

Intensity fluctuation of a gain-switched semiconductor laser for quantum key distribution systems

Security certification of quantum key distribution (QKD) systems under practical conditions is necessary for social deployment. This article focused on the transmitter, and, in particular, investigated the intensity fluctuation of the optical pulses emitted by a gain-switched semiconductor laser used in QKD systems implementing decoy-BB84 protocol. A large intensity fluctuation was observed for low excitation, showing strong negative correlation between the adjacent pulses, which would affect the final key rate. The fluctuation decreased and the correlation disappeared as excitation increased. Simulation with rate equations successfully reproduced the experimental results and revealed that the large fluctuation originates from an intrinsic instability of gain-switched lasers driven periodically at a rate comparable to the inverse of carrier lifetime, as in GHz-clock QKD systems. Methods for further reduction of the intensity fluctuation were also discussed.

Quantum key distribution with an efficient countermeasure against correlated intensity fluctuations in optical pulses

Quantum key distribution (QKD) allows two distant parties to share secret keys with the proven security even in the presence of an eavesdropper with unbounded computational power. Recently, GHz-clock decoy QKD systems have been realized by employing ultrafast optical communication devices. However, security loopholes of high-speed systems have not been fully explored yet. Here we point out a security loophole at the transmitter of the GHz-clock QKD, which is a common problem in high-speed QKD systems using practical band-width limited devices. We experimentally observe the inter-pulse intensity correlation and modulation pattern-dependent intensity deviation in a practical high-speed QKD system. Such correlation violates the assumption of most security theories. We also provide its countermeasure which does not require significant changes of hardware and can generate keys secure over 100u2009km fiber transmission. Our countermeasure is simple, effective and applicable to wide range of high-speed QKD systems, and thus paves the way to realize ultrafast and security-certified commercial QKD systems.Quantum key distribution: enhancing the security of high-speed key transmissionsA potential security loophole and its countermeasure have been discovered in practical implementations of high-speed quantum key distribution (QKD). Ken-ichiro Yoshino from NEC corporation and a team of researchers from Japan investigated the intensity fluctuations of optical pulses in a GHz-clocked QKD system, revealing that the limited bandwidth of the ultrafast optical transmitter’s electronics generates deviations from the ideal signal. These perturbations have been shown to carry signatures of previous modulation patterns - effectively introducing correlations between individual pulses. As the strength of QKD relies on its proof-of-principle security, which in many cases is derived under the assumption of independent pulses, these correlations constitute a loophole that might compromise the whole protocol. Fortunately, the researchers developed two countermeasures: pattern sifting and alternate key distillation, which recover security and do not impact performances too severely.

Analog Quantum Error Correction with Encoding a Qubit into an Oscillator

To implement fault-tolerant quantum computation with continuous variables, Gottesman-Kitaev-Preskill (GKP) qubits have been recognized as an important technological element. However, the analog outcome of GKP qubits, which includes beneficial information to improve the error tolerance, has been wasted, because the GKP qubits have been treated as only discrete variables. In this Letter, we propose a hybrid quantum error correction approach that combines digital information with the analog information of the GKP qubits using a maximum-likelihood method. As an example, we demonstrate that the three-qubit bit-flip code can correct double errors, whereas the conventional method based on majority voting on the binary measurement outcome can correct only a single error. As another example, we show that a concatenated code known as Knills C_{4}/C_{6} code can achieve the hashing bound for the quantum capacity of the Gaussian quantum channel (GQC). To the best of our knowledge, this approach is the first attempt to draw both digital and analog information to improve quantum error correction performance and achieve the hashing bound for the quantum capacity of the GQC.

Progressive phase conjugation and its application in reconfigurable spatial-mode extraction and conversion

We develop a new technology, which is referred to as progressive phase conjugation (PPC), in which phase conjugation is electrically performed without requiring a coherent reference beam by fusion using a reference-free spatial phase detection and spatial phase modulation. This method enables remote setting of a phase detector from the signal transmitter without an additional transmission line for the reference beam. It also enables realization of high-speed and dynamic wavefront compensation owing to its open-loop architecture using the single-shot phase detection method. Therefore, the PPC is applicable to a wide range of optical communication technologies, including the reconfigurable spatial-mode extraction and conversion of mode transmission in a multi-mode fiber (MMF). In our experiment, spatial modes are generated by directing a laser beam into a MMF with a 50-micron core diameter. At the output side of the optical fiber, the phase distributions of the spatial modes are detected using the reference-free phase detector constructed by combining a spatial filtering method with holographic diversity interferometry using two CCD imagers. Then, the phase conjugate distribution of the detected phase pattern is displayed on a LCOS-type SLM. We confirm that the PPC system can extract a specific mode pattern with a considerably low crosstalk of less than 1% by displaying the corresponding phase-conjugation pattern on the SLM. In addition, we demonstrated a reconfigurable spatial-mode conversion by the phase control technology using the SLM. By applying the spatial phase modulation to an optical beam incident on the SLM, the spatial mode of the output beam is flexibly changed.

Entanglement generation by communication using phase-squeezed light with photon loss

Graduate School of Information Science and Technology,Hokkaido University, Kita14-Nishi9, Kita-ku, Sapporo 060-0814, JapanIn order to implement fault-tolerant quantum computation, entanglement generation with lowerror probability and high success probability is required. We have proposed the use of squeezedcoherent light as a probe to generate entanglement between two atoms by communication, and shownthat the error probability is reduced well below the threshold of fault-tolerant quantum computation[Phys. Rev. A. 88, 022313 (2013)]. Here, we investigate the effect of photon loss mainly due tofinite coupling efficiency to the cavity. The error probability with the photon loss is calculatedby the beam-splitter model for homodyne measurement on probe light. Optimum condition onthe amplitude of probe light to minimize the error probability is examined. It is shown that thephase-squeezed probe light yields lower error probability than coherent-light probe. A fault-tolerantquantum computation algorithm can be implemented under 0.59 dB loss by concatenating five-qubiterror correction code.PACSnumbers: 03.65.Ud, 03.67.Lx, 03.67.Hk, 42.50.DvI. INTRODUCTION

Experiment on three-dimensional display using spatial cross modulation method

We performed an experiment on a three-dimensional (3D) display using the spatial cross modulation (SCM) method with a single phase-only spatial light modulator and an optical random diffuser. The experimental result revealed the SCM has great potential to realize the 3D display with unprecedented high resolution and light utilization efficiency.