• Journal of the Optical Society of Korea
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• 제12권1호
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• pp.42-48
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• 2008

The relative intensity noise (RIN) characteristics in 405 nm blue laser diodes grown on wurtzite AlGaInN multiple quantum well structures were investigated using the rate equations with the quantum Langevin noise model. The device parameters were extracted from the optical gain properties of the MQW active region using the self-consistent numerical method developed for calculating the multiband Hamiltonian in the strained wurtzite crystal. These methods have been applied to laser diodes for various conditions including the external feedback and the high frequency current injection.

• 대한의용생체공학회:학술대회논문집
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• 대한의용생체공학회 1998년도 추계학술대회
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• pp.297-298
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• 1998

Prototype portal imaging device (EPID) based on CCD camera, which has a $20\times20cm^2$ field of view (FOV), has been developed and then tested by acquiring phantom images for 6 MV x-ray beam. While, among the captured images, each frame suffered notorious quantum noise, the frame averaging largely enhanced the image quality against quantum noise. Over 60 frames averaging, the signal-to-noise ratio (SNR) was increased by $\sim20$ times and contrast was increased about 2 times in the skull-region of the acquired head-phantom image.

• 대한수학회지
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• 제38권2호
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• pp.275-281
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• 2001

The trajectory of a classical dynamics is determined by the least action principle. As soon as we come to quantum dynamics, we have to consider all possible trajectories which are proposed to be a sum of the classical trajectory and Brownian fluctuation. Thus, the action involves the square of the derivative B(t) (white noise) of a Brownian motion B(t). The square is a typical example of a generalized white noise functional. The Feynman propagator should therefore be an average of a certain generalized white noise functional. This idea can be applied to a large class of dynamics with various kinds of Lagrangians.

• 한국초전도ㆍ저온공학회논문지
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• 제24권2호
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• pp.1-6
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• 2022

This paper reviews the literature that has been published since 1980s related to cryogenic refrigeration systems for quantum computing. The reason why such a temperature level of 10-20 mK is necessary for quantum computing is that the superconducting qubit is sensitive to even very small thermal disturbances. The entanglement of the qubits may not be sustained due to thermal fluctuations and mechanical vibrations beyond their thresholds. This phenomenon is referred to as decoherence, and it causes an computation error in operation. For the stable operation of the quantum computer, a low-vibration cryogenic refrigeration system is imperative as an enabling technology. Conventional dilution refrigerators (DR), so called 'wet' DR, are precooled by liquid helium, but a more convenient and economical precooling method can be achieved by using a mechanical refrigerator instead of liquid cryogen. These 'dry' DRs typically equip pulse-tube refrigerators (PTR) for precooling the DRs around 4 K because of its particular advantage of low vibration characteristic. In this review paper, we have focused on the development status of 4 K PTRs and further potential development issues will be also discussed. A quiet 4 K refrigerator not only serves as an indispensable precooler of DR but also immediately enhances the characteristics of low noise amplifiers (LNA) or other cryo-electronics of various type quantum computers.

• 천문학회보
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• 제46권1호
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• pp.37.1-37.1
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• 2021

For the first time in Korea, we are developing technology for gravitational wave (GW) detectors as a major R&D program. Our main research target is quantum noise reduction technology which can enhance the sensitivity of a GW detector beyond its limit by classical physics. Technology of generating squeezed vacuum state of light (SQZ) can suppress quantum noise (shot noise at higher frequencies and radiation pressure noise at lower frequencies) of laser interferometer type GW detectors. Squeezing technology has recently started being used for GW detectors and becoming necessary and key components. Our ultimate goal is to participate and make contribution to international collaborations for upgrade of existing GW detectors and construction of next generation GW detectors. This presentation will summarize our results in 2020 and plan for the upcoming years. Technical details will be presented in other family talks.

• Journal of Radiation Protection and Research
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• 제24권3호
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• pp.161-170
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• 1999

비정질 실리콘 방사선 촬영기와 기존 X-ray filmrh과의 영상질 비교를 통하여 시스템의 성능을 평가하였다. 다양한 영상질 평가를 위하여 MTF (Modulation Transfer Function), NPS (Noise Power Spectrum), Contrast를 측정하여 계산하였다. 실험 결과 DR의 MTF는 기존 film-screen system과 유사하였다. Noise 특성은 두 시스템 모두 quantum noise가 주를 이뤘으며, 특히 DR 에서는 전기적인 noise가 발견되었다. 또한 우수한 sensitivity 특성과 영상처리를 통하여 DR 시스템이 기존의 film-screen 시스템보다 놀은 대조도를 보였다. 이와 같이 DR에서는 기존 film-screen 시스템과 유사한 해상도와 영상처리를 통해 같은 촬영조건에서도 향상된 대조도의 영상을 얻을 수 있었다. 본 연구를 통하여 의학적 활용과 관련된DR 시스템의성능에 대한 유용한 정보를 제공할 수 있으리라 기대한다.

• Current Optics and Photonics
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• 제3권6호
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• pp.531-540
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• 2019

Spectral Amplitude Coding Optical Code Division Multiple Access (SAC OCDMA) is an advanced technique in asynchronous environments. This paper proposes design and implementation of a novel quantum logic gate (QLG) code, with code construction algorithm generated without following any code mapping procedures for SAC system. The proposed code has a unitary matrices property with maximum overlap of one chip for various clients and no overlaps in spectra for the rest of the subscribers. Results indicate that a single algorithm produces the same length increment for codes with weight greater than two and follows the same signal to noise ratio (SNR) and bit error rate (BER) calculations for a higher number of users. This paper further examines the performance of a QLG code based SAC-OCDMA system with NAND and direct detection techniques. BER analysis was carried out for the proposed code and results were compared with existing MDW, RD and GMP codes. We demonstrate that the QLG code based system performs better in terms of cardinality, which is followed by improved BER. Numerical analysis reveals that for error free transmission (10^-9), the suggested code supports approximately 170 users with code weight 4. Our results also conclude that the proposed code provides improvement in the code construction, cross-correlation and minimization of noises.

• 한국초전도학회:학술대회논문집
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• 한국초전도학회 2009년도 Korea Superconductivity Society Meeting 2009
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• pp.75-75
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• 2009

• 한국초전도학회:학술대회논문집
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• 한국초전도학회 2009년도 Korea Superconductivity Society Meeting 2009
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• pp.68-68
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• 2009

• 한국초전도ㆍ저온공학회논문지
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• 제22권4호
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• pp.1-5
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• 2020

Heart consists of myocardium cells and the electrophysiological activity of the cells generate magnetic fields. By measuring this magnetic field, magnetocardiogram (MCG), functional diagnosis of the heart diseases is possible. Since the strength of the MCG signals is weak, typically in the range of 1-10 pT, we need sensitive magnetic sensors. Conventionally, superconducting quantum interference devices (SQUID)s were used for the detection of MCG signals due to its superior sensitivity to other magnetic sensors. However, drawback of the SQUID is the need for regular refill of a cryogenic liquid, typically liquid helium for cooling low-temperature SQUIDs. Efforts to eliminate the need for the refill in the SQUID system have been done by using cryocooler-based conduction cooling or use of non-cryogenic sensors, or room-temperature sensors. Each sensor has advantage and disadvantage, in terms of magnetic field sensitivity and complexity of the system, and we review the recent trend of MCG technology.