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Dive into the research topics where Young-Sik Ra is active.

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Featured researches published by Young-Sik Ra.


Optics Express | 2009

Reversing the weak quantum measurement for a photonic qubit

Yong-Su Kim; Young-Wook Cho; Young-Sik Ra; Yoon-Ho Kim

We demonstrate the conditional reversal of a weak quantum measurement on a photonic qubit. The state recovery fidelity, determined by quantum process tomography, is shown to be over 94% for partial-collapse strength up to 0.9. We also experimentally study information gain due to the weak measurement and discuss the role of the reversing operation as an information erasure.


Nature Communications | 2013

Observation of detection-dependent multi-photon coherence times

Young-Sik Ra; Malte C. Tichy; Hyang-Tag Lim; Osung Kwon; Florian Mintert; Andreas Buchleitner; Yoon-Ho Kim

The coherence time constitutes one of the most critical parameters that determines whether or not interference is observed in an experiment. For photons, it is traditionally determined by the effective spectral bandwidth of the photon. Here we report on multi-photon interference experiments in which the multi-photon coherence time, defined by the width of the interference signal, depends on the number of interfering photons and on the measurement scheme chosen to detect the particles. A theoretical analysis reveals that all multi-photon interferences with more than two particles feature this dependence, which can be attributed to higher-order effects in the mutual indistinguishability of the particles. As a striking consequence, a single, well-defined many-particle quantum state can exhibit qualitatively different degrees of interference, depending on the chosen observable. Therefore, optimal sensitivity in many-particle quantum interferometry can only be achieved by choosing a suitable detection scheme.


conference on lasers and electro optics | 2013

Nonmonotonicity in quantum-to-classical transition in multiparticle interference

Young-Sik Ra; Malte C. Tichy; Hyang-Tag Lim; Osung Kwon; Florian Mintert; Andreas Buchleitner; Yoon-Ho Kim

Quantum-mechanical wave–particle duality implies that probability distributions for granular detection events exhibit wave-like interference. On the single-particle level, this leads to self-interference—e.g., on transit across a double slit—for photons as well as for large, massive particles, provided that no which-way information is available to any observer, even in principle. When more than one particle enters the game, their specific many-particle quantum features are manifested in correlation functions, provided the particles cannot be distinguished. We are used to believe that interference fades away monotonically with increasing distinguishability—in accord with available experimental evidence on the single- and on the many-particle level. Here, we demonstrate experimentally and theoretically that such monotonicity of the quantum-to-classical transition is the exception rather than the rule whenever more than two particles interfere. As the distinguishability of the particles is continuously increased, different numbers of particles effectively interfere, which leads to interference signals that are, in general, nonmonotonic functions of the distinguishability of the particles. This observation opens perspectives for the experimental characterization of many-particle coherence and sheds light on decoherence processes in many-particle systems.


Physical Review A | 2011

Four-photon indistinguishability transition

Malte C. Tichy; Hyang-Tag Lim; Young-Sik Ra; Florian Mintert; Yoon-Ho Kim; Andreas Buchleitner; Hermann-Herder Strasse

We demonstrate the conspiration of many-particle interferences of different degree to determine the transmission of four photons of tunable indistinguishability through a four-port beam splitter array. The probability of certain output events depends non-monotonically on the degree of distinguishability, due to distinct multi-particle interference contributions to the transmission signal.


Optics Express | 2009

Coherence properties of spontaneous parametric down-conversion pumped by a multi-mode cw diode laser

Osung Kwon; Young-Sik Ra; Yoon-Ho Kim

Coherence properties of the photon pair generated via spontaneous parametric down-conversion pumped by a multi-mode cw diode laser are studied with a Mach-Zehnder interferometer. Each photon of the pair enters a different input port of the interferometer and the biphoton coherence properties are studied with a two-photon detector placed at one output port. When the photon pair simultaneously enters the interferometer, periodic recurrence of the biphoton de Broglie wave packet is observed, closely resembling the coherence properties of the pump diode laser. These properties could be useful for building engineered entangled photon sources based on diode laser-pumped spontaneous parametric down-conversion.


Optics Express | 2013

Time-bin entangled photon pairs from spontaneous parametric down-conversion pumped by a cw multi-mode diode laser.

Osung Kwon; Kwang-Kyoon Park; Young-Sik Ra; Yong-Su Kim; Yoon-Ho Kim

Generation of time-bin entangled photon pairs requires the use of the Franson interferometer which consists of two spatially separated unbalanced Mach-Zehnder interferometers through which the signal and idler photons from spontaneous parametric down-conversion (SPDC) are made to transmit individually. There have been two SPDC pumping regimes where the scheme works: the narrowband regime and the double-pulse regime. In the narrowband regime, the SPDC process is pumped by a narrowband cw laser with the coherence length much longer than the path length difference of the Franson interferometer. In the double-pulse regime, the longitudinal separation between the pulse pair is made equal to the path length difference of the Franson interferometer. In this paper, we propose another regime by which the generation of time-bin entanglement is possible and demonstrate the scheme experimentally. In our scheme, differently from the previous approaches, the SPDC process is pumped by a cw multi-mode (i.e., short coherence length) laser and makes use of the coherence revival property of such a laser. The high-visibility two-photon Franson interference demonstrates clearly that high-quality time-bin entanglement source can be developed using inexpensive cw multi-mode diode lasers for various quantum communication applications.


Physical Review Letters | 2011

Experimental Realization of an Approximate Partial Transpose for Photonic Two-Qubit Systems

Hyang-Tag Lim; Yong-Su Kim; Young-Sik Ra; Joonwoo Bae; Yoon-Ho Kim

The partial transpose by which a subsystem’s quantum state is solely transposed is of unique importance in quantum information processing from both fundamental and practical point of view. In this work, we present a practical scheme to realize a physical approximation to the partial transpose using local measurements on individual quantum systems and classical communication. We then report its linear optical realization and show that the scheme works with no dependence on local basis of given quantum states. A proof-of-principle demonstration of entanglement detection using the physical approximation of the partial transpose is also reported.


New Journal of Physics | 2010

Weak value measurement with an incoherent measuring device

Young-Wook Cho; Hyang-Tag Lim; Young-Sik Ra; Yoon-Ho Kim

In the Aharonov-Albert-Vaidman (AAV) weak measurement, it is assumed that the measuring device or the pointer is in a quantum mechanical pure state. In reality, however, it is often not the case. In this paper, we generalize the AAV weak measurement scheme to include more generalized situations in which the measuring device is in a mixed state. We also report an optical implementation of the weak value measurement in which the incoherent pointer is realized with the pseudo-thermal light. The theoretical and experimental results show that the measuring device under the influence of partial decoherence could still be used for amplified detection of minute physical changes and are applicable for implementing the weak value measurement for massive particles.


Physical Review A | 2010

Observing photonic de Broglie waves without the maximally-path-entangled |N,0〉 + |0,N〉 state

Osung Kwon; Young-Sik Ra; Yoon-Ho Kim

We report the observation of the photonic de Broglie wave for a pair of photons, generated by spontaneous parametric down-conversion, that are not photon number-path entangled. The experimental and theoretical results suggest that the photonic de Broglie wave is, in fact, not related to the entanglement of the photons, rather it is related to the indistinguishable pathways established by the measurement scheme.


Physics Letters A | 2010

Experimental verification of the commutation relation for Pauli spin operators using single-photon quantum interference

Yong-Su Kim; Hyang-Tag Lim; Young-Sik Ra; Yoon-Ho Kim

Abstract We report experimental verification of the commutation relation for Pauli spin operators using the single-photon polarization state. The experimental quantum operation corresponding to the commutator, [ σ z , σ x ] = k σ y , showed process fidelity of 0.94 compared to the ideal σ y operation and | k | is determined to be 2.12 ± 0.18 .

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Yoon-Ho Kim

Pohang University of Science and Technology

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Hyang-Tag Lim

Pohang University of Science and Technology

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Yong-Su Kim

Pohang University of Science and Technology

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Osung Kwon

Pohang University of Science and Technology

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Joonwoo Bae

Korea Institute for Advanced Study

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Claude Fabre

PSL Research University

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