Sun-Hyun Youn

A balanced homodyne detector for high-rate Gaussian-modulated coherent-state quantum key distribution

We discuss the excess noise contributions of a practical balanced homodyne detector (BHD) in Gaussian-modulated coherent-state (GMCS) quantum key distribution (QKD). We point out that the key generated from the original realistic model of GMCS QKD may not be secure. In our refined realistic model, we take into account excess noise due to the finite bandwidth of the homodyne detector and the fluctuation of the local oscillator (LO). A high-speed BHD suitable for GMCS QKD in the telecommunication wavelength region is built and experimentally tested. The 3 dB bandwidth of the BHD is found to be 104 MHz and its electronic noise level is 13 dB below the shot noise at an LO level of 8.5×108 photons per pulse. The secure key rate of a GMCS QKD experiment with this homodyne detector is expected to reach Mbits s−1 over a few kilometers.

Instant single-photon Fock state tomography

Heralded single photons are prepared at a rate of approximately 100 kHz via conditional measurements on polarization-nondegenerate biphotons produced in a periodically poled potassium-titanyl phosphate crystal. The single-photon Fock state is characterized using high-frequency pulsed optical homodyne tomography with a fidelity of (57.6+/-0.1)%. The state preparation and detection rates allowed us to perform on-the-fly alignment of the apparatus based on real-time analysis of the quadrature measurement statistics.

Spatial and temporal characterization of a Bessel beam produced using a conical mirror

We experimentally analyze a Bessel beam produced with a conical mirror, paying particular attention to its superluminal and diffraction‐free properties. We spatially characterized the beam in the radial and on‐axis dimensions, and verified that the transverse profile is constant over a distance of 73 cm. In addition, we measured the superluminal phase and group velocities of the beam in free space. Both spatial and temporal measurements show good agreement with the theoretical predictions.

Double pendulum model for a tennis stroke including a collision process

By means of adding a collision process between the ball and racket in the double pendulum model, we analyzed the tennis stroke. The ball and the racket system may be accelerated during the collision time; thus, the speed of the rebound ball does not simply depend on the angular velocity of the racket. A higher angular velocity sometimes gives a lower rebound ball speed. We numerically showed that the proper time-lagged racket rotation increased the speed of the rebound ball by 20%. We also showed that the elbow should move in the proper direction in order to add the angular velocity of the racket.

Conditional generation scheme for high-fidelity Yurke-Stoler states by mixing two coherent beams with a squeezed vacuum state

The numerical conditions to generate high-fidelity Yurke-Stoler states (|α > +eiψ| − α >)were found for two cascade-placed beam splitters with one squeezed state input and two coherent state inputs. Controlling the amplitudes and the phases of beams allows for various Yurke-Stoler states to be manipulated with ultra-high fidelity, and the expected theoretical fidelity is more than 0.9999.

Conditional generation scheme for a high-purity superposed state of single and two-photon states by mixing two coherent beams with squeezed vacuum states

The numerical conditions to generate high-purity |1 >, |2 >, and |1 > + reiψ|2 > states were found from the explicit form of the probability amplitudes for two cascade-placed beam splitters with one squeezed state input and two coherent state inputs. The expected theoretical signal-tonoise ratio is more than 1000. Controlling the amplitudes and the phases of two coherent beams, as well as the transmittances of two beam splitters, allow various quantum states to be manipulated with a high signal-to-noise ratio. High-purity nonclassical states are expected to be a key element in quantum information science.

Quantum Tomography of the Single‐Photon State Generated by Down Conversion in a Periodically Poled KTP Crystal

We report the preparation of the single‐photon Fock state |1〉 by conditional measurements on the bi‐photon generated in a periodically poled KTP crystal. We employ time‐resolved homodyne detection to measure the quadrature fluctuations and characterize the single photon state with an overall efficiency of 53%. The photon production rate is over 9000 s−1, which is much higher than in previous experiment of the kind. Additionally, the experiment employs a new balanced detector whose bandwidth permits time‐resolved measurement at the repetition rate of the mode‐locked master laser, without the need for a pulse picker.

Going beyond the kinetic chain process in a swing by using a triple pendulum model

An efficient way to transfer input potential energy to the kinetic energy of a racket or bat was analyzed by using two coupled harmonic oscillators and the triple pendulum. We find the most efficient way to transfer energy based on the kinetic chain process. Using control parameters, such as the release times and the lengths and masses of the triple pendulum, we showed how to optimize the kinetic chain process. We also introduce a new method to get an efficient way to transfer initial energy to the kinetic energy of the third rod in the triple pendulum without a time delay, which is considered an essential part of a kinetic chain process.

Measuring the polarization state of the weak signal by the homodyne detection

Optical homodyne detection was adopted to eavesdrop the quantum key in quantum cryptography [1]. We measured the polarization state of the weak signal field by means of the polarization-controllable homodyne detector that we proposed[2]. Homodyne detection is a standard method to measure the quantum mechanical properties of light[3-6]. In this method, a weak signal field is combined with a strong local oscillator field at a beam splitter, and the resulting signal is measured as a current. When the local oscillator field is much stronger than the signal field, the difference between the output of the two ports is proportional to the signal field. In a balanced homodyne detection method, when a 50-50 beam splitter is used, the overall fluctuations of a local oscillator field can be subtracted. With the homodyne detection method, the quadrature amplitude of the squeezed light is easily measured[7-9]. Also, the quasiprobability distribution function can be measured using so-called optical homodyne tomography[10-13]. And optical homodyne detection was adopted to eavesdrop the quantum key in quantum cryptography[1]. In the polarization controllable balanced homodyne detection(PCBHD) method the polarization of the signal field and the local oscillator field are different from each other and we can obtain various information by varying the polarization state of the local oscillator field(Fig. 1). Fig. 1 schematic diagram of the PCBHD The polarization vectors of the signal field and local oscillator field are can be expressed in general as 1 2 1 2 ˆ i i S e a e i a e j δ δ = + (1) ˆ cos sin L i L L L e i e j φ θ θ = + (2) where ˆS e , ˆL e are the polarization vector of the signal field and the local oscillator field, 1 a , 2 a are amplitude factor of the signal field and satisfy 2 2 1 2 1 a a + = , 1 δ , 2 δ are phase factor of the signal field, L θ is the inclination angle of the local oscillator field and L φ is the phase difference between two perpendicular components of the polarization vector of the local oscillator field. The output current ( ) I t of the detector is the difference between two detector currents and is expressed as

Ghost Imaging With Classically Correlated Beams

Quantum ghost imaging uses quantum mechanically entangled photons to form an image of an object. The quantum ghost image is also obtained by means of classical coincidence measurements with a classically correlated light source[1,2]. In this work we performed classical coincidence imaging experiments with classically correlated beams in their direction of propagation. We observed the ghost interference patterns which were usually made by quantum mechanically entangled states and we also analyze in detail the mechanism of the ghost imaging with classically correlated lights. We made? the classically correlated source with an Ar laser and controlled the direction of the light by a mirror? mounted on a small speaker.