Theoretical analysis and implementation on QKD with the decoy-state method

 

Masahito Hayashi

Japan Science and Technology Agency

 

Our project demonstrated an improved decoy state quantum key distribution incorporating finite statistics due to the finite code length and report on its demonstration. For this purpose, we implemented key distillation system as well as quantum communication system equiped with weak coherent pulses with variable intensities. Further, in order to keep its security, we consider the following three problems.

 

1) Security of known channel with finite-length code and imperfect resources: Security formulas of quantum key distribution (QKD) with imperfect resources are obtained for finite-length code when the decoy method is applied. This analysis is useful for guaranteeing the security of implemented QKD systems. Our formulas take into account the effect of the vacuum state and dark counts in the detector. Our security is proved even if we use the threshold detector.

 

2) Asymptotic key generation formulas in decoy-state method with arbitrary number of intensities: We developed a general theory for quantum key distribution (QKD) in both the forward error correction and the reverse error correction cases when the QKD system is equipped with phase-randomized coherent light with arbitrary number of decoy intensities. For this purpose, generalizing Wang's expansion, we derive a convex expansion of the phase-randomized coherent state. We also numerically check that the asymptotic key generation rates are almost saturated when the number of decoy intensities is three. (If we include signal state, the number of intensities is four.)

 

3) Security of unknown channel with finite-length code and imperfect resources incorporating finite statistics: In this part, we described all random variables appearing our QKD system, and numerically evaluated the security by taking into account the statistical fluctuation of estimator of channel. Moreover, in our experiment, four different intensities including the vacuum state and the

signal state for optimal pulses are used and the key generation rate of 200 bps is achieved in the 20 km telecom optical fiber transmission keeping the eavesdropper's mutual information with the final key less than 2^{-9}.

 

This talk contains joint research with Jun Hasegawa, Tohya Hiroshima, Akihiro Tanaka, and Akihisa Tomita.

 

References

Masahito Hayashi, "Upper bounds of eavesdropper's performances in finite-length code with decoy method," quant-ph/0702250; To appear in PRA

 

Masahito Hayashi "General theory for decoy-state quantum key distribution with arbitrary number of intensities," quant-ph/0702251

 

Jun Hasegawa, Masahito Hayashi, Tohya Hiroshima, Akihiro Tanaka, Akihisa Tomita "Experimental Decoy State Quantum Key Distribution with Unconditional Security Incorporating

Finite Statistics," quant-ph/0705.3081

 

Jun Hasegawa, Masahito Hayashi, Tohya Hiroshima, Akihisa Tomita in preparation.

 

 

 

 

 

 

Quantum Key Distribution Using Quantum Faraday Rotators

Mahn-Soo Choi and Taeseung Choi

Korea University

 

 

We propose a new quantum key distribution protocol based on the fullyquantum mechanical states of the Faraday rotators.  The protocol is unconditionally secure against collective attacks for multi-photon source up to two photons on a noisy environment.  It is also robust against impersonation attacks.  The protocol may be implemented experimentally with the current spintronics technology on semiconductors.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Quest for the experimental evidence of the working principle of Kane's Quantum computer model

 

Soonchil Lee

Department of Physics, Korea Advanced Institute of Science and Technology

 

One of the best practical quantum computer designs reported so far is the system consisting of phosphorus atoms regularly spaced inside silicon crystal as proposed by B. Kane. The hyperfine interaction between the nuclear spin and the electron spin of the donor, and the indirect exchange interaction between donor nuclear spins can be controlled by electric field realizing one and two-qubit operations, respectively. Experimental demonstration, however, has not been done yet, since the technologies required for manufacturing of the device and detection of single spin state are not fully developed yet. We plan to get the experimental evidence of the electric control of hyperfine field by NMR. The first part of the experiment is to observe P NMR signal, which has not been observed yet. The second part of the experiment is show that the hyperfine interaction can be controlled by external electric field. An external electric field will push away the electron wave function of the donor atom from the nucleus position. Then, it is expected that NMR resonance frequency of the donor nucleus decreases due to the reduced hyperfine interaction.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Remote application of operators

 

Nguyen Ba An

Institute of Physics, Vietnam and

Korea Institute for Advanced Study, Korea

 

Alice and Bob are two distant parties. We deal with remote application on Bob's arbitrary quantum state of an operator which is immersed in a lump operator given to Alice. For such kind of task the usual method of bidirectional quantum state teleportation does not help. Shared EPR pairs with entanglement swapping do not either. We show however that the task can be performed using a GHZ state shared between Alice and Bob. We also propose a quantum scheme to control the task by a number of third parties based on sharing a multipartite GHZ state.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

What can we do with entangled photons?

 

Sang-Kyung Choi

Korea Research Institute of Standards and Science

 

A brief introduction covers the concept of entanglement in the context of photonic quantum information. The process of spontaneous parametric down-conversion (SPDC) is shown to be a practical method for generating an entangled pair of photons. Furthermore SPDC in conjunction with interferometry can generate entanglement of a higher number of photons. Such multiphoton entanglement can be applied to demonstrations of quantum computation and quantum lithography. Strongly correlated photon pairs can also find potential applications in quantum metrology.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Teleportation capability, distillability, and nonlocality
on three-qubit states

 

Soojoon Lee

Department of Mathematics, Kyung Hee University

 

In this work, we consider teleportation capability, distillability, and nonlocality on three-qubit states. In order to investigate some relations among them, we first find the explicit formulas of the quantities about the maximal teleportation fidelity on three-qubit states. We show that if any three-qubit state is useful for three-qubit teleportation then the three-qubit state is distillable into a Greenberger-Horne-Zeilinger state, and that if any three-qubit state violates a specific form of Mermin inequality then the three-qubit state is useful for three-qubit teleportation.

 

This is a joint work with Jaewoo Joo and Jaewan Kim

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Many-qubit interactions in superconducting flux qubits

 

Sam Young Cho

 

Centre for Modern Physics and Department of Physics, Chongqing University, Chongqing 400044, The People¡¯s Republic of China

 

 

The last decade has seen rapidly developing advanced material technologies that make it possible to investigate previously inaccessible quantum systems for quantum information and computation in solid-state systems. Especially, coherent manipulation of quantum states in tunable superconducting devices has enabled to demonstrate macroscopic qubits and entangled states of qubits. Experimentally, it has been shown that, in terms of pseudo-spins, various types of exchange interactions between two artificial-spins such as an Ising interaction for charge qubits and flux qubits and an XY interaction for phase qubits, can be realized and controlled by the system parameters. We discuss, in a general framework, how artificial-multiple spin interactions are possible and realizable in superconducting qubit systems. In fact, flux qubit systems are shown to have an intrinsic property which is multiple artificial-spin interactions. Accordingly, flux qubit systems enable to study various artificial-spin systems corresponding to many-body systems unlikely found naturally. Examples are discussed to include artificial spin interactions in two-flux qubit systems and multiple-artificial spin interactions in four-flux qubit system.

 

[1] S. Y. Cho and M. D. Kim, arXiv:cond-mat/0703505 (2007).

[2] M. D. Kim and S. Y. Cho, arXiv:0705.399 [cond-mat.super-con] (2007). 

[3] Q. Q. Shi, S. Y. Cho, B. Li, and M. D. Kim, arXiv:0706.2402 [cond-mat.mes-hall] (2007).

 

Acknowledgements: This work was supported in part by the Australian Research Council.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Building higher-dimensional photonic quantum states and entanglement: A bottom-up approach

 

Yoon-Ho Kim

Department of Physics, Pohang University of Science and Technology

 

 

Two-dimensional quantum states and their entanglement (i.e., qubits and entangled qubits) have long been important in quantum information research. Recently, higher-dimensional quantum states and their entanglement (i.e., quDits and entangled quDits) have been found to

be important for better understanding the nature of quantum entanglement and in several areas of quantum information research.

 

In photonic quantum information research, qubits are almost always implemented using the polarization states of a photon, even though choosing another two-dimensional photonic degree of freedom should make no in-principle difference. This is due to the fact that the

polarization state of a photon is much easier to generate, manipulate, and measure than any other photonic degrees of freedom.

 

For higher-dimensional photonic quantum states, the situation has been a bit different. Degrees of freedom that are difficult to manipulate, such as, orbital angular momentum, spatial path, etc, have been used to demonstrate higher-dimensional single- and

entangled-photon states. Only recently, pure qutrit (three- dimensional) and ququart (four-dimensional) states based on the polarization degree of freedom has been reported.

 

In this talk, we first discuss why such odd choices have been made for implementing higher-dimensional photonic quantum states. We then introduce schemes for generating a general four-dimensional photonic quantum state (ququart) using the polarization state of a pair of

photons and discuss how one can entangle two such ququarts linear optically.

 

Unlike other higher-dimensional two-photon entanglement schemes, the state is built bottom-up in the proposed method: ququart states are built using pairs of photons and these ququarts get entangled linear optically (modulo coincidence post-selection). Potential benefits of the bottom-up approach to quantum state generation will be discussed and an on-going experiment in our laboratory to implement the proposal will be presented.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Generating new mesoscopic object via conditional measurement and its properties

 

Sergey A. Podoshvedov1, Jaewan Kim1, Juhui Lee2 and Sung Dahm Oh2

 

1School of Computational Sciences, Korea Institute for Advanced Study, Seoul, Korea

2Department of Physics, Sookmyung Women¡¯s University, Seoul, South Korea

 

 

We propose a scheme for generating Schrodinger cat states of a single-mode optical field by means of conditional measurement. Combining one mode prepared in a state of a single-mode qubit and another mode in a coherent state, the state of one of the output modes of the beam splitter exhibits a number of properties similar to those of superposition of displaced vacuum and single photon, provided that no photons are detected in the other output channel. We present analytical and numerical results concerning the superposition of macroscopic vacuum and macroscopic one-photon, with special emphasis on interference properties of the state. We show the state has both microscopic and macroscopic properties. Finally, we discuss the effect realistic photocounting on generation of the state.       

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Role of quantum noise of atoms and photons on quantum correlations

 

Raymond Ooi

Department of Physics, Korea Advanced Institute of Science and Technology

 

 

 

I present the physical origin of quantum noise in atom-field interactions based on Heisenberg operator approach. The approach is used to study photon correlation in extended medium or amplifier. For a double Raman configuration, I identify the conditions where the boundary solutions alone give qualitatively correct two-photon correlation, without the need for quantum noise. However, in general the quantum noise is required to obtain correct correlation. Physical significances of the quantum noise and boundary operators are discussed.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Addressing Individual Atoms in Optical Lattices with Standing-Wave Driving Fields

 

Jaeyoon Cho

Korea Research Institute of Standards and Science

 

A scheme for addressing individual atoms in one- or two-dimensional optical lattices loaded with one atom per site is proposed. The scheme is based on position-dependent atomic population transfer induced by several standing-wave driving fields. This allows various operations important in quantum information processing, such as manipulation and measurement of any single atom, two-qubit operations between any pair of adjacent atoms, and patterned loading of the lattice with one atom per every nth site for arbitrary n. The proposed scheme is robust against considerable imperfections and actually within reach of current technology.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Atomic superposition state measurements with the Cavity-QED Microlaser

 

Kyungwon An

School of Physics and Astronomy, Seoul National University

 

Quantum mechanical superposition state of a two-level atom is measured by using the cavity-QED microlaser as a state detector. It has been found that atoms initially prepared in various superposition states result in different frequency shifts and asymmetry in the laser output so that arbitrary initial superposition states could be identified. In addition, the polarity of the atom-cavity coupling in the microlaser can be manipulated by employing high-order cavity modes in order to induce new types of atom-field interactions. Certain geometrical structures are observed in the lasing gain, associated with the spatially modulated atom-cavity coupling constant.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Relation between the degree of the entanglement and the resolution of a quantum interferometer

 

Taesoo Kim

Department of Physics, University of Ulsan

 

It is well known that maximally entangled states, so called, NOON states, inside the interferomter lead to the Heisenberg limited precision.  We have analyzed the effect of the partial entanglement in a Mach-Zehnder type quantum interferometer on sensitivities of the interferometer. The calculation results show that the visibility of the interference fringe is proportional to the degree of entanglement, and that the phase uncertainty of the interferometer is inversely proportional to the degree of entanglement for both a Fock state and a coherent state inputs.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Control of black hole evaporation?

 

Doyeol Ahn

Department of Physics, University of Seoul

 

Contradiction between Hawking's semi-classical arguments and string theory on the evaporation of black hole has been one of the most intriguing problems in fundamental physics. A final-state boundary condition inside the black hole was proposed by Horowitz and Maldacena to resolve this contradiction. The Horowitz and Maldacena conjecture on the black hole final state is proven for the Schwarzschild black with collapsing gravitaional shell. I also point out that original Hawking effect can be also regarded as a separate boundary condition at the event horizon for this scenario. Here, I found that the change of Hawking boundary condition may affect the information transfer from the initial collapsing matter to the outgoing Hawking radiation during evaporation process and as a result the evaporation process itself, significantly.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Entanglement generation between stationary qubits separated by a remote distance

 

Hyuk Jae Lee

Department of Physics, Kookmin University

 

 

 

It is very important to make the entangled state of spatially separated qubits for the scalable quantum computer. We present how to entangle the qubits which are apart with each others. Controlling the interaction time with a auxiliary moving particle, two remote particles prepared initially by the separable state become the maximally entangled state. This suggest the XOR gate for remote particles.  Furthermore, we show that the N-qubit prepared initially by the separable state can become the W-state from the same interaction as the two-qubit case.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Device and System Technologies for Quantum Information Processing

 

Jungsang Kim

FCIEMAS, ECE Department, Duke University, Durham, NC 27708, USA

 

 

Quantum information processing is moving rapidly from fundamental science to the practical world. The most significant example is quantum key distribution technology, where commercial grade field trials have successfully been completed. High performance single photon sources and detectors remain to be bottleneck for improving system performance. Engineering approach to quantum computers are necessary to take the next step in demonstrating the feasible path to scalability. I will discuss some of the recent developments in hardware technology and architectural concepts for a quantum information processor.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Coupling to environment: Atoms in electromagnetic field

Jae-Seung Lee

Department of Chemistry, Kent State University, USA


The problem of spontaneous emission has been studied on a combined system of an atom and radiation field. The dynamics has been numerically calculated by the direct diagonalization of the Hamiltonian for the system with up to 15000 field oscillators. The parameters of the discrete finite model were optimized by comparing results with the exact solution when the oscillators have equidistant frequencies and equal coupling constants. We can addresse some problems too complex for analytical treatment with this numerical approach, which involve perturbation by a train of composite laser pulses and emission by multi-atom systems.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Application of Quantum Cellular Automata in Quantum Information: A Brief Introduction 

 

Young Soon Kim

Department of Physics, Myongji University, Korea

 

 

 

Cellular automata will be defined with illustrative examples. Some quantum cellular automation rules will be introduced to demonstrate that desired quantum states can be generated in a simple one-dimensional array of qubits utilizing the notion of quantum cellular automata. 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Storage of spin squeezing in a two component BEC

 

Sang Wook Kim

Department of Physic Education, Pusan National University

 

We have investigated the coherent control of spin squeezing of two component Bose-Einstein condensates (TBEC) focusing on long time maintenance of the spin squeezing. We find that by rapidly turning off the external field at a time when the maximal squeezing occurs in the TBEC the spin squeezing can be maintained in a nearly fixed direction for very long time. We explain the underlying physics by using phase model, and provide analytic expressions for the maximal squeezing time and the corresponding squeezing parameter.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Finite States Teleportation and Quantum Repeater for QKD

 

Jae-Weon Lee

School of Computational Sciences, Korea Institute for Advanced Study

 

 

 

Although there are already many practical QKD protocols such as the decoy state protocol, the maximum distance of conventional QKD systems is limited to about 200 km by photon absorption. On the other hand, building a quantum repeater for global QKD is a very challenging task. Fortunately, for QKD purposes, it is not necessary to teleport an arbitrary qubit state. Teleporting a state among a finite set is enough for QKD. We suggest a quantum repeater that teleports only finite states and hence requires less quantum resources. We compare required entanglements for teleportation of arbitrary states with that of finite states. A QKD project done by ETRI-KIAS-Korea university is also reviewed.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Helstrom Theorem by No-Signalling Condition

 

Won-Young Hwang

Department of Physics Education, Chonnam National University

 

 

We prove a special case of Helstrom theorem by using no-signaling condition in the special theory of relativity that faster-than-light communication is impossible.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Asymptotic quantum cloning is state estimation

 

Joonwoo Bae

School of Computational Sciences, Korea Institute for Advanced Study

 

The impossibility of perfect cloning and state estimation are two fundamental results in Quantum Mechanics. It has been conjectured that quantum cloning becomes equivalent to state estimation in the asymptotic regime where the number of clones tends to infinity. We prove this conjecture using two known results of Quantum Information Theory: the monogamy of quantum correlations and the properties of entanglement breaking channels.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Do the homework of quantum mechanics with quantum computers

 

Sangchul Oh

Max Planck Institute for the Physics of Complex Systems, Noethnitzer Str. 38, D-01187, Dresden, Germany

 

Quantum simulation is one of the most promising applications of quantum computers. It has been believed that a quantum computer can simulate a quantum system better than a classical computer because a quantum computer itself is a quantum system [1]. However, it seems to be unclear how to simulate an interacting quantum system and to obtain its physical properties with quantum computers. In this talk, we present a new method to obtain the ground state energy and expectation values of interacting quantum systems with quantum computers. Our method is based on the combination of adiabatic quantum evolution and quantum phase estimation algorithm. Adiabatic turning on the interaction makes the ground state of a non-interacting system evolve to the ground state of an interacting system. By applying the quantum phase estimation algorithm, the phase of an evolving quantum system can be measured continuously without collapse of a quantum state. So the ground energy of an interacting system can be obtained as a function of coupling strength. With the help of the Hellman-Feyman theorem, the expectation value of an observable for the ground state of an interacting system is obtained. As real applications of our method, we solve three problems of quantum mechanics by implementing our method on a classical computer. First, we consider the one-dimensional finite XY spin model. From the paramagnetic ground state, we obtain the ferromagnetic ground state and its spin correlation functions. Second, we study a harmonic oscillator with the quartic term, for which the usual perturbation theory cannot be applied. With several qubits, our method gives us very precise energy spectrum in agreement with Bender and Wu's result [2]. Finally, we study the one-dimensional potential scattering model. This can be solved exactly but the conventional perturbation theory breaks down. We obtain the energy spectrum and the bound state for attractive potential in agreement with the exact result [3]. We note that our method is similar to quantum Monte Carlo methods. But it does not suffer from the sign problem which the quantum Monte Carlo method has. We also discuss how our method can be applied to other quantum many-body systems such as lattice fermions and bosons, atoms, and molecules.

 

[1] R.P. Feynman, Int. J. Theor. Phys. 21, 467 (1982);  Found. Phys. 16, 507 (1986).

[2] C.M. Bender and T.T. Wu, Phys. Rev.  184, 1231 (1969).

[3] S. Kehrein, ``The Flow Equation Approach to Many-Particle Systems" (Springer-Verlag, Berlin, 2006).

 

 

 

 

 

 

 

 

 

 

 

 

Complementarity and nonlocality test in electronic Mach-Zehnder interferometers

 

Kicheon Kang

Chonnam National University, Korea

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Entanglements in superconducting flux qubits

 

Mun Dae Kim

School of Physics, Korea Institute for Advanced Study, Korea

 

 

 

Superconducting flux qubits are considered theoretically in order to generate the quantum entanglement. A phase coupling is proposed in a way of superconducting loops connecting the qubits, which can offer enough strength of interactions between qubits. For two coupled flux qubits, Bell states are obtained at the ground and excited states of the coupled qubits system, as the energy levels are split  and the two qubit tunnelling channel is opened due to the strong coupling strength. We also investigate two types of genuine three-qubit entanglement, known as the Greenberger-Horne-Zeilinger (GHZ) and W states. While an excited state can be the W state, the ground state of the three-flux qubits is shown to be the GHZ state. Specific ranges of system parameters are discussed to realize the GHZ state and the W state experimentally.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Incompatibility between Self-Observing Consciousness and the Axioms of Quantum theory

 

Daegene Song

School of Computational Sciences, Korea Institute for Advanced Study, Korea

 

Based on the standard axioms of quantum theory, we provide a counter-example which invalidates the full compatibility between consciousness and quantum theory. In particular, we present an example of a natural phenomenon where the conscious status of an observer can be fully described in mathematical terms analogous to the state vector that is being observed without relying on any analysis through a brain or biological means. This mathematical description of the observer's conscious state enables one to examine consciousness within the standard axioms of quantum theory. The separation between the observing party and the physical system being observed, imposed in the axiom of quantum theory, poses a problem when the observer is observing one's own conscious state, i.e., self-observing consciousness.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Which-path detection in a closed-loop Aharonov-Bohm interferometer

 

Yunchul Chung1, Dong-In Chang2, Gyong Luck Khym3, Kicheon Kang3,  Hu-Jong Lee2, Minky Seo1, Diana Mahalu4, Vladimir Umansky4, Moty Heiblum4

 

1Department of Physics, Pusan National University, Korea

2Department of Physics, Pohang University of Science and Technology, Korea

3Department of Physics, Chonnam National University, Korea

4Department of Condensed Matter Physics, Weizmann Institute of Science, Rehovot, Israel

 

The wave-particle duality of electrons was tested in a closed-loop-type Aharonov-Bohm interferometer fabricated on GaAs/AlGaAs heterojunction two-dimensional electron gas system. The which-path electron detector consisted of a quantum-dot (QD) structure imbedded in one arm of the interferometer and a quantum point contact (QPC) located in proximity to the QD. The quantum point contact coupled to the quantum dot detects the presence of the charge inside the quantum dot to give the path information of the electron inside the interferometer. The which-path detection is inevitably coupled to the interferometer and the environments, leading to dephasing in the traversing electron state. This thus results in the suppression of the interference of conducting electrons, the manifestation of a wave nature of the particles.. In this work, observation similar to the one using an open-loop-type electron interferometer [1] was carried out in a closed-loop-type electron interferometer to obtain more detailed information on the dephasing mechanism and its origin. Especiallly, dephasing of electrons circulating multiple times inside the interferometer was studied in depth. This study confirms that the which-path detection is the real cause of the dephasing.

            

[1] E. Buks, R. Schuster, M. Heiblum, D. Mahalu and V. Umansky, Nature 391, 871 (1998)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Relative particle nature and nonclassicality of light

 

Ki-Sik Kim

 

 Inha  University, Korea

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Engineering nonclssical light for quantum information processing

 

Hyunseok Jeong

Centre for Quantum Computer Technology, Department of Physics, University of Queensland, Brisbane, Australia

 

I will present several schemes to engineer non-classical states in free-traveling optical fields, which are useful for quantum information processing, using experimentally available resources. In particular, I will describe a protocol which allows to produce Schrodinger cat states (quantum superpositions of macroscopically distinguishable coherent states) in free-traveling optical fields, using a homodyne detection and photon number states as resources. This protocol was experimentally implemented with light pulses containing two photons, producing squeezed Schrodinger cat states with a negative Wigner function.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Entanglement generation via beam splitter and its detection

 

Hyunchul Nha

School of Computational Sciences, Korea Institute for Advanced Study, Korea

 

 

Two-mode entangled states can be generated by injecting a single-mode "nonclassical" state into a beam splitter, but in general, it is a hard task to determine whether a two-mode state is entangled or separable. In this talk, I will show how to derive a generalized class of separability conditions that can detect entanglement generated via a beam splitter using the previously known nonclassical states. These nonclassical properties include (1) quadrature squeezing and higher-order squeezing, and (2) sub-Poissonian and higher-order photon-number statistics.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

                    POSTER SESSION

 

1) ¡°Theoretical analysis and implementation on QKD with the decoy-state method¡±

by Masahito Hayashi

(Japan Science and Technology Agency, Japan)

 

2) ¡°Nonclassical properties of photon-added and photon-subtracted states¡±

by Su-Yong Lee (KAIST, Korea)

 

3) ¡°Thoery of single and two modes superposition states consisting from macroscopic to microscopic states¡±

by Juhui Lee (Sookmyung Women¡¯s University, Seoul, South Korea)

 

4) ¡°The role of entanglement in quantum lithography  ¡±

by Sun-Kyung Lee (KAIST, Korea)

 

5) ¡°Generating entangled states of two ququarts using linear optical elements¡± by So-Young Baek  (POSTECH, Korea)

 

6) ¡°Preparation of general single-ququart states using ultrafast spontaneous parametric down-conversion¡± by So-Young Baek  (POSTECH, Korea)

 

7) ¡°Group velocity dispersion effect on entangled photon transmission through optical fibers¡± by Osung Kwon (POSTECH, Korea)

 

8) ¡°Free-space quantum cryptography testbed : Weak pulse implementations of BB84 and B92 protocols¡± by Youn-Chang Jeong (POSTECH, Korea)