Kimin Park

Entangled coherent states versus entangled photon pairs for practical quantum-information processing

We compare effects of decoherence and detection inefficiency on entangled coherent states (ECSs) and entangled photon pairs (EPPs), both of which are known to be particularly useful for quantum-information processing (QIP). When decoherence effects caused by photon losses are heavy, the ECSs outperform the EPPs as quantum channels for teleportation both in fidelities and in success probabilities. On the other hand, when inefficient detectors are used, the teleportation scheme using the ECSs suffers undetected errors that result in the degradation of fidelity, while this is not the case for the teleportation scheme using the EPPs. Our study reveals the merits and demerits of the two types of entangled states in realizing practical QIP under realistic conditions.

Nearly deterministic bell measurement for multiphoton qubits and its application to quantum information processing

We propose a Bell-measurement scheme by employing a logical qubit in Greenberger-Horne-Zeilinger entanglement with an arbitrary number of photons. Remarkably, the success probability of the Bell measurement as well as teleportation of the Greenberger-Horne-Zeilinger entanglement can be made arbitrarily high using only linear optics elements and photon on-off measurements as the number of photons increases. Our scheme outperforms previous proposals using single-photon qubits when comparing the success probabilities in terms of the average photon usages. It has another important advantage for experimental feasibility in that it does not require photon-number-resolving measurements. Our proposal provides an alternative candidate for all-optical quantum information processing.

Quantum metrology for nonlinear phase shifts with entangled coherent states

We investigate the phase enhancement of quantum states subject to nonlinear phase shifts. The optimal phase estimation of even entangled coherent states (ECSs) is shown to be better than that of NOON states with the same average particle numbernand nonlinearity exponent k. We investigate the creation of an approximate entangled coherent state (AECS) from a photon-subtracted squeezed vacuum with current optical technology methods and show that a pure AECS is even better than an even ECS for largen� . Finally, we examine the simple, but physically relevant, cases of loss in the nonlinear interferometer for a fixed average photon number � n� .

Evaluation of graphene-wrapped LiFePO4 as novel cathode materials for Li-ion batteries

Graphene-wrapped LiFePO4 (LiFePO4/G) is introduced as a cathode material for Li-ion batteries with an excellent rate capability. A straightforward solid-state reaction between graphene oxide-wrapped FePO4 and a lithium precursor resulted in highly conducting LiFePO4/G composites, which feature ∼70 nm-sized LiFePO4 crystallites with robust connection to the external graphene network. This unique morphology enables all LiFePO4 particles to be readily accessed by electrons during battery operation, leading to a remarkably enhanced rate capability. The in situ electrochemical impedance spectra were studied in detail throughout charge and discharge processes, by which enhanced electronic conductance and thereby reduced charge transfer resistance was confirmed as the origin of the superior performance in the novel LiFePO4/G.

Nearly deterministic Bell measurement with multiphoton entanglement for efficient quantum-information processing

We present a detailed analysis of the Bell measurement scheme proposed in Lee et al. [Phys. Rev. Lett. 114, 113603 (2015)] based on a logical qubit using Greenberger-Horne-Zeilinger entanglement of photons. The success probability of the proposed Bell measurement can be made arbitrarily high using only linear optics as the number of photons in a logical qubit increases. We compare our scheme with all the other proposals, using single-photon qubits, coherent-state qubits, or hybrid qubits, suggested to enhance the efficiency of the Bell measurement. As a remarkable advantage, our scheme requires only photon on-off measurements, while photon number resolving detectors are necessary for all the other proposals. We find that the scheme based on coherent-state qubits shows the best performance with respect to the attained success probability in terms of the average number of photons used in the process, while our scheme outperforms the schemes using single-photon qubits. We finally show that efficient quantum communication and fault-tolerant quantum computation can be realized using our approach.

Nonlinear potential of a quantum oscillator induced by single photons

Experimental investigation of the nonlinear dynamics of a quantum oscillator is a long standing goal of quantum physics. We propose a conditional method for inducing an arbitrary nonlinear potential on a quantum oscillator weakly interacting with light. Such an arbitrary nonlinear potential can be implemented by sequential repetition of an elementary conditional

Deterministic nonlinear phase gates induced by a single qubit

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Optimum Morphology of Mixed-Olivine Mesocrystals for a Li-Ion Battery

-gate. To implement the

Qubit-mediated deterministic nonlinear gates for quantum oscillators

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Near-deterministic Bell measurement for multiphoton quantum information processing

-gate, a single photon is linearly coupled to the oscillator and is subsequently detected by optical homodyne detection.