arXiv subjects

Kazuki Koshino

Publications and source records attributed to Kazuki Koshino.

32 records · Page 2Linked to original sources

arXiv2018

Nonreciprocal microwave transmission based on Gebhard-Ruckenstein hopping

We study nonreciprocal microwave transmission based on the Gebhard-Ruckenstein hopping. We consider a superconducting device that consists of microwave resonators and a coupler. The Gebhard-Ruckenstein hopping between the resonators gives rise to a linear energy dispersion which manifests chiral propagation of microwaves in the device. This device functions as an on-chip circulator with a wide bandwidth when transmission lines are attached.

cond-mat.mes-hall↗

arXiv2016

Single microwave-photon detector using an artificial $Λ$-type three-level system

Single photon detection is a requisite technique in quantum-optics experiments in both the optical and the microwave domains. However, the energy of microwave quanta are four to five orders of magnitude less than their optical counterpart, making the efficient detection of single microwave photons extremely challenging. Here, we demonstrate the detection of a single microwave photon propagating through a waveguide. The detector is implemented with an "impedance-matched" artificial $Λ$ system comprising the dressed states of a driven superconducting qubit coupled to a microwave resonator. We attain a single-photon detection efficiency of $0.66 \pm 0.06$ with a reset time of $\sim 400$~ns. This detector can be exploited for various applications in quantum sensing, quantum communication and quantum information processing.

quant-ph↗

arXiv2015

Dressed-state engineering for continuous detection of itinerant microwave photons

We propose a scheme for continuous detection of itinerant microwave photons in circuit quantum electrodynamics. In the proposed device, a superconducting qubit is coupled dispersively to two resonators: one is used to form an impedance-matched $Λ$ system that deterministically captures incoming photons, and the other is used for continuous monitoring of the event. The present scheme enables efficient photon detection: for realistic system parameters, the detection efficiency reaches 0.9 with a bandwidth of about ten megahertz.

quant-ph↗

arXiv2015

Theory of microwave single-photon detection using an impedance-matched $Λ$ system

By properly driving a qubit-resonator system in the strong dispersive regime, we implement an "impedance-matched" $Λ$ system in the dressed states, where a resonant single photon deterministically induces a Raman transition and excites the qubit. Combining this effect and a fast dispersive readout of the qubit, we realize a detector of itinerant microwave photons. We theoretically analyze the single-photon response of the $Λ$ system and evaluate its performance as a detector. We achieve a high detection efficiency close to unity without relying on precise temporal control of the input pulse shape and under a conservative estimate of the system parameters. The detector can also be reset quickly by applying microwave pulses, which allows a short dead time and a high repetition rate.

quant-ph↗

arXiv2014

Dephasing in single-electron generation due to environmental noise probed by Hong Ou Mandel interferometry

We consider the effect of dephasing on a quantum dot which injects single electrons on a chiral edge channel of the quantum Hall effect. Dephasing is described by the coupling of the dot to a bosonic bath which represents the electromagnetic environment. Using the input-output formalism of quantum optics, we derive the density matrix of the edge degrees of freedom. Results are illustrated by computing the zero frequency current-current correlations when two such single electron emitters achieve a collision at the location of a quantum point contact, in the same spirit as the Hong Ou Mandel experiment of quantum optics. Such correlations are directly linked to the quantum mechanical purity. We show that as observed in a recent experiment, the effect of dephasing leads to a non-vanishing of the Hong Ou Mandel dip when the time delay between the two electron wave packets is zero. Generalizations to time filtered wave packets as well as to asymmetric, detuned injection between opposite edges are obtained.

cond-mat.mes-hall↗

arXiv2013

Implementation of an impedance-matched Λsystem by dressed-state engineering

In one-dimensional optical setups, light-matter interaction is drastically enhanced by the interference between the incident and scattered fields. Particularly, in the impedance-matched Λ-type three-level systems, a single photon deterministically induces the Raman transition and switches the electronic state of the system. Here we show that such a Λsystem can be implemented by using dressed states of a driven superconducting qubit and a resonator. The input microwave photons are perfectly absorbed and are down-converted into other frequency modes in the same waveguide. The proposed setup is applicable to single-photon detection in the microwave domain.

quant-ph↗

arXiv2012

Properties of a single photon generated by a solid-state emitter: effects of pure dephasing

We investigate the properties of a single photon generated by a solid-state emitter subject to strong pure dephasing. We employ a model in which all the elements of the system, including the propagating fields, are treated quantum mechanically. We analytically derive the density matrix of the emitted photon, which contains full information about the photon, such as its pulse profile, power spectrum, and purity. We visualize these analytical results using realistic parameters and reveal the conditions for maximizing the purity of generated photons.

quant-ph↗

arXiv2012

Entangling homogeneously broadened matter qubits in the weak-coupling cavity-QED regime

In distributed quantum information processing, flying photons entangle matter qubits confined in cavities. However, when a matter qubit is homogeneously broadened, the strong-coupling regime of cavity QED is typically required, which is hard to realize in actual experimental setups. Here, we show that a high-fidelity entanglement operation is possible even in the weak-coupling regime in which dampings (dephasing, spontaneous emission, and cavity leakage) overwhelm the coherent coupling between a qubit and the cavity. Our proposal enables distributed quantum information processing to be performed using much less demanding technology than previously.

quant-ph↗

arXiv2009

Deterministic photon-photon (SWAP)^{1/2} gate using a lambda system

We theoretically present a method to realize a deterministic photon-photon (SWAP)^{1/2} gate using a three-level lambda system interacting with single photons in reflection geometry. The lambda system is used completely passively as a temporary memory for a photonic qubit; the initial state of the lambda system may be arbitrary, and active control by auxiliary fields is unnecessary throughout the gate operations. These distinct merits make this entangling gate suitable for deterministic and scalable quantum computation.

quant-ph↗

arXiv2004

Drastic effects of damping mechanisms on the third-order optical nonlinearity

We have investigated the optical response of superradiant atoms, which undergoes three different damping mechanisms: radiative dissipation ($γ_r$), dephasing ($γ_d$), and nonradiative dissipation ($γ_n$). Whereas the roles of $γ_d$ and $γ_n$ are equivalent in the linear susceptibility, the third-order nonlinear susceptibility drastically depends on the ratio of $γ_d$ and $γ_n$: When $γ_d \ll γ_n$, the third-order susceptibility is essentially that of a single atom. Contrarily, in the opposite case of $γ_d \gg γ_n$, the third-order susceptibility suffers the size-enhancement effect and becomes proportional to the system size.

quant-ph↗

arXiv2004

Two-photon nonlinearity in general cavity QED systems

We have investigated the two-photon nonlinearity at general cavity QED systems, which covers both weak and strong coupling regimes and includes radiative loss from the atom. The one- and two-photon propagators are obtained in analytic forms. By surveying both coupling regimes, we have revealed the conditions on the photonic wavepacket for yielding large nonlinearity depending on the cavity Q-value. We have also discussed the effect of radiative loss on the nonlinearity.

quant-ph↗

arXiv2003

Quantum Zeno Effect for Exponentially Decaying Systems

The quantum Zeno effect -- suppression of decay by frequent measurements -- was believed to occur only when the response of the detector is so quick that the initial tiny deviation from the exponential decay law is detectable. However, we show that it can occur even for exactly exponentially decaying systems, for which this condition is never satisfied, by considering a realistic case where the detector has a finite energy band of detection. The conventional theories correspond to the limit of an infinite bandwidth. This implies that the Zeno effect occurs more widely than expected so far.

quant-ph↗

arXiv2002

Quantum Zeno and anti-Zeno effects by indirect measurement with finite errors

We study the quantum Zeno effect and the anti-Zeno effect in the case of `indirect' measurements, where a measuring apparatus does not act directly on an unstable system, for a realistic model with finite errors in the measurement. A general and simple formula for the decay rate of the unstable system under measurement is derived. In the case of a Lorentzian form factor, we calculate the full time evolutions of the decay rate, the response of the measuring apparatus, and the probability of errors in the measurement. It is shown that not only the response time but also the detection efficiency plays a crucial role. We present the prescription for observing the quantum Zeno and anti-Zeno effects, as well as the prescriptions for avoiding or calibrating these effects in general experiments.

quant-ph↗

arXiv1998

Quantum Optics in Dispersive and Absorptive Media

Using microscopic models in which both photons and excitons are treated as microscopic degrees of freedom, we discuss polaritons of two cases: One is the case when excitonic parameters are time dependent. The time dependence causes creation of polaritons from a "false vacuum." % We present the creation spectra of both % the lower- and upper-branch polaritons. It is shown that both the creation sepctra and the creation efficiency are much different from the results of the previous studies. The other is polaritons in absorptive and inhomogeneous cavities. A polariton in such a system cannot be viewed as a back and forth oscillation between a photon state and an exciton state.

quant-ph↗