By combining a tilted-pulse-intensity-front scheme using a LiNbO3 crystal and a chirped-pulsebeating method, we generated a narrowband intense terahertz (THz) pulse, which had a maximum electric field of more than 10 kV/cm at around 2 THz, a bandwidth of similar to 50 GHz, and frequency tunability from 0.5 to 2 THz. By performing THz-pump and near-infrared-probe experiments on GaAs quantum wells, we observed that the resonant excitation of the intraexcitonic 1s-2p transition induces a clear and large Autler-Townes splitting. Our time-resolved measurements show that the splitting energy observed in the rising edge region of electric field is larger than in the constant region. This result implies that the splitting energy depends on the time-averaged THz field over the excitonic dephasing time rather than that at the instant of the exciton creation by a probe pulse. (C) 2015 AIP Publishing LLC.

Secrecy issues of free-space optical links realizing information theoretically secure communications and high transmission rates are discussed. We numerically study secrecy communication rates of optical wiretap channel based on on-off keying (OOK) modulation under typical conditions met in satellite-ground links. It is shown that, under reasonable degraded conditions on a wiretapper, information theoretically secure communications should be possible in a much wider distance range than a range limit of quantum key distribution, enabling secure optical links between geostationary Earth orbit satellites and ground stations with currently available technologies. We also provide the upper bounds on the decoding error probability and the leaked information to estimate a necessary code length for given required levels of performances. This result ensures that a reasonable length of wiretap channel code for our proposed scheme must exist.

Secrecy issues of free-space optical links realizing information theoretically secure communications and high transmission rates are discussed. We numerically study secrecy communication rates of optical wiretap channel based on on-off keying (OOK) modulation under typical conditions met in satellite-ground links. It is shown that, under reasonable degraded conditions on a wiretapper, information theoretically secure communications should be possible in a much wider distance range than a range limit of quantum key distribution, enabling secure optical links between geostationary Earth orbit satellites and ground stations with currently available technologies. We also provide the upper bounds on the decoding error probability and the leaked information to estimate a necessary code length for given required levels of performances. This result ensures that a reasonable length of wiretap channel code for our proposed scheme must exist.

© 2015 American Physical Society.We demonstrate an all-fiber cavity quantum electrodynamics system with a trapped single atom in the strong coupling regime. We use a nanofiber Fabry-Perot cavity, that is, an optical nanofiber sandwiched by two fiber-Bragg-grating mirrors. Measurements of the cavity transmission spectrum with a single atom in a state-insensitive nanofiber trap clearly reveal the vacuum Rabi splitting.

We demonstrate time-bin entanglement generation in telecom wavelength using a 7 mu m radius Si micro-ring resonator pumped by a continuous wave laser. The resonator structure can enhance spontaneous four wave mixing, leading to a photon pair generation rate of about 90-100 Hz with a laser pump power of as low as -3.92 dBm (0.41 mW). We succeed in observing time-bin entanglement with the visibility over 92%. Moreover, wavelength-tunability of the entangled photon pair is demonstrated by changing the operation temperature. (C) 2015 Optical Society of America

We propose a scheme for single-atom, quantum control of the direction of propagation of a coherent field incident, via a tapered fiber, upon a microtoroidal whispering-gallery-mode (WGM) resonator. The scheme involves overcoupling of the fiber taper to the resonator and strong coupling of an atom to the evanescent field of the WGM, i.e., an atom-field coupling that exceeds the total WGM linewidth. In contrast to previous, related schemes that operate in the bad-cavity regime, the proposed scheme can operate effectively with much stronger incident fields, while also preserving their coherent nature. It can also serve to prepare an entangled state of the atom and coherent optical pulses propagating in opposite directions along the fiber. We evaluate the fidelity of preparation of such a state taking into account absorption and atomic spontaneous emission and demonstrate that high fidelities should be possible with realistic parameters.

We design and fabricate ultra-low-loss tapered optical fibers (TOFs) with minimal lengths. We first optimize variations of the torch scan length using the flame-brush method for fabricating TOFs with taper angles that satisfy the adiabaticity criteria. We accordingly fabricate TOFs with optimal shapes and compare their transmission to TOFs with a constant taper angle and TOFs with an exponential shape. The highest transmission measured for TOFs with an optimal shape is in excess of 99.7% with a total TOF length of only 23 mm, whereas TOFs with a constant taper angle of 2 mrad reach 99.6% transmission for a 63 mm TOF length. (C) 2014 Optical Society of America

We study the elimination of the chirp of narrowband terahertz pulses generated by chirped laser pulse beating using a laser pulse stretcher with two grating pairs that cancel out the third-order spectral phase. First, we show that positively chirped terahertz pulses can be generated using a pulse stretcher with a grating pair and internal lenses. We then combine this with a second grating pair, the spectral phase of which has the opposite sign to that of the first one. By varying the separation of the second grating pair, we experimentally verify that the chirp of the generated terahertz pulses can be eliminated. (C) 2014 Optical Society of America

Single-photon sources are important elements in quantum optics and quantum information science. It is crucial that such sources be able to couple photons emitted from a single quantum emitter to a single propagating mode, preferably to the guided mode of a single-mode optical fiber, with high efficiency. Various photonic devices have been successfully demonstrated to efficiently couple photons from an emitter to a single mode of a cavity or a waveguide. However, efficient coupling of these devices to optical fibers is sometimes challenging. Here we show that up to 38% of photons from an emitter can be directly coupled to a single-mode optical fiber by utilizing the flat tip of a silica nanofiber. With the aid of a metallic mirror, the efficiency can be increased to 76%. The use of a silicon waveguide further increases the efficiency to 87%. This simple device can be applied to various quantum emitters.

We demonstrate a method for controlling the chirp of the frequency-tunable narrowband terahertz pulses that are generated by photomixing with nonlinearly chirped laser pulse pairs. We find that in a grating-based laser-pulse stretcher, the frequency sweep rates of the generated terahertz pulses can be controlled by simply changing the incident angle. This method is also applicable to other mechanisms of terahertz pulse generation. (C) 2014 AIP Publishing LLC.

We theoretically investigated a microtoroidal cavity quantum electrodynamics system in which radiation from the emitter is nearly perfectly guided into a fiber mode, and analyzed the resonance fluorescence from the emitter after pulsed excitation. We derived analytic formulas to rigorously evaluate the photon statistics of the pulse emitted into the fiber, and clarified the conditions needed for the excitation pulse to generate single- and two-photon pulses.

We show that a hemispherically shaped tip on an air-clad optical fiber simultaneously works as a high-numerical-aperture lens and efficiently collects photons from an emitter placed near the beam waist into the fundamental guided mode. Numerical simulations show that the coupling efficiency reaches about 25%. We have constructed a confocal microscope with such a lensed fiber. The measurements are in good agreement with the numerical simulation. The monolithic structure with a high-photon-collection efficiency will provide a flexible substitute for a conventional lens system in various experiments such as single-atom trapping with a tightly focused optical trap. (C) 2014 Optical Society of America

:Single-photon sources are important elements in quantum optics and quantum information science. It is crucial that such sources be able to couple photons emitted from a single quantum emitter to a single propagating mode, preferably to the guided mode of a single-mode optical fiber, with high efficiency. Various photonic devices have been successfully demonstrated to efficiently couple photons from an emitter to a single mode of a cavity or a waveguide. However, efficient coupling of these devices to optical fibers is sometimes challenging. Here we show that up to 38% of photons from an emitter can be directly coupled to a single-mode optical fiber by utilizing the flat tip of a silica nanofiber. With the aid of a metallic mirror, the efficiency can be increased to 76%. The use of a silicon waveguide further increases the efficiency to 87%. This simple device can be applied to various quantum emitters.

We theoretically show that it is possible to generate chirp-free terahertz (THz) pulses with a chirped-pulse beating method by using an optical fiber as a pulse stretcher. Proper choices of the core radius and the dopant fraction of the core material of a step-index single-mode optical fiber eliminate the third-order spectral phase of the fiber, thus giving the pump laser pulse a purely linear chirp. We also show that even a standard commercial single-mode optical fiber can give THz pulses of lower chirp than the lower limit for a grating pair. We perform experiments to verify our theory. © 2013 AIP Publishing LLC.

Efficient coupling of photons from single-photon source to the single-mode fiber is requied for quantum information and cryptography technology. We have performed simulations of photon-collecting devices utilizing a nanofiber tip. Up to 38% of light from point dipole source is coupled to the fundamental guided mode of the nanofiber. These devices utilizing a nanofiber tip are promising to achive efficient single-photon sources.

The spectral linewidth and intensity of narrowband terahertz pulses generated by photomixing with beating of nonlinearly chirped laser pulse pairs are studied. The direct relationship between the dispersion of the pump pulse stretcher and the chirp of the terahertz pulse is shown. Simple analytical expressions for the linewidth and intensity of the terahertz spectrum are obtained and their validity is confirmed by numerical simulations. Generation of narrowband terahertz pulses with minimal chirp is demonstrated. (C) 2013 The Japan Society of Applied Physics

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.

Cavity quantum electrodynamics provides the setting for quantum control of strong interactions between a single atom and one photon. Many such atom-cavity systems interacting by coherent exchanges of single photons could be the basis for scalable quantum networks. However, moving beyond current proof-of-principle experiments involving just one or two conventional optical cavities requires the localization of individual atoms at distances less than or similar to 100 nm from a resonator's surface. In this regime an atom can be strongly coupled to a single intracavity photon while at the same time experiencing significant radiative interactions with the dielectric boundaries of the resonator. Here, we report using real-time detection and high-bandwidth feedback to select and monitor single caesium atoms located similar to 100 nm from the surface of a microtoroidal optical resonator. Strong radiative interactions of atom and cavity field probe atomic motion through the evanescent field of the resonator and reveal both the significant role of Casimir-Polder attraction and the manifestly quantum nature of the atom-cavity dynamics.

Quantum computation and communication rely on the ability to manipulate quantum states robustly and with high fidelity. To protect fragile quantum-superposition states from corruption through so-called decoherence noise, some form of error correction is needed. Therefore, the discovery of quantum error correction(1,2) (QEC) was a key step to turn the field of quantum information from an academic curiosity into a developing technology. Here, we present an experimental implementation of a QEC code for quantum information encoded in continuous variables, based on entanglement among nine optical beams(3). This nine-wave-packet adaptation of Shor's original nine-qubit scheme(1) enables, at least in principle, full quantum error correction against an arbitrary single-beam error.

Beyond traditional nonlinear optics with large numbers of atoms and photons, qualitatively new phenomena arise in a quantum regime of strong interactions between single atoms and photons. By using a microscopic optical resonator, we achieved such interactions and demonstrated a robust, efficient mechanism for the regulated transport of photons one by one. With critical coupling of the input light, a single atom within the resonator dynamically controls the cavity output conditioned on the photon number at the input, thereby functioning as a photon turnstile. We verified the transformation from a Poissonian to a sub- Poissonian photon stream by photon counting measurements of the input and output fields. The results have applications in quantum information science, including for controlled interactions of single light quanta and for scalable quantum processing on atom chips.

Over the past decade, strong interactions of light and matter at the single-photon level have enabled a wide set of scientific advances in quantum optics and quantum information science. This work has been performed principally within the setting of cavity quantum electrodynamics(1-4) with diverse physical systems(5), including single atoms in Fabry-Perot resonators(1,6), quantum dots coupled to micropillars and photonic bandgap cavities(7,8) and Cooper pairs interacting with superconducting resonators(9,10). Experiments with single, localized atoms have been at the forefront of these advances(11-15) with the use of optical resonators in high-finesse Fabry-Perot configurations(16). As a result of the extreme technical challenges involved in further improving the multilayer dielectric mirror coatings(17) of these resonators and in scaling to large numbers of devices, there has been increased interest in the development of alternative microcavity systems(5). Here we show strong coupling between individual caesium atoms and the fields of a high-quality toroidal microresonator. From observations of transit events for single atoms falling through the resonator's evanescent field, we determine the coherent coupling rate for interactions near the surface of the resonator. We develop a theoretical model to quantify our observations, demonstrating that strong coupling is achieved, with the rate of coherent coupling exceeding the dissipative rates of the atom and the cavity. Our work opens the way for investigations of optical processes with single atoms and photons in lithographically fabricated microresonators. Applications include the implementation of quantum networks(18,19), scalable quantum logic with photons(20), and quantum information processing on atom chips(21).

We report generation of squeezed vacuum in sideband modes of continuous-wave light at 946nm using a periodically poled KTiOPO4 crystal in an optical parametric oscillator. At the pump power of 250mW, we observe the squeezing level of -5.6 +/- 0.1dB and the anti- squeezing level of + 12.7 +/- 0.1dB. The pump power dependence of the observed squeezing/anti- squeezing levels agrees with theoretically calculated values when phase fluctuation of locking is taken into account. (c) 2006 Optical Society of America.

Quantum teleportation(1-8) involves the transportation of an unknown quantum state from one location to another, without physical transfer of the information carrier. Although quantum teleportation is a naturally bipartite process, it can be extended to a multipartite protocol known as a quantum teleportation network(9). In such a network, entanglement is shared between three or more parties. For the case of three parties ( a tripartite network), teleportation of a quantum state can occur between any pair, but only with the assistance of the third party. Multipartite quantum protocols are expected to form fundamental components for larger-scale quantum communication and computation(10,11). Here we report the experimental realization of a tripartite quantum teleportation network for quantum states of continuous variables ( electromagnetic field modes). We demonstrate teleportation of a coherent state between three different pairs in the network, unambiguously demonstrating its tripartite character.

The polarization dependence of quantum beats from the A-exciton and the B-exciton in a GaN sample of exceptional quality is studied with four-wave mixing experiments. When changing the incident polarizations from collinear to crossed linear, a pi-phase shift of the beats is observed while the decay rate remains unchanged. This confirms previous theoretical predictions. (C) 1997 Optical Society of America

We made experimental studies on manipulation of quantum entanglement and super-dense coding wjth continuous variables.In detail,I.We succeeded in creating multipartite entanglement with 3 squeezed vacuums and making a quantum teleportation network.II.We succeeded in high-fidelity quantum teleportation.III.We succeeded in teleporting a squeezed state.IV.We succeeded in the experiment of super-dense coding with continuous variables

前年度において生成に成功した連続量3者間量子エンタングルメントを用いて、3者間の量子テレポーテーション・ネットワークを構築した。連続量3者間量子エンタングルメントをアリス、ボブ、クレアの3者に分配し、この3者の間で任意の送信者・受信者の組み合わせにおいて量子テレポーテーションに成功すれば、3者間の量子テレポーテーション・ネットワークの構築に成功したといえる。ただし、2者間の量子テレポーテーションの場合と異なり、送受信に関わらない残りの1者も3者間量子エンタングルメントを共有しているため、他の2者の間の量子テレポーテーションが成功するためにはこの1者もエンタングルメントの測定を行い、その結果を受信者に伝達する必要がある。例としてアリスが送信者、ボブが受信者の場合、アリスは送信状態であるコヒーレント状態を自分の量子エンタングルメントビームとビームスプリッターを用いて合波し、その出力をそれぞれ直交した位相成分x、pについてホモダイン測定する。クレアは自分の量子エンタングルメントビームのみをp成分についてホモダイン測定する。ボブはこれらの結果をそれぞれ受け取り、自分の量子エンタングルメントビームに変位操作を行うことで、送信状態を再現する。クレアからボブへの情報伝達路のゲインを0から1の間で変化させると、ゲインが約0.1以下では再現された状態のフィデリティは古典限界を下回り、約0.1以上では古典限界を上回った。これは3者間量子エンタングルメントを用いて初めて可能な量子操作である。ゲインが約0.9の時に最大のフィデリティ0.63を得た。このような実験を他の2つの組み合わせについても行い、同様の結果を得た。これにより、初めて3者間の量子テレポーテーションに成功した

We succeeded in creating highly squeezed states of light and our record of squeezing level was 9 dB. By using such highly squeezed states of light, we succeeded in creating high level of quantum entanglement. Moreover, we succeeded in building a universal squeezer with the high level of quantum entanglement and measurement and feed-forward" technique.By combining two universal squeezers with two beam splitters, we succeeded in building a setup for quantum non-demolition interaction. Finally, we succeeded in quantum non-demolition measurement in two conjugate variables and checked the existence of quantum entanglement between outputs

前年度に引き続き、超低損失なトロイド型微小光共振器およびサブ波長径テーパー・ファイバーを作製した。特にテーパー・ファイバーについては、任意のウエスト径、ウエスト長、テーパー角を持ったファイバーの作製技術を確立した。これにより例えば、共振器へ光を入出力するための外部導波路に適した、短い(数mm程度の)ウエスト長のファイバーから、シリカガラスの非線形性を利用した非線形光学ファイバーに適した、長い(数10mm程度の)ウエスト長のファイバーまで、用途に応じて最適な設計のテーパー・ファイバーを作製することができるようになった。後者のデモンストレーションとして、長いウエスト長のテーパー・ファイバーに受動モード同期チタン・サファイアレーザーを入力し、シリカガラスの非線形性とテーパー・ファイバーの特殊な群速度分散による白色光発生に成功した。これは、現在白色光発生に広く用いられているフォトニック結晶ファイバーと比較して格段に安価に作製できる点、さらに通常の単一モード光ファイバーとシームレスに結合した構造である点において有利である。次に、単一半導体量子ドットのコヒーレント量子分光を行うため、上記の技術で作製したテーパー・ファイバーの表面上にコロイド型半導体量子ドットを配置し、テーパー・ファイバーの伝搬モードのエヴァネッセント波と半導体量子ドットの蛍光とを結合させた。この系を用いることで、単一のコロイド型半導体量子ドットの共鳴蛍光を世界で初めて観測した

我々は、Q値が極めて高く（＝損失が低く）、モード体積Vが極めて小さな（＝光のエネルギー密度が大きな）トロイド型微小光共振器を開発した。共振器の光カー効果はQ値とモード体積の比（Q/V値）に比例するが、我々の開発したトロイド型微小光共振器はQ/V値が極めて高いため、この共振器を用いることで、微弱光による相互位相変調が発現する。本研究では、実際にこの相互位相変調を観測した。

本研究では、光ファイバーに直接結合した微小共振器を用いて、光子の量子非破壊測定の実現を目指し、その基盤技術開発を実施した。すなわち、高Q/V値のトロイド型微小光共振器が持つ極めて強い光閉じ込めを利用して単一光子レベルでの巨大非線形光学効果（光カー効果）を発現させ、信号光の光子数に比例したプローブ光の位相変化を誘起させることで、信号光の光子数とプローブ光の位相の間に量子相関を生じさせ、量子非破壊測定条件の検証を実施するために必要な技術を開発した。

共振器量子電気力学系は、光共振器に閉じ込められた光子と、それと相互作用する単一原子からなる系であり、特に強結合領域の共振器量子電気力学系では、光と原子のコヒーレントな相互作用による様々な量子現象が観測できる。研究代表者らは、ごく最近、ナノ光ファイバーとファイバーブラッグ格子を組み合わせた新奇な全ファイバー共振器を開発し、トラップされた単一原子と全ファイバー共振器の共振器量子電気力学系を実現した。本研究では、これらの成果をもとに、全ファイバー共振器量子電気力学系を複数独立に構築し、それらをファイバーで直接結合した連結共振器量子電気力学系の実現に向けた予備的な研究を実施した。

ナノ光ファイバーの両端を単一モード光ファイバーと連続的に接続するテーパー部において、基本導波モードと高次導波モードの結合による損失は局所的なファイバー径と各モードの伝搬定数に強く依存する。本研究では、損失を押さえながらも全長を最短にするテーパー形状を設計するとともに、その作製方法を確立した。具体的には、99.7%を超える透過率を持ちながら全長わずか23 mmのナノ光ファイバーの作製に成功した。この結果はOptics Express誌に発表した[R. Nagai and T. Aoki, Opt.Express 22, 28427 (2014)]。今後は、作製した超低損失ナノ光ファイバーを用いた単一原子のレーザー冷却・トラップとその量子測定・操作を目指す。

量子光学の実験的研究において、高Q値微小光共振器を用いて光を波長スケールの微小体積中に強く閉じ込めることで、光の量子性が増強され、通常の系では困難な非古典的光学現象の観測が可能となる。我々は過去の研究において107～108程度の高Q値微小トロイド型光共振器を用いてさまざまな非古典的光学現象の観測に成功した。しかし、これらの研究で観測された非古典的光学現象あるいは生成された光の量子状態の量子性は依然として低く、量子通信をはじめとした光学的量子情報への応用には不十分である。そのため、より高いQ値の微小共振器の開発が求められている。上記の研究で用いた微小トロイド型光共振器は、エレクトロニクス用途のシリコン基板上のシリコン酸化膜を材料とするが、低損失光ファイバーを材料とすることでさらにQ値の高い共振器の実現が期待される。そこで本研究では、低損失光ファイバーを材料として微小球型光共振器を作成し、109～1010の超高Q値微小光共振器の実現を目指した。まず、光ファイバーを溶融し、表面張力により真球形状の微小光共振器を作製する技術を開発した。具体的には、被覆を除去しクラッド表面を洗浄した光ファイバーの先端を、CO2レーザーを用いて溶融した。シリカガラスはCO2レーザーの発振波長である中赤外領域に大きな吸収係数を持つため、CO2レーザーの照射によって局所的に加熱することができる。ただしこの方法では、流入熱量は加熱領域の体積に比例するが放射による熱の流出は表面積に比例するため、数μmスケールの微小な体積の高温加熱は困難であることも予想された。しかし、開口数の大きなレンズを用いてレーザーを集光することで、直径数μm程度の極細光ファイバーの先端であっても容易に溶融させることができた。上記の方法で作製した微小球共振器のQ値を周波数領域において測定した。テーパーファイバーを外部導波路として結合した微小球共振器に対して狭線幅の外部共振器型半導体レーザーを入力し、レーザー波長を掃引することで共振スペクトルを測定し、その幅からQ値を得た。結合損失を考慮し、共振器の真性Q値を見積もった結果、1×109を達成した。作製条件の最適化によって、更なるQ値の向上が見込まれる。