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Keiji Sasaki

Publications and source records attributed to Keiji Sasaki.

17 recordsLinked to original sources
arXiv2026

Orbital Optical Chirality as the Origin of Vortex Dichroism

Optical chirality quantifies the geometrical twisting of electromagnetic fields and underlies chiral light-matter interactions, yet its conventional formulation captures only the spin-associated chiral geometry of light. We introduce orbital optical chirality and derive its continuity equation, revealing physical properties distinct from those of angular momentum. For vortex beams, spin and orbital optical chiralities follow the spin and orbital indices, respectively. Applied to a twisted nanorod dimer, orbital optical chirality gives rise to vortex dichroism through quadrupolar hybridized modes, while dipolar modes exhibit only circular dichroism. These results establish a unified framework for optical chirality and provide access to chiral geometries beyond those resolved by spin optical chirality alone.

physics.optics↗
arXiv2024

Chirality enhancement using topology-designed 3D nanophotonic antennas

We explore chiroptical phenomena in 3D chiral nano-gap antennas using topology optimization. The characteristic helical geometries of the topology-designed antennas exhibit giant chiral dissymmetry (g=-1.70) considering the gap intensity, circular-to-linear polarization conversion, and circularly polarized light emission from a linear dipole coupled with the antenna. We observed that the spin angular momentum of light, flowing into the nanogap with opposite signs, locally amplifies optical chirality. These findings carry profound implications for the nanoscale control of complex light-matter interactions with structured light.

physics.optics↗
arXiv2019

Selective excitation of multipolar spoof plasmons using orbital angular momentum of light

The nature of light-matter interaction is governed by the spatial-temporal structures of a light field and material wavefunctions. The emergence of the light beam with transverse phase vortex, or equivalently orbital angular momentum (OAM) has been providing intriguing possibilities to induce unconventional optical transitions beyond the framework of the electric dipole interaction. The uniqueness stems from the OAM transfer from light to material, as demonstrated using the bound electron of a single trapped ion. However, many aspects of the vortex light-matter interaction are still unexplored especially in solids with extended electronic states. Here, we unambiguously visualized dipole-forbidden multipolar excitations in a solid-state electron system; spoof localized surface plasmon, selectively induced by the terahertz vortex beam. The results obey the selection rules governed by the conservation of the total angular momentum, which is numerically confirmed by the electromagnetic field analysis. Our results show light's OAM can be efficiently transferred to an elementary excitation in solids.

physics.optics↗
arXiv2009

An Entanglement Filter

The ability to filter quantum states is a key capability in quantum information science and technology, in which one-qubit filters, or polarizers, have found wide application. Filtering on the basis of entanglement requires extension to multi-qubit filters with qubit-qubit interactions. We demonstrated an optical entanglement filter that passes a pair of photons if they have the desired correlations of their polarization. Such devices have many important applications to quantum technologies.

quant-ph↗
arXiv2008

Beating the standard quantum limit: Phase super-sensitivity of N-photon interferometers

Quantum metrology promises greater sensitivity for optical phase measurements than could ever be achieved classically. Here we present a theory of the phase sensitivity for the general case where the detection probability is given by an $N$ photon interference fringe. We find that the phase sensitivity has a complex dependence on both the intrinsic efficiency of detection $η$ and the interference fringe visibility $V$. Most importantly, the phase that gives maximum phase sensitivity is in general not the same as the phase at which the slope of the interference fringe is a maximum, as has previously been assumed. We determine the parameter range where quantum enhanced sensitivity can be achieved. In order to illustrate these theoretical results, we perform a four photon experiment with $η=3/4$ and $V=82\pm6$% (an extension of our previous work [Science \textbf{316}, 726 (2007)]) and find a phase sensitivity 1.3 times greater than the standard quantum limit at a phase different to that which gives maximum slope of the interference fringe.

quant-ph↗
arXiv2007

Beating the Standard Quantum Limit with Four Entangled Photons

Precision measurements are important across all fields of science. In particular, optical phase measurements can be used to measure distance, position, displacement, acceleration and optical path length. Quantum entanglement enables higher precision than would otherwise be possible. We demonstrate an optical phase measurement with an entangled four photon interference visibility greater than the threshold to beat the standard quantum limit--the limit attainable without entanglement. These results open the way for new high-precision measurement applications.

quant-ph↗
arXiv2006

Determining the optical axes of entangled Laguerre Gauss modes

A method for determining the positions of hologram dislocations relative to the optical axes of entangled Laguerre Gaussian modes is proposed. In our method, the coincidence count rate distribution was obtained by scanning the position of one of the holograms in two dimensions. Then, the relative position of the hologram dislocation was determined quantitatively from the positions of the minimum and maximum coincidence count rates. The validity of the method was experimentally verified, and in addition, an experiment demonstrating the violation of the Clauser-Horne-Shimony-Holt inequality was performed using the well-identified optical axes of the entangled modes.

quant-ph↗
arXiv2005

Demonstration of an optical quantum controlled-NOT gate without path interference

We report the first experimental demonstration of an optical quantum controlled-NOT gate without any path interference, where the two interacting path interferometers of the original proposals (Phys. Rev. A {\bf 66}, 024308 (2001), Phys. Rev. A {\bf 65}, 012314 (2002)) have been replaced by three partially polarizing beam splitters with suitable polarization dependent transmittances and reflectances. The performance of the device is evaluated using a recently proposed method (Phys. Rev. Lett. {\bf 94}, 160504 (2005)), by which the quantum process fidelity and the entanglement capability can be estimated from the 32 measurement results of two classical truth tables, significantly less than the 256 measurement results required for full quantum tomography.

quant-ph↗
arXiv2004

A high-yield single photon source using gated spontaneous parametric down conversion

The construction of a single photon source using gated parametric fluorescence is reported with the measurement results of the photon number distribution. A beamlike twin-photon method is used in order to achieve high collection efficiency. The estimated probability P(1) to find a single photon in a collimated output pulse is 26.5 % at a repetition rate of 10 kHz when the effective quantum efficiency of 27.4 % in the detection setup is compensated.

quant-ph↗
arXiv2004

Efficiencies for the single mode operation of a quantum optical nonlinear shift gate

We investigate the single mode operation of a quantum optical nonlinear πphase shift gate implemented by a single two-level atom in one-dimensional free space. Since the single mode property of the input photons at the atom is not preserved in the interaction at the atom, we analyze the effeciency of single mode operation that can still be achieved. We show how the input pulse shape can be optimized to obtain high efficiencies for the nonlinear single mode operation. With this analysis, we obtain an optimal single mode transmittance per photon of 78% for the successful nonliner πphase shift operation.

quant-ph↗
arXiv2004

A study on the shape of two-photon wavefunctions after the nonlinear interaction with a one-dimensional atom

We study the interaction of Gaussian one- and two-photon pulses with a single two-level atom based on a one-dimensional model of pulse propagation to and from the atom. The characteristic time scale of the atomic response is the dipole relaxation time 1/Gamma. We therefore compare the effect of the non-linear two-photon interaction for a long pulse length of 10/Gamma with a short pulse of $1/Γ$. Our results indicate that the effect of the non-linear interaction is particularly strong for the short pulse length of 1/Gamma.

quant-ph↗
arXiv2004

Distinguishing genuine entangled two-photon-polarization states from independently generated pairs of entangled photons

A scheme to distinguish entangled two-photon-polarization states (ETP) from two independent entangled one-photon-polarization states (EOP) is proposed. Using this scheme, the experimental generation of ETP by parametric down-conversion is confirmed through the anti-correlations between three orthogonal two-photon-polarization states. The estimated fraction of ETP among the correlated photon pairs is 37% in the present experimental setup.

quant-ph↗
arXiv2003

Entanglement and four wave mixing effects in the dissipation free nonlinear interaction of two photons at a single atom

We investigate the nonlinear interaction between two photons in a single input pulse at an atomic two level nonlinearity. A one dimensional model for the propagation of light to and from the atom is used to describe the precise spatiotemporal coherence of the two photon state. It is shown that the interaction generates spatiotemporal entanglement in the output state similar to the entanglement observed in parametric downconversion. A method of generating photon pairs from coherent pump light using this quantum mechanical four wave mixing process is proposed.

quant-ph↗
arXiv2003

Nonlinear interaction of two photons at a one-dimensional atom: spatiotemporal quantum coherence in the emitted field

The nonlinear photon-photon interaction mediated by a single two-level atom is studied theoretically based on a one-dimensional model of the field-atom interaction. This model allows us to determine the effects of an atomic nonlinearity on the spatiotemporal coherence of a two photon state. Specifically, the complete two photon output wave function can be obtained for any two photon input wave function. It is shown that the quantum interference between the components of the output state associated with different interaction processes causes bunching and anti-bunching in the two photon statistics. This theory may be useful for various applications in photon manipulation, e.g. quantum information processing using photonic qubits, quantum nondemolition measurements, and the generation of entangled photons.

quant-ph↗
arXiv2003

Optimized phase switching using a single atom nonlinearity

We show that a nonlinear phase shift of pi can be obtained by using a single two level atom in a one sided cavity with negligible losses. This result implies that the use of a one sided cavity can significantly improve the pi/18 phase shift previously observed by Turchette et al. [Phys. Rev. Lett. 75, 4710 (1995)].

quant-ph↗
arXiv2002

Realization of a resonant non-linear phase flip in cavity quantum electrodynamics

Optical nonlinearities sensitive to individual photons may be extremely useful as elements in quantum logic circuits for photonic qubits. A much cited example is the work of Turchette et al. [Phys. Rev. Lett. 75, 4710 (1995)], in which a phase shift of about 10 degrees was reported. To improve this result, we propose a single sided cavity geometry with minimal cavity losses. It should then be possible to achieve a nonlinear phase shift of 180 degrees.

quant-ph↗