Gen Taguchi

Measurement and control of spatial qubits generated by passing photons through double slits

We present an experimental study of the non-classical correlations of a pair of spatial qubits formed by passing two down-converted photons through a pair of double slits. After confirming the entanglement generated in our setup by quantum tomography using separate measurements of the slit images and the interference patterns, we show that the complete Hilbert space of the spatial qubits can be accessed by measurements performed in a single plane between the image plane and the focal plane of a lens. Specifically, it is possible to obtain both the which-path and the interference information needed for quantum tomography in a single scan of the transversal distribution of photon coincidences. Since this method can easily be extended to multi-dimensional systems, it may be a valuable tool in the application of spatial qudits to quantum information processes.

Weak measurement of photon polarization by back-action-induced path interference

An essential feature of weak measurements on quantum systems is the reduction of measurement back-action to negligible levels. To observe the non-classical features of weak measurements, it is therefore more important to avoid additional back-action errors than it is to avoid errors in the actual measurement outcome. In this paper, it is shown how an optical weak measurement of diagonal (PM) polarization can be realized by path interference between the horizontal (H) and vertical (V) polarization components of the input beam. The measurement strength can then be controlled by rotating the H and V polarizations towards each other. This well-controlled operation effectively generates the back-action without additional decoherence, while the visibility of the interference between the two beams only limits the measurement resolution. As the experimental results confirm, we can obtain extremely high weak values, even at rather low visibilities. Our method therefore provides a realization of weak measurements that is extremely robust against experimental imperfections.

Reconstruction of spatial qutrit states based on realistic measurement operators

Spatial qudit states can be realized by using multislits to discretize the transverse momentum of a photon. The merit of this kind of spatial qudit states is that the implementation of higher-dimensional qudits is relatively easy. As we have recently shown, the quantum states of these spatial qudits can be analyzed by scanning a single interference pattern. This method of single scan tomography can also be applied at higher dimensions, but the reconstruction becomes more sensitive to smaller details of the scanned patterns as the dimensions increase. In this paper, we investigate the effect of finite measurement resolution on the single scan tomography of spatial qutrits. Realistic measurement operators describing the spatial resolution of the measurement are introduced and the corresponding pattern functions for quantum state reconstruction are derived. We use the pattern functions to analyze experimental results for entangled pairs of spatial qutrits generated by spontaneous parametric downconversion. It is shown that a reliable reconstruction of the quantum state can be achieved with finite measurement resolution if this limitation of the measurement is included in the pattern functions of single scan tomography.

Mechanical active control of surface plasmon properties

We present a multilayer device which allows the control of Surface Plasmon (SP) propagation properties (propagation length and extension). A simple modification on an inner air gap thickness strongly affects SP propagation mode due to coupling with Parallel-Plate (PP) mode.

Experimental investigation of the role of measurement uncertainties in the violation of Leggett-Garg inequalities

We show that the violation of Leggett-Garg inequalities can be confirmed by intermediate measurements of arbitrary strength if both measurement resolution and back-action uncertainties are taken into account.

A method for weak measurement of photon polarization robust against experimental imperfections

We realized a simple optical weak measurement of diagonal (PM) polarization of photons by interference between the horizontal and vertical polarization components. This measurement can achieve small back-action even at low visibilities of the interference.

Interferometric weak measurement of photon polarization

We realize a minimum back‐action quantum non‐demolition measurement of variable strength on photon polarization in the diagonal(PM) basis by two‐mode path interference. This method uses the phase difference between the positive (P) and negative (M) superpositions in the interference between the horizontal (H) and vertical (V) polarized paths in the input beam. Although the interference can not occur when the H and V polarizations are distinguishable, a well‐controlled amount of interference is induced by erasing the H and V information using a coherent rotation of polarization toward a common diagonal polarization. This method is particularly suitable for the realization of weak measurements, where the control of the back‐action is essential.

Analysis of spatial qutrit states using position measurements with finite resolution

We show that the quantum states of spatial qudits generated by passing photons through multi-slits can be reliably reconstructed from position measurements with finite spatial resolution if the correct measurement operators are used.

EXPERIMENTAL ANALYSIS OF SPATIAL QUTRIT ENTANGLEMENT OF DOWN-CONVERTED PHOTON PAIRS

As was first demonstrated by Neves and co-workers, entangled spatial qudits can be obtained by passing photons generated in spontaneous parametric down-conversion (SPDC) through multi-slits . Recently, we have investigated the possibility of accessing the complete Hilbert space of these multidimensional quantum systems by measurements between the focal and image planes of a lens . It then becomes possible to perform quantum state tomography in a single scan in one plane. In our initial work, we demonstrated this single scan tomography for the case of a double-slit qubit. However, our method can easily be extended to general multi-slit qudits. In this paper, we report the first experimental results using entangled triple-slit qutrits. As in the previous work on qubits, several qutrit states were prepared in one of two arms, A, through the measurements in the other arm, B, and the 3× 3 density matrices were reconstructed from the scans in arm A. The complete 9 × 9 density matrix of the entangled state can then be obtained from a sufficiently large number of settings in arm B.

Analysis of entanglement in the double slit interference patterns of down converted photon pairs

We investigate the non-classical correlation of two entangled photons passing through a pair of double slits. The quantum state is reconstructed using quantum tomography based on the correlated interference patterns and slit images.