Fumihiro Kaneda

Experimental violation and reformulation of the Heisenberg's error-disturbance uncertainty relation

The uncertainty principle formulated by Heisenberg in 1927 describes a trade-off between the error of a measurement of one observable and the disturbance caused on another complementary observable such that their product should be no less than the limit set by Plancks constant. However, Ozawa in 1988 showed a model of position measurement that breaks Heisenbergs relation and in 2003 revealed an alternative relation for error and disturbance to be proven universally valid. Here, we report an experimental test of Ozawas relation for a single-photon polarization qubit, exploiting a more general class of quantum measurements than the class of projective measurements. The test is carried out by linear optical devices and realizes an indirect measurement model that breaks Heisenbergs relation throughout the range of our experimental parameter and yet validates Ozawas relation.

Experimental Test of Error-Disturbance Uncertainty Relations by Weak Measurement

We experimentally test the error-disturbance uncertainty relation (EDR) in generalized, strength-variable measurement of a single photon polarization qubit, making use of weak measurement that keeps the initial signal state practically unchanged. We demonstrate that the Heisenberg EDR is violated, yet the Ozawa and Branciard EDRs are valid throughout the range of our measurement strength.

Entangled photon generation in two-period quasi-phase-matched parametric down-conversion.

We proposed and demonstrated a simple but deterministic scheme for generating polarization-entangled photon pairs at telecommunication wavelengths with type-II quasi-phase-matched spontaneous parametric down-conversion (QPM-SPDC) having two poling periods. We fabricated a LiNbO3 crystal having two poling periods so as to generate entangled photons at two wavelengths, i.e., 1506 nm and 1594 nm. We characterized the two-photon polarization state with state tomography and confirmed that the state was highly entangled.

Entangled-state generation with an intrinsically pure single-photon source and a weak coherent source

We report on the experimental generation of an entangled state with a spectrally pure heralded single-photon state and a weak coherent state. By choosing group-velocity matching in the nonlinear crystal, our system for producing entangled photons was 60 times brighter than that in the earlier experiment [Phys. Rev. Lett. 90, 240401 (2003)], with no need of bandpass filters. This entanglement system is useful for quantum information protocols that require indistinguishable photons from independent sources.

Experimental test of error-disturbance relations in generalized photon-polarization measurements

We report an experimental test of Ozawars relation for a single-photon polarization qubit, exploiting an indirect measurement model that breaks Heisenbergrs relation throughout the range of our experimental parameter and yet validates Ozawars relation.

Generation and superactivation of bound entanglement

We report the experimental realization of the four-partite bound-entangled state, i.e., Smolin state [1] and demonstrate the ‘superactivation’ of the bound entanglement. Bound entanglement is a class of multipartite entanglement that cannot be distilled into pure entanglement under local operations and classical communication (LOCC). Despite its undistillable property, this class of entanglement has interesting properties that are expected to be useful in quantum information processing (QIP). For instance, in the Smolin state, the bound entanglement can be activated into distillable entanglement when two of the parties get together. The process is called “unlocking” [1], and has been demonstrated experimentally [2]. More importantly, the bound entanglement can be activated with the help of entanglement in other systems [3]. The activation of bound entanglement, i.e., superactivation, plays an essential role in the QIP protocols using bound entangled states (e.g., remote information concentration [4]).

Efficient generation and activation of bound entanglement in optical qubits

We report the efficient generation of a four-qubit bound entangled Smolin state. We also demonstrate the activation of the bound entanglement; we make the bound entanglement in the Smolin state distillable with the help of an additional Bell pair.