Kaoru Sanaka

Hybrid quantum repeater using bright coherent light.

We describe a quantum repeater protocol for long-distance quantum communication. In this scheme, entanglement is created between qubits at intermediate stations of the channel by using a weak dispersive light-matter interaction and distributing the outgoing bright coherent-light pulses among the stations. Noisy entangled pairs of electronic spin are then prepared with high success probability via homodyne detection and postselection. The local gates for entanglement purification and swapping are deterministic and measurement-free, based upon the same coherent-light resources and weak interactions as for the initial entanglement distribution. Finally, the entanglement is stored in a nuclear-spin-based quantum memory. With our system, qubit-communication rates approaching 100 Hz over 1280 km with fidelities near 99% are possible for reasonable local gate errors.

Lasing of donor-bound excitons in ZnSe microdisks

Excitons bound to flourine atoms in ZnSe have the potential for several quantum optical applications. Examples include optically accessible quantum memories for quantum information processing and lasing without inversion. These applications require the bound-exciton transitions to be coupled to cavities with high cooperativity factors, which results in the experimental observation of low-threshold lasing. We report such lasing from fluorine-doped ZnSe quantum wells in 3 and 6 micron microdisk cavities. Photoluminescence and selective photoluminescence spectroscopy confirm that the lasing is due to bound-exciton transitions.

Optical pumping of a single electron spin bound to a fluorine donor in a ZnSe nanostructure.

Here we demonstrate optical pumping of a single electron within a semiconductor nanostructure comprised of a single fluorine donor located within a ZnSe/ZnMgSe quantum well. Experiments were performed to detect optical pumping behavior by observing single photons emitted from the nanostructure when the electron changes spin state. These results demonstrate initialization and read-out of the electron spin qubit and open the door for coherent optical manipulation of a spin by taking advantage of an unconventional nanostructure.

Optically controlled semiconductor spin qubits for quantum information processing

Implementations of quantum information processing systems based on optically controlled electron spins in semiconductor quantum dots are particulary appealing due to several features. These features include inherent ultrafast gate operation times, reasonably long decoherence times, small optical control power and a natural ability to link to optical fiber communication networks. We will discuss the current state of the art in the experimental implementations of the main elements of semiconductor spin qubits: qubit initialization, single-qubit gates, two-qubit gates, entanglement distribution, projective measurement, quantum memory and indistinguishable single-photon generation.

Investigation of excitons bound to fluorine donors in ZnSe

The electrical and optical properties of fluorine impurities in ZnSe have been studied. Bulk and delta-doped ZnSe:F layers were grown by molecular beam epitaxy. Electrical measurements reveal evidence that a majority of fluorine impurities act as shallow donors. In delta-doped samples, we find strong photoluminescence due to the radiative recombination of fluorine donor bound excitons. We measured a fluorescence lifetime of less than 250 ps and determined the inhomogeneous line width to be below 2 meV at liquid helium temperature.

Entangling single photons from independently tuned semiconductor nanoemitters.

Quantum communication systems based on nanoscale semiconductor devices is challenged by inhomogeneities from device to device. We address this challenge using ZnMgSe/ZnSe quantum-well nanostructures with local laser-based heating to tune the emission of single impurity-bound exciton emitters in two separate devices. The matched emission in combination with photon bunching enables quantum interference from the devices and allows the postselection of polarization-entangled single photons. The ability to entangle single photons emitted from nanometer-sized sources separated by macroscopic distances provides an essential step for a solid-state realization of a large-scale quantum optical network. This paves the way toward measurement-based entanglement generation between remote electron spins localized at macroscopically separated fluorine impurities.

Optical properties of fluorine implanted ZnMgSe/ZnSe quantum-well nanostructures

Here we demonstrate the ion-implantation of fluorine as an alternative doping method for ZnMgSe/ZnSe QWs. The photoluminescence measurements of F-implanted ZnSe QWs show the correlation between the number of sharp recombination peaks of F-donor bound-excitons and the implantation dose as well as the saturation of the luminescence intensity related to a donor. When special techniques such as selective implantation through a mask and registration of single ion impacts are applied on micro-, nano-cavities, the ion implantation can be an attractive alternative fluorine doping method for quantum information technology based on fluorine impurities in ZnSe.

Quantum interference between photons emitted by independent semiconductor single-photon devices

Many schemes for optical quantum computation and long-distance quantum communication require quantum interference between indistinguishable single-photon states generated from large numbers of independent sources. Solidstate systems allow integration of such sources on a chip. It is therefore desirable to achieve multiple solid-state singlephoton sources for practical applications. We show a promising candidate for this in the single photons generated by the radiative decay processes of excitons that are bound to isolated fluorine donor impurities in ZnSe/ZnMgSe quantum well nanostructures. Donor-bound-exciton single-photon sources typically have a narrow distribution of center wavelengths, and they overcome dipole dephasing due to their fast radiative decay time. The emitter we introduce here demonstrates these advantages, showing strong potential for allowing quantum interference between single photons emitted by independent solid-state single-photon sources.