Yoshiaki Rikitake

Coherent transfer of light polarization to electron spins in a semiconductor.

We demonstrate that the superposition of light polarization states is coherently transferred to electron spins in a semiconductor quantum well. By using time-resolved Kerr rotation, we observe the initial phase of Larmor precession of electron spins whose coherence is transferred from light. To break the electron-hole spin entanglement, we utilized the big discrepancy between the transverse g factors of electrons and light-holes. The result encourages us to make a quantum media converter between flying photon qubits and stationary electron-spin qubits in semiconductors.

Spin state tomography of optically injected electrons in a semiconductor

Spin is a fundamental property of electrons, with an important role in information storage. For spin-based quantum information technology, preparation and read-out of the electron spin state are essential functions. Coherence of the spin state is a manifestation of its quantum nature, so both the preparation and read-out should be spin-coherent. However, the traditional spin measurement technique based on Kerr rotation, which measures spin population using the rotation of the reflected light polarization that is due to the magneto-optical Kerr effect, requires an extra step of spin manipulation or precession to infer the spin coherence. Here we describe a technique that generalizes the traditional Kerr rotation approach to enable us to measure the electron spin coherence directly without needing to manipulate the spin dynamics, which allows for a spin projection measurement on an arbitrary set of basis states. Because this technique enables spin state tomography, we call it tomographic Kerr rotation. We demonstrate that the polarization coherence of light is transferred to the spin coherence of electrons, and confirm this by applying the tomographic Kerr rotation method to semiconductor quantum wells with precessing and non-precessing electrons. Spin state transfer and tomography offers a tool for performing basis-independent preparation and read-out of a spin quantum state in a solid.

Polarization transfer from photon to electron spin in g factor engineered quantum wells

The authors demonstrate polarization transfer from a photon to an electron spin intermediated by a light-hole exciton in a GaAs∕AlGaAs quantum well, which has an engineered electron g factor of less than 0.01 for an in-plane magnetic field. Negative spin polarization was clearly observed at the selective excitation of the light-hole exciton from two-color time-resolved Kerr rotation. This demonstration is a necessary step towards demonstrating coherence transfer from a photon to an electron spin, which is necessary for building a quantum repeater used for long distance quantum communications.

Decoherence of localized spins interacting via RKKY interaction

We theoretically study decoherence of two localized spins interacting via the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction in one-, two-, and three-dimensional electron gas. We derive the kinetic equation for the reduced density matrix of the localized spins and show that energy relaxation caused by singlet-triplet transition is suppressed when the RKKY interaction is ferromagnetic. We also estimate the decoherence time of the system consisting of two quantum dots embedded in a two-dimensional electron gas.

Effect of exchange interaction on the fidelity of quantum state transfer from a photon qubit to an electron-spin qubit

We analyzed the fidelity of the quantum state transfer (QST) from a photon-polarization qubit to an electron-spin-polarization qubit in a semiconductor quantum dot, with special attention to the exchange interaction between the electron and the simultaneously created hole. In order to realize a high-fidelity QST we had to separate the electron and hole as soon as possible, since the electron-hole exchange interaction modifies the orientation of the electron spin. Thus, we propose a double-dot structure to separate the electron and hole quickly, and show that the fidelity of the QST can reach as high as 0.996 if the resonant tunneling condition is satisfied.

Measurement of Electron Spin States in a Semiconductor Quantum Well Using Tomographic Kerr Rotation

Spin coherence is essential for spin-based quantum information technology. The conventional spin measurement technique, however, requires an extra step of spin manipulation or precession to measure the electron spin coherence. To measure the electron spin coherence directly, we have developed the tomographic Kerr rotation (TKR) method, which is based on the magneto-optical Kerr effect on the condition of the coherent transfer of light polarization states into electron spin states in a GaAs/AlGaAs quantum well. The TKR method allows measurement of the coherent superposition state of electron spins |±y>e = (|↑>e±i|↓>e)/√2 in addition to conventionally measured ±z states (|↑>e or |↓>e) (which merely indicate the population of electron spins). Here we demonstrate that electron spin coherence can be measured by linearly polarized probe light to show that TKR is independent of the choice of probe light polarization. We also describe a method to distinguish TKR from the conventional magnetic circular dichroism (MCD) effect.

Time Evolution of an Exciton and a Photon Qubit Coupled with Each Other

We theoretically study the time evolution of an exciton qubit (quantum dot) and a photon qubit (photonic cavity) coupled with each other. Special attention is paid to the effect of pure dephasing and energy dissipation in the quantum dot. We find that the pure dephasing brings about the oscillation of the entropy of the quantum dot with the half period of Rabi oscillation. Finite entanglement remains in the steady state. This state can be used for preparing the Bell state between photon and exciton qubits. The energy dissipation suppresses the amplitude of the oscillation of the entanglement between the exciton and photon qubits. We also find that the energy dissipation in the quantum dot changes the oscillation period of the entropy of the quantum dot.

Time-bin state transfer to electron spin coherence in solids

We demonstrate that a coherent superposition state of two temporally separated optical pulses, called a time-bin state, can be transferred to that of up/down electron spins in a semiconductor by synchronizing the time separation to the precession period of either electrons or holes. The time-bin transfer scheme does not require polarization mode degeneracy and can map the time-bin state to the electron spin state that is not accessible directly using only polarization. The scheme offers a new approach for quantum interfaces between photons and electron spins.

Spin coherent read, write, manipulation of electrons with light in solids

Spin is a quantum property of electrons. For spin‐based quantum information technology, preparation and read‐out of the electron spin state should be spin coherent. We demonstrate that the polarization coherence of light can be transferred to the spin coherence of electrons in a semiconductor quantum nanostructure, and the prepared coherence of the electron spin can also be read out with light by the developed tomographic Kerr rotation method. We also demonstrate that a single photon is efficiently converted (∼27%) into a single electron trapped in a gate‐defined quantum dot, where the g‐factor of electrons is tuned to zero, and the charge state is detected with an adjacent quantum point contact without destructing the spin state. We further demonstrate that the spin coherence of a single electron trapped in one of double quantum dots is electrically manipulated with a microwave applied to the gate and read out via the Pauli spin blockade phenomenon. All of these functions are needed to build all semicond...

Quantum media conversion from a photon to an electron spin

1 Research Institute of Electrical Communication, Tohoku University, Sendai 980-8577, Japan E-mail: kosaka@riec.tohoku.ac.jp 2 CREST-JST, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan 3 Low Temperature Physics Laboratory, RIKEN, Saitama 351-0198, Japan 4 Department of Information Engineering, Sendai National College of Technology, Sendai 989-3128, Japan 5 Nanotechnology Research Institute, AIST, Tsukuba 305-8568, Japan