R. Stockill

Direct Photonic Coupling of a Semiconductor Quantum Dot and a Trapped Ion

Coupling individual quantum systems lies at the heart of building scalable quantum networks. Here, we report the first direct photonic coupling between a semiconductor quantum dot and a trapped ion and we demonstrate that single photons generated by a quantum dot controllably change the internal state of a Yb^{+} ion. We ameliorate the effect of the 60-fold mismatch of the radiative linewidths with coherent photon generation and a high-finesse fiber-based optical cavity enhancing the coupling between the single photon and the ion. The transfer of information presented here via the classical correlations between the σ_{z} projection of the quantum-dot spin and the internal state of the ion provides a promising step towards quantum-state transfer in a hybrid photonic network.

Phase-Tuned Entangled State Generation between Distant Spin Qubits

Entanglement between distant nodes is essential for the successful realisation of a distributed quantum network. Photon-mediated entanglement has recently been demonstrated in atomic [1] and diamond defect systems [2] and, very recently, using heavy hole spins in quantum dots [3]. However, until now controlled creation of entangled states with arbitrary phase had yet to be demonstrated in these systems. Here we demonstrate the high frequency creation of Bell states with arbitrary phase using electron-spin qubits confined to distant self-assembled InGaAs quantum dots (QD).

Quantum dot spin coherence governed by a strained nuclear environment

The interaction between a confined electron and the nuclei of an optically active quantum dot provides a uniquely rich manifestation of the central spin problem. Coherent qubit control combines with an ultrafast spin–photon interface to make these confined spins attractive candidates for quantum optical networks. Reaching the full potential of spin coherence has been hindered by the lack of knowledge of the key irreversible environment dynamics. Through all-optical Hahn echo decoupling we now recover the intrinsic coherence time set by the interaction with the inhomogeneously strained nuclear bath. The high-frequency nuclear dynamics are directly imprinted on the electron spin coherence, resulting in a dramatic jump of coherence times from few tens of nanoseconds to the microsecond regime between 2 and 3 T magnetic field and an exponential decay of coherence at high fields. These results reveal spin coherence can be improved by applying large magnetic fields and reducing strain inhomogeneity.

Extension of electron spin coherence in a quantum dot via CPT-locking of the Overhauser field

Semiconductor quantum dots (QD) offer the current state-of-the-art in intensity and coherence for single photon generation, as well as serving as a testbed for various novel approaches to quantum networking. Their high brightness provides a means to produce high-bandwidth spin-photon entanglement [1]. However, the purity and usefulness of the generated entangled state is severely limited by the coherence time of the QD charge carrier spin, typically on the order of 2ns for electrons in a self-assembled InGaAs QD. This limit is imposed by low-frequency fluctuations of the Overhauser field generated by host nuclear spins, affecting the electron spin resonance frequency.

Phase-tuned entangled state generation between distant spin qubits

Entanglement between distant nodes is essential for the successful realisation of a distributed quantum network. Photon-mediated entanglement has recently been demonstrated in atomic [1] and diamond defect systems [2] and, very recently, using heavy hole spins in quantum dots [3]. However, until now controlled creation of entangled states with arbitrary phase had yet to be demonstrated in these systems. Here we demonstrate the high frequency creation of Bell states with arbitrary phase using electron-spin qubits confined to distant self-assembled InGaAs quantum dots (QD).