Chengyong Hu

Giant optical Faraday rotation induced by a single-electron spin in a quantum dot : Applications to entangling remote spins via a single photon

We propose a quantum nondemolition method---a giant optical Faraday rotation near the resonant regime to measure a single-electron spin in a quantum dot inside a microcavity where a negatively charged exciton strongly couples to the cavity mode. Left-circularly and right-circularly polarized lights reflected from the cavity obtain different phase shifts due to cavity quantum electrodynamics and the optical spin selection rule. This yields giant and tunable Faraday rotation that can be easily detected experimentally. Based on this spin-detection technique, a deterministic photon-spin entangling gate and a scalable scheme to create remote spin entanglement via a single photon are proposed.

Deterministic photon entangler using a charged quantum dot inside a microcavity

We present two novel schemes to generate photon polarization entanglement via single electron spins confined in charged quantum dots inside microcavities. One scheme is via entangled remote electron spins followed by negatively-charged exciton emissions, and another scheme is via a single electron spin followed by the spin state measurement. Both schemes are based on giant circular birefringence and giant Faraday rotation induced by a single electron spin in a microcavity. Our schemes are deterministic and can generate an arbitrary amount of multi-photon entanglement. Following similar procedures, a scheme for a photon-spin quantum interface is proposed.

Proposed entanglement beam splitter using a quantum-dot spin in a double-sided optical microcavity

Received 7 August 2009; revised manuscript received 1 October 2009; published 30 November 2009We propose an entanglement beam splitter EBS using a quantum-dot spin in a double-sided opticalmicrocavity. In contrast to the conventional optical beam splitter, the EBS can directly split a photon-spinproduct state into two constituent entangled states via transmission and reflection with high fidelity and highefficiency up to 100 percent . This device is based on giant optical circular birefringence induced by a singlespin as a result of cavity quantum electrodynamics and the spin-selection rule of trion transition Pauli block-ing . The EBS is robust and it is immune to the fine-structure splitting in a realistic quantum dot. This quantumdevice can be used for deterministically creating photon-spin, photon-photon, and spin-spin entanglements aswell as a single-shot quantum nondemolition measurement of a single spin. Therefore, the EBS can find wideapplications in quantum information science and technology.DOI: 10.1103/PhysRevB.80.205326 PACS number s : 78.67.Hc, 03.67.Mn, 42.50.Pq, 78.20.Ek

Loss-resistant state teleportation and entanglement swapping using a quantum-dot spin in an optical microcavity

We present a scheme for efficient state teleportation and entanglement swapping using a single quantum-dot spin in an optical microcavity based on giant circular birefringence. State teleportation or entanglement swapping is heralded by the sequential detection of two photons, and is finished after the spin measurement. The spin-cavity unit works as a complete Bell-state analyzer with a built-in spin memory allowing loss-resistant repeater operation. This device can work in both the weak coupling and the strong coupling regime, but high efficiencies and high fidelities are only achievable when the side leakage and cavity loss is low. We assess the feasibility of this device, and show it can be implemented with current technology. We also propose a spin manipulation method using single photons, which could be used to preserve the spin coherence via spin echo techniques.

Cavity enhanced spin measurement of the ground state spin of an NV center in diamond

A key step in the use of diamond nitrogen vacancy (NV) centers for quantum computational tasks is a single shot quantum non-demolition measurement of the electronic spin state. Here, we propose a high fidelity measurement of the ground state spin of a single NV center, using the effects of cavity quantum electrodynamics. The scheme we propose is based in the one-dimensional atom or Purcell regime, removing the need for high Q cavities that are challenging to fabricate. The ground state spin of the NV center has a splitting of 6-10µeV, which can be resolved in a high-resolution absorption measurement. By incorporating the center in a low-Q and low volume cavity we show that it is possible to perform single shot readout of the ground state spin using a weak laser with an error rate of 7◊10 3 , when realistic experimental parameters are considered. Since very low levels of light are used to probe the state of the spin we limit the number of florescence cycles, which dramatically reduces the measurement induced decoherence approximating a non-demolition measurement of ground state spin.

Quantum dot induced phase shift in a pillar microcavity

A.B. Young, ∗ R. Oulton, 2 C.Y. Hu, A.C.T. Thijssen, C. Schneider, S. Reitzenstein, M. Kamp, S. Höfling, L. Worschech, A. Forchel, and J.G. Rarity Merchant Venturers School of Engineering, Woodland Road Bristol, BS8 1UB H.H. Wills Physics Laboratory, Tyndall Avenue, Bristol BS8 1TL, UK Technische Physik, Physikalisches Institut and Wilhelm Conrad Röntgen-Center for Complex Material Systems, Universität Würzburg, Am Hubland, 97474 Würzburg, Germany (Dated: November 2, 2010)

SINGLE PHOTON SOURCES BASED UPON SINGLE QUANTUM DOTS IN SEMICONDUCTOR MICROCAVITY PILLARS

Semiconductor microcavity pillars with both circular and elliptical cross-section containing semiconductor quantum dots are shown to be good candidates for efficient single photon sources. Pillars with small diameters are shown to have exceptionally high quality factors and the reduction in the measured quality factor as the pillar diameter is reduced is shown to agree well with finite difference time domain simulation. These pillars exhibit a Purcell enhancement of the quantum dot emission when the dots are on-resonance with the cavity mode and strong photon antibunching. The use of the polarized modes of an elliptical micropillar allows the polarization of the emitted single photons to be selected.

Charged quantum dot micropillar system for deterministic light-matter interactions

This work was funded by the Future Emerging Technologies (FET) programme within the Seventh Framework Programme for Research of the European Commission, FET-Open, FP7-284743 [project Spin Photon Angular Momentum Transfer for Quantum Enabled Technologies (SPANGL4Q)] and the German Ministry of Education and research (BMBF) and Engineering and Physical Sciences Research Council (EPSRC) [project Solid State Quantum Networks (SSQN)]. J.G.R. is sponsored by the EPSRC fellowship EP/M024458/1.

Focused ion beam etching for the fabrication of micropillar microcavities made of III-V semiconductor materials

The authors demonstrate a simple approach for the construction of single photon sources utilizing focused ion beam (FIB) etching, a maskless fabrication technique. Here they use FIB with gas-assisted etching to fabricate micropillar microcavities from a GaAs∕AlGaAs distributed Bragg reflector planar cavity containing self-assembled InAs quantum dots. Using a 1.5μm square pillar, they demonstrate a single photon source where the two photon emission is suppressed by a factor of 3.8. They believe this to be the first example of a FIB fabricated pillar single photon source.The authors demonstrate a simple approach for the construction of single photon sources utilizing focused ion beam (FIB) etching, a maskless fabrication technique. Here they use FIB with gas-assisted etching to fabricate micropillar microcavities from a GaAs∕AlGaAs distributed Bragg reflector planar cavity containing self-assembled InAs quantum dots. Using a 1.5μm square pillar, they demonstrate a single photon source where the two photon emission is suppressed by a factor of 3.8. They believe this to be the first example of a FIB fabricated pillar single photon source.

Generating entanglement with low-Q-factor microcavities

We propose a method of generating entanglement using single photons and electron spins in the regime of resonance scattering. The technique involves matching the spontaneous emission rate of the spin dipole transition in bulk dielectric to the modified rate of spontaneous emission of the dipole coupled to the fundamental mode of an optical microcavity. We call this regime resonance scattering where interference between the input photons and those scattered by the resonantly coupled dipole transition result in a reflectivity of zero. The contrast between this and the unit reflectivity when the cavity is empty allow us to perform a non demolition measurement of the spin and to non deterministically generate entanglement between photons and spins. The chief advantage of working in the regime of resonance scattering is that the required cavity quality factors are orders of magnitude lower than is required for strong coupling, or Purcell enhancement. This makes engineering a suitable cavity much easier particularly in materials such as diamond where etching high quality factor cavities remains a significant challenge.