Observation of Entanglement Between a Quantum Dot Spin and a Single Photon
Authors: Wei-bo Gao, Parisa Fallahi, Emre Togan, Javier Miguel-Sanchez, Atac Imamoglu
Affiliation: Institute of Quantum Electronics, ETH Zurich, Switzerland
Entanglement deepens our understanding of fundamental physics. A well-known example is the study of non-local interpretations of quantum mechanics by testing the violation of Bell’s inequality . In the practical side, an interface between a stationary qubit (spin) and a flying qubit (photon) is a basic element to build a distributed quantum network. Moreover, such a network can be used for building a quantum computer, that offers significant speedups in solving certain technologically relevant classes of problems. A realization of distributed quantum computation will use few-qubit quantum processor nodes (spins) connected by photons . A particularly interesting platform is a spin based semiconductor system in which photonic circuits that interconnect the nodes can be fabricated on the same semiconductor chip .
In the past few years, considerable efforts have been made to demonstrate entanglement between a spin and a photon. In 2004, the first observation of entanglement between a single trapped atom and a single photon was realized in the group of C. Monroe in Michigan . In 2006 and 2007, the entanglement between a neutral atom and a single photon was realized in the groups of H. Weinfurter  G. Rempe  in Germany. In 2010, entanglement between an N-V center and a single photon was reported . Despite the appeal of semiconductor based systems the realization of spin-photon entanglement in these systems had been very challenging mainly due to fast transitions and strong decoherence of the quantum dot spins and has only been achieved in 2012. In our work as well as the complementary work of the Yamamoto group at Stanford, the difficulty of measurement was overcome and the first semiconductor spin-photon entanglement was reported [8, 9].
Our experiment focuses on a single electron spin trapped in an InGaAs self-assembled quantum dot. A magnetic field of 0.7 Tesla applied perpendicular to the sample growth direction. The ground states of the quantum dot are identified by the orientation of the electron spin. The basic principle behind the deterministic generation of a spin–photon entangled state is straightforward: following the excitation of the quantum dot into an excited (trion) state, radiative recombination will project the system into an entangled state where the color and polarization of the emitted photon are entangled with the spin of the electron in the ground state. Our measurement process entails suppression of the laser background using cross polarized excitation and collection, thus erasing the polarization information of the quantum dot photons. We therefore focus on the entanglement between the spin qubit and the color (frequency) of the photonic qubit.
To demonstrate entanglement we measure both classical and quantum correlations between the electron spin and the color of the emitted photon. Quantum correlations are demonstrated through observation of oscillations in the emitted photon counts conditioned on detecting a spin in a superposition of up and down spins. By reversing the spin projection direction, we observe a Pi-phase change in the oscillation. These oscillations originate from a phase shift between the two components of the entangled state as the time between photon emission and spin measurement is changed. We calculate an overall entanglement fidelity with a lower bound of 0.67±0.05 , which is above 0.5 and thus constitutes a proof for entanglement in our system.
Semiconductor quantum dots have many advantages compared to the other candidates for optically accessible qubits. For example, majority of the photons are emitted into the zero phonon line making them bright, narrow bandwidth single-photon sources. Also fast spin manipulation of the spin states (~4ps) is possible, and the spontaneous emission time is short (~600ps), making a repetition rate of 76MHz possible in our experiment. There are, however, also drawbacks: the dephasing time of the spin states is about 1ns, limiting the useful time window in the entanglement generation. Moreover, lack of a cycling transition, reduces the likelihood of being able to determine the spin state at a single experimental run, limiting efficient scaling to more spins. Luckily these drawbacks can be overcome by using coupled quantum dot systems that have much richer transition structure and the spin qubits can be more robust against the noisy environment . We aim to achieve spin-spin entanglement in the coupled quantum dot systems in the near future.
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