Showing papers by "Chao-Yang Lu published in 2010"

Experimental demonstration of a hyper-entangled ten-qubit Schrödinger cat state

Abstract: Creating entangled photon states becomes technologically ever more difficult as the number of particles increases, and the current record stands at six entangled photons. However, using both their polarization and momentum degrees of freedom, up to ten-qubit states can be encoded in ‘only’ five photons, as has now been demonstrated.

Observation of spin-dependent quantum jumps via quantum dot resonance fluorescence

Abstract: A promising approach to realizing a practical qubit scheme for quantum computation involves the optical control of single electron spins in semiconductor quantum dots. Rapid progress towards the reliable preparation and manipulation of the quantum states of such spins has been achieved in recent years. The final challenge is to carry out 'single shot' measurements of the electron spin without interfering with it. Vamivakas et al. have now developed a technique that enables such a measurement through coupling of one quantum dot to another. This quantum dot 'molecule', unlike its single quantum dot counterpart, allows separate and independent optical transitions for state preparation, manipulation and measurement, avoiding the dilemma of relying on the same transition to address the spin state of an electron. As a result, the authors show, it is possible to observe spin quantum jumps in real time. A promising approach to realizing a practical quantum bit scheme is the optical control of single electron spins in quantum dots. The reliable preparation and manipulation of the quantum states of such spins have been demonstrated recently. The final challenge is to carry out single-shot measurements of the electron spin without interfering with it. A technique has now been developed that enables such measurement, by coupling one quantum dot to another to produce a quantum dot molecule. Reliable preparation, manipulation and measurement protocols are necessary to exploit a physical system as a quantum bit1. Spins in optically active quantum dots offer one potential realization2,3 and recent demonstrations have shown high-fidelity preparation4,5 and ultrafast coherent manipulation6,7,8. The final challenge—that is, single-shot measurement of the electron spin—has proved to be the most difficult of the three and so far only time-averaged optical measurements have been reported9,10,11,12. The main obstacle to optical spin readout in single quantum dots is that the same laser that probes the spin state also flips the spin being measured. Here, by using a gate-controlled quantum dot molecule13,14,15, we present the ability to measure the spin state of a single electron in real time via the intermittency of quantum dot resonance fluorescence12,16. The quantum dot molecule, unlike its single quantum dot counterpart, allows separate and independent optical transitions for state preparation, manipulation and measurement, avoiding the dilemma of relying on the same transition to address the spin state of an electron.

Experimental realization of a controlled-NOT gate with four-photon six-qubit cluster states.

Abstract: We experimentally demonstrate an optical controlled-NOT (CNOT) gate with arbitrary single inputs based on a 4-photon 6-qubit cluster state entangled both in polarization and spatial modes. We first generate the 6-qubit state, and then, by performing single-qubit measurements, the CNOT gate is applied to arbitrary single input qubits. To characterize the performance of the gate, we estimate its quantum process fidelity and prove its entangling capability. In addition, our results show that the gate cannot be reproduced by local operations and classical communication. Our experiment shows that such hyper-entangled cluster states are promising candidates for efficient optical quantum computation.

Direct measurement of spin dynamics in InAs/GaAs quantum dots using time-resolved resonance fluorescence

Abstract: We temporally resolve the resonance fluorescence from an electron spin confined to a single self-assembled quantum dot to measure directly the spin's optical initialization and natural relaxation time scales at 4 K. Our measurements demonstrate that spin initialization occurs on the order of microseconds in the Faraday configuration when a laser resonantly drives the quantum dot transition. We show that the mechanism mediating the optically induced spin-flip changes from electron-nuclei interaction to hole-mixing interaction at 0.6 T external magnetic field. Spin relaxation measurements result in times on the order of milliseconds and suggest that a ${B}^{\ensuremath{-}5}$ magnetic field dependence, due to spin-orbit coupling, is sustained all the way down to 2.2 T.

Teleportation-based realization of an optical quantum two-qubit entangling gate

Abstract: In recent years, there has been heightened interest in quantum teleportation, which allows for the transfer of unknown quantum states over arbitrary distances. Quantum teleportation not only serves as an essential ingredient in long-distance quantum communication, but also provides enabling technologies for practical quantum computation. Of particular interest is the scheme proposed by D. Gottesman and I. L. Chuang [(1999) Nature 402:390–393], showing that quantum gates can be implemented by teleporting qubits with the help of some special entangled states. Therefore, the construction of a quantum computer can be simply based on some multiparticle entangled states, Bell-state measurements, and single-qubit operations. The feasibility of this scheme relaxes experimental constraints on realizing universal quantum computation. Using two different methods, we demonstrate the smallest nontrivial module in such a scheme—a teleportation-based quantum entangling gate for two different photonic qubits. One uses a high-fidelity six-photon interferometer to realize controlled-NOT gates, and the other uses four-photon hyperentanglement to realize controlled-Phase gates. The results clearly demonstrate the working principles and the entangling capability of the gates. Our experiment represents an important step toward the realization of practical quantum computers and could lead to many further applications in linear optics quantum information processing.

Bell inequality tests of four-photon six-qubit graph states

Abstract: We now experimentally demonstrate a Y-shaped graph state with photons' polarization and spatial modes as qubits Based on this state and a linear-type graph state, we report on the experimental realization of two different Bell inequality tests, which represent higher violation than previous Bell tests

Optical Readout of Quantum Dot Electron Spin

Abstract: By monitoring the intermittency of resonance fluorescence from a quantum dot molecule we perform optical single-shot measurements of a resident electron’s spin. Two-laser experiments confirm the conditionality of the resonance fluorescence on the spin orientation.