Showing papers by "C. J. Ballance published in 2015"

Hybrid quantum logic and a test of Bell's inequality using two different atomic isotopes.

Abstract: Web summaryHarnessing the entanglement of different ionic species could bring new flexibility in quantum computing, and now two groups independently demonstrate entanglement between different atomic species; Ballance et al. achieve entanglement between different atomic isotopes, whereas the related paper by Tan et al. shows entanglement between different elements, together demonstrating a first step towards mixed-species quantum logic. In quantum-computing architectures, not all physical systems are equally good at completing each task. For example, in trapped-ion quantum computers, one specific element might be an excellent memory qubit, while another element is more suited to transporting information between nodes. However, a crucial prerequisite to harness these advantages is the entanglement of different atomic species. Now, two groups have independently achieved this. Ting Rei Tan et al. showed entanglement between different elements 9Be+ and 25Mg+, and Christopher Ballance et al. achieved entanglement between different atomic isotopes, 40Ca+ and 43Ca+. These studies represent a first step towards mixed-species quantum logic, and from a fundamental perspective they show that particles that are distinguishable by many internal properties can indeed be entangled and violate Bell's inequality. Entanglement is one of the most fundamental properties of quantum mechanics1,2,3, and is the key resource for quantum information processing4,5 (QIP). Bipartite entangled states of identical particles have been generated and studied in several experiments, and post-selected or heralded entangled states involving pairs of photons, single photons and single atoms, or different nuclei in the solid state, have also been produced6,7,8,9,10,11,12. Here we use a deterministic quantum logic gate to generate a ‘hybrid’ entangled state of two trapped-ion qubits held in different isotopes of calcium, perform full tomography of the state produced, and make a test of Bell’s inequality with non-identical atoms. We use a laser-driven two-qubit gate13, whose mechanism is insensitive to the qubits’ energy splittings, to produce a maximally entangled state of one 40Ca+ qubit and one 43Ca+ qubit, held 3.5 micrometres apart in the same ion trap, with 99.8 ± 0.6 per cent fidelity. We test the CHSH (Clauser–Horne–Shimony–Holt)14 version of Bell’s inequality for this novel entangled state and find that it is violated by 15 standard deviations; in this test, we close the detection loophole8 but not the locality loophole7. Mixed-species quantum logic is a powerful technique for the construction of a quantum computer based on trapped ions, as it allows protection of memory qubits while other qubits undergo logic operations or are used as photonic interfaces to other processing units15,16. The entangling gate mechanism used here can also be applied to qubits stored in different atomic elements; this would allow both memory and logic gate errors caused by photon scattering to be reduced below the levels required for fault-tolerant quantum error correction, which is an essential prerequisite for general-purpose quantum computing.

Laser-driven quantum logic gates with precision beyond the fault-tolerant threshold

Abstract: A quantum computer could solve problems intractable on any conventional machine, but the physical resources necessary to build one depend strongly on the precision with which its constituent qubits can be manipulated. We demonstrate laser-driven two-qubit and single-qubit logic gates with fidelities 99.9(1)% and 99.9934(3)% respectively, using qubits stored in long-lived hyperfine states of calcium-43 ions held in a room-temperature device. Calcium-43 ions have exhibited the longest coherence time ($T_2^*\approx 50$sec) and the highest state preparation and measurement fidelity of any single physical qubits. We study the speed/fidelity trade-off for the two-qubit gate, for gate times between 3.8$\mu$s and 520$\mu$s, and develop a theoretical model allowing us to identify the principal technical sources of error. We conclude that trapped-ion quantum logic is possible with the precision necessary for a practical quantum computer.

Dark-resonance Doppler cooling and high fluorescence in trapped Ca-43 ions at intermediate magnetic field

Abstract: We demonstrate simple and robust methods for Doppler cooling and obtaining high fluorescence from trapped 43Ca+ ions at a magnetic field of 146 Gauss. This field gives access to a magnetic-field-independent "atomic clock" qubit transition within the ground level hyperfine structure of the ion, but also causes the complex internal structure of the 64 states relevant to Doppler cooling to be spread over many times the atomic transition line-width. Using a time-dependent optical Bloch equation simulation of the system we develop a simple scheme to Doppler-cool the ion on a two-photon dark resonance, which is robust to typical experimental variations in laser intensities, detunings and polarizations. We experimentally demonstrate cooling to a temperature of 0.3 mK, slightly below the Doppler limit for the corresponding two-level system, and then use Raman sideband laser cooling to cool further to the ground states of the ion's radial motional modes. These methods will enable two-qubit entangling gates with this ion, which is one of the most promising qubits so far developed.

Optical injection and spectral filtering of high-power ultraviolet laser diodes.

Abstract: We demonstrate injection locking of high-power laser diodes operating at 397 nm. We achieve stable operation with an injection power of ∼100 μW and a slave laser output power of up to 110 mW. We investigate the spectral purity of the slave laser light via photon scattering experiments on a single trapped (40)Ca(+) ion. We show that it is possible to achieve a scattering rate indistinguishable from that of monochromatic light by filtering the laser light with a diffraction grating to remove amplified spontaneous emission.