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Atomic and Laser Physics
Credit: Jack Hobhouse

Derek Stacey

Emeritus Professor

Sub department

  • Atomic and Laser Physics

Research groups

  • Ion trap quantum computing
Derek.Stacey@physics.ox.ac.uk
  • About
  • Publications

High-fidelity spatial and polarization addressing of 43Ca+ qubits using near-field microwave control

Physical Review A American Physical Society 95:2 (2017) 022337

Authors:

Diana PL Aude Craik, Norbert M Linke, Martin A Sepiol, Thomas Harty, Joseph Goodwin, Christopher J Ballance, Derek Stacey, Andrew Steane, David M Lucas, David TC Allcock

Abstract:

Individual addressing of qubits is essential for scalable quantum computation. Spatial addressing allows unlimited numbers of qubits to share the same frequency, whilst enabling arbitrary parallel operations. We demonstrate addressing of long-lived $^{43}\text{Ca}^+$ "atomic clock" qubits held in separate zones ($960\mu$m apart) of a microfabricated surface trap with integrated microwave electrodes. Such zones could form part of a "quantum CCD" architecture for a large-scale quantum information processor. By coherently cancelling the microwave field in one zone we measure a ratio of Rabi frequencies between addressed and non-addressed qubits of up to 1400, from which we calculate a spin-flip probability on the qubit transition of the non-addressed ion of $1.3\times 10^{-6}$. Off-resonant excitation then becomes the dominant error process, at around $5 \times 10^{-3}$. It can be prevented either by working at higher magnetic field, or by polarization control of the microwave field. We implement polarization control with error $2 \times 10^{-5}$, which would suffice to suppress off-resonant excitation to the $\sim 10^{-9}$ level if combined with spatial addressing. Such polarization control could also enable fast microwave operations.
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High-Fidelity Preparation, Gates, Memory, and Readout of a Trapped-Ion Quantum Bit.

Physical review letters 113:22 (2014) 220501

Authors:

TP Harty, DTC Allcock, CJ Ballance, L Guidoni, HA Janacek, NM Linke, DN Stacey, DM Lucas

Abstract:

We implement all single-qubit operations with fidelities significantly above the minimum threshold required for fault-tolerant quantum computing, using a trapped-ion qubit stored in hyperfine "atomic clock" states of ^{43}Ca^{+}. We measure a combined qubit state preparation and single-shot readout fidelity of 99.93%, a memory coherence time of T_{2}^{*}=50  sec, and an average single-qubit gate fidelity of 99.9999%. These results are achieved in a room-temperature microfabricated surface trap, without the use of magnetic field shielding or dynamic decoupling techniques to overcome technical noise.
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A microfabricated ion trap with integrated microwave circuitry

ArXiv 1210.3272 (2012)

Authors:

DTC Allcock, TP Harty, CJ Ballance, BC Keitch, NM Linke, DN Stacey, DM Lucas

Abstract:

We describe the design, fabrication and testing of a surface-electrode ion trap, which incorporates microwave waveguides, resonators and coupling elements for the manipulation of trapped ion qubits using near-field microwaves. The trap is optimised to give a large microwave field gradient to allow state-dependent manipulation of the ions' motional degrees of freedom, the key to multiqubit entanglement. The microwave field near the centre of the trap is characterised by driving hyperfine transitions in a single laser-cooled 43Ca+ ion.
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Background-free detection of trapped ions

Applied Physics B: Lasers and Optics 107:4 (2012) 1175-1180

Authors:

NM Linke, DTC Allcock, DJ Szwer, CJ Ballance, TP Harty, HA Janacek, DN Stacey, AM Steane, DM Lucas

Abstract:

We demonstrate a Doppler cooling and detection scheme for ions with low-lying D levels which almost entirely suppresses scattered laser light background, while retaining a high fluorescence signal and efficient cooling. We cool a single ion with a laser on the 2S1/2 ?2P1/2 transition as usual, but repump via the 2P3/2 level. By filtering out light on the cooling transition and detecting only the fluorescence from the 2P3/2 → 2S1/2 decays, we suppress the scattered laser light background count rate to 1 s-1 while maintaining a signal of 29000 s-1 with moderate saturation of the cooling transition. This scheme will be particularly useful for experiments where ions are trapped in close proximity to surfaces, such as the trap electrodes in microfabricated ion traps, which leads to high background scatter from the cooling beam.
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Background-free detection of trapped ions

Applied Physics B: Lasers and Optics (2011) 1-6

Authors:

NM Linke, DTC Allcock, DJ Szwer, CJ Ballance, TP Harty, HA Janacek, DN Stacey, AM Steane, DM Lucas

Abstract:

We demonstrate a Doppler cooling and detection scheme for ions with low-lying D levels which almost entirely suppresses scattered laser light background, while retaining a high fluorescence signal and efficient cooling. We cool a single ion with a laser on the {Mathematical expression} transition as usual, but repump via the {Mathematical expression} level. By filtering out light on the cooling transition and detecting only the fluorescence from the {Mathematical expression} decays, we suppress the scattered laser light background count rate to 1 s -1 while maintaining a signal of 29000 s -1 with moderate saturation of the cooling transition. This scheme will be particularly useful for experiments where ions are trapped in close proximity to surfaces, such as the trap electrodes in microfabricated ion traps, which leads to high background scatter from the cooling beam. © 2011 Springer-Verlag.
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