Publications by Derek Stacey


High-Fidelity Preparation, Gates, Memory, and Readout of a Trapped-Ion Quantum Bit.

Physical review letters 113 (2014) 220501-

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

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.


A microfabricated ion trap with integrated microwave circuitry

ArXiv (0)

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

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.


Background-free detection of trapped ions

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

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

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.


Background-free detection of trapped ions

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

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

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.


Implementation of a symmetric surface electrode ion trap with field compensation using a modulated Raman effect

ArXiv (0)

DTC Allcock, JA Sherman, DN Stacey, AH Burrell, MJ Curtis, G Imreh, NM Linke, DJ Szwer, SC Webster, AM Steane, DM Lucas

We describe the fabrication and characterization of a new surface-electrode Paul ion trap designed for experiments in scalable quantum information processing with Ca+. A notable feature is a symmetric electrode pattern which allows rotation of the normal modes of ion motion, yielding efficient Doppler cooling with a single beam parallel to the planar surface. We propose and implement a technique for micromotion compensation in all directions using an infrared repumper laser beam directed into the trap plane. Finally, we employ an alternate repumping scheme that increases ion fluorescence and simplifies heating rate measurements obtained by time-resolved ion fluorescence during Doppler cooling.


Memory coherence of a sympathetically cooled trapped-ion qubit

Physical Review A - Atomic, Molecular, and Optical Physics 79 (2009)

JP Home, MJ McDonnell, DJ Szwer, BC Keitch, DM Lucas, DN Stacey, AM Steane

We demonstrate sympathetic cooling of a C 43 a+ trapped-ion "memory" qubit by a C 40 a+ "coolant" ion sufficiently near the ground state of motion for fault-tolerant quantum logic, while maintaining coherence of the qubit. This is an essential ingredient in trapped-ion quantum computers. The isotope shifts are sufficient to suppress decoherence and phase shifts of the memory qubit due to the cooling light which illuminates both ions. We measure the qubit coherence during ten cycles of sideband cooling, finding a coherence loss of 3.3% per cooling cycle. The natural limit of the method is O (10-4) infidelity per cooling cycle. © 2009 The American Physical Society.


Memory coherence of a sympathetically cooled trapped-ion qubit

PHYSICAL REVIEW A 79 (2009) ARTN 050305

JP Home, MJ McDonnell, DJ Szwer, BC Keitch, DM Lucas, DN Stacey, AM Steane


High-fidelity readout of trapped-ion qubits

PHYS REV LETT AMER PHYSICAL SOC 100 (2008) 200502

AH Myerson, DJ Szwer, SC Webster, DTC Allcock, MJ Curtis, G Imreh, JA Sherman, DN Stacey, AM Steane, DM Lucas

We demonstrate single-shot qubit readout with a fidelity sufficient for fault-tolerant quantum computation. For an optical qubit stored in Ca-40(+) we achieve 99.991(1)% average readout fidelity in 10(6) trials, using time-resolved photon counting. An adaptive measurement technique allows 99.99% fidelity to be reached in 145 mu s average detection time. For Ca-43(+), we propose and implement an optical pumping scheme to transfer a long-lived hyperfine qubit to the optical qubit, capable of a theoretical fidelity of 99.95% in 10 mu s. We achieve 99.87(4)% transfer fidelity and 99.77(3)% net readout fidelity.