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Bulletin of the American Physical Society

High-fidelity spatial addressing of $^{43}\text{Ca}^+$ qubits using near-field microwave control

Diana Prado Lopes Aude Craik, Norbert M. Linke, D. T. C. Allcock, M. A. Sepiol, T. P. Harty, Christopher J. Ballance, Derek N. Stacey, Andrew M. Steane, David Lucas

Published May 24, 2016• 0 citations
OpenAlex

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 of a microfabricated surface trap with integrated microwave electrodes. 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, implying an addressing error of $1.3\times 10^{-6}$. Off-resonant excitation prevents this error level being directly demonstrated, but we also show polarization control of the microwave field with error $2\times 10^{-5}$, sufficient to suppress off-resonant excitation out of the qubit states to the $\sim 10^{-9}$ level. Such polarization control could enable fast microwave operations.

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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 of a microfabricated surface trap with integrated microwave electrodes. 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, implying an addressing error of $1.3\times 10^{-6}$. Off-resonant excitation prevents this error level being directly demonstrated, but we also show polarization control of the microwave field with error $2\times 10^{-5}$, sufficient to suppress off-resonant excitation out of the qubit states to the $\sim 10^{-9}$ level. Such polarization control could enable fast microwave operations.

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Individual addressing of qubits is essential for scalable quantum computation. Spatial addressing allows unlimited numbers of qubits to share the same frequency, while enabling arbitrary parallel operations. We demonstrate addressing of long-lived $^{43}\mathrm{Ca}^{+}$ ``atomic clock'' qubits held in separate zones ($960\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\text{m}$ apart) of a microfabricated surface trap with integrated microwave electrodes. Such zones could form part of a ``quantum charge-coupled device'' architecture for a large-scale quantum information processor. By coherently canceling the microwave field in one zone we measure a ratio of Rabi frequencies between addressed and nonaddressed qubits of up to 1400, from which we calculate a spin-flip probability on the qubit transition of the nonaddressed ion of $1.3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$. Off-resonant excitation then becomes the dominant error process, at around $5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$. 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 $2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$, which would suffice to suppress off-resonant excitation to the $\ensuremath{\sim}{10}^{\ensuremath{-}9}$ level if combined with spatial addressing. Such polarization control could also enable fast microwave operations.

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arXiv:1606.08409•Physical Review Letters•Sep 27, 2016•144 citations
High-fidelity spatial addressing of 43Ca+^{43}\text{Ca}^+43Ca+ qubits using near-field microwave control
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 43Ca+^{43}\text{Ca}^+43Ca+ atomic clock qubits held in separate zones of a microfabricated surface trap with integrated microwave electrodes. 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, implying an addressing error of 1.3×10−61.3\times 10^{-6}1.3×10−6. Off-resonant excitation prevents this error level being directly demonstrated, but we also show polarization control of the microwave field with error 2×10−52\times 10^{-5}2×10−5, sufficient to suppress off-resonant excitation out of the qubit states to the ∼10−9\sim 10^{-9}∼10−9 level. Such polarization control could enable fast microwave operations.
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 43Ca+^{43}\text{Ca}^+43Ca+ atomic clock qubits held in separate zones of a microfabricated surface trap with integrated microwave electrodes. 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, implying an addressing error of 1.3×10−61.3\times 10^{-6}1.3×10−6. Off-resonant excitation prevents this error level being directly demonstrated, but we also show polarization control of the microwave field with error 2×10−52\times 10^{-5}2×10−5, sufficient to suppress off-resonant excitation out of the qubit states to the ∼10−9\sim 10^{-9}∼10−9 level. Such polarization control could enable fast microwave operations.
Individual addressing of qubits is essential for scalable quantum computation. Spatial addressing allows unlimited numbers of qubits to share the same frequency, while enabling arbitrary parallel operations. We demonstrate addressing of long-lived 43Ca+^{43}\mathrm{Ca}^{+}43Ca+ ``atomic clock'' qubits held in separate zones ($960\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\text{m}$ apart) of a microfabricated surface trap with integrated microwave electrodes. Such zones could form part of a ``quantum charge-coupled device'' architecture for a large-scale quantum information processor. By coherently canceling the microwave field in one zone we measure a ratio of Rabi frequencies between addressed and nonaddressed qubits of up to 1400, from which we calculate a spin-flip probability on the qubit transition of the nonaddressed ion of $1.3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$. Off-resonant excitation then becomes the dominant error process, at around $5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$. 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 $2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$, which would suffice to suppress off-resonant excitation to the $\ensuremath{\sim}{10}^{\ensuremath{-}9}$ level if combined with spatial addressing. Such polarization control could also enable fast microwave operations.
This thesis reports high-fidelity near-field spatial microwave addressing of long-lived 43Ca+ "atomic clock" qubits performed in a two-zone single-layer surface-electrode ion trap. Addressing is implemented by using two of the trap's integrated microwave electrodes, one in each zone, to drive single-qubit rotations in the zone we choose to address whilst interferometrically cancelling the microwave field at the neighbour (non-addressed) zone. Using this field-nulling scheme, we measure a Rabi frequency ratio between addressed and non-addressed zones of up to 1400, from which we calculate an addressing error (or a spin-flip probability on the qubit transition) of 1e-6. Off-resonant excitation out of the qubit state is a more significant source of error in this experiment, but we also demonstrate polarisation control of the microwave field at an error level of 2e-5, which, if combined with individual-ion addressing, would be sufficient to suppress off-resonant excitation errors to the 1e-9 level. Further, this thesis presents preliminary results obtained with a micron-scale coupled-microstrip differential antenna probe that can be scanned over an ion-trap chip to map microwave magnetic near fields. The probe is designed to enable the measurement of fields at tens of microns above electrode surfaces and to act as an effective characterisation tool, speeding up design-fabrication-characterisation cycles in the production of new prototype microwave ion-trap chips. Finally, a new multi-layer design for an ion-trap chip which displays, in simulations, a 100-fold improvement in addressing performance, is presented. The chip electrode structure is designed to use the cancelling effect of microwave return currents to produce Rabi frequency ratios of order 1000 between trap zones using a single microwave electrode (i.e. without the need for nulling fields). If realised, this chip could be used to drive individually addressed single-qubit operations on arrays of memory qubits in parallel and with high fidelity.
We use electronic microwave control methods to implement addressed single-qubit gates with high speed and fidelity, for 43Ca+^{43}{\mathrm{Ca}}^{+}43Ca+ hyperfine ``atomic clock'' qubits in a cryogenic (100 K) surface trap. For a single qubit, we benchmark an error of $1.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$ per Clifford gate (implemented using 600 ns $\ensuremath{\pi}/2$ pulses). For 2 qubits in the same trap zone (ion separation $5\text{ }\text{ }\mathrm{\ensuremath{\mu}}\mathrm{m}$), we use a spatial microwave field gradient, combined with an efficient four-pulse scheme, to implement independent addressed gates. Parallel randomized benchmarking on both qubits yields an average error $3.4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$ per addressed $\ensuremath{\pi}/2$ gate. The scheme scales theoretically to larger numbers of qubits in a single register.
We report the achievement of single-qubit gates with sub-part-per-million error rates, in a trapped-ion ^{43}Ca^{+} hyperfine clock qubit. We explore the speed and fidelity trade-off for gate times 4.4≤t_{g}≤35 μs, and benchmark a minimum error per Clifford gate of 1.5(4)×10^{-7}. Calibration errors are suppressed to <10^{-8}, leaving qubit decoherence (T_{2}≈70 s), leakage, and measurement as the dominant error contributions. The ion is held above a microfabricated surface-electrode trap that incorporates a chip-integrated microwave resonator for electronic qubit control; the trap is operated at room temperature without magnetic shielding.
We demonstrate a two-qubit logic gate driven by near-field microwaves in a room-temperature microfabricated surface ion trap. We introduce a dynamically decoupled gate method, which stabilizes the qubits against fluctuating energy shifts and avoids the need to null the microwave field. We use the gate to produce a Bell state with fidelity 99.7(1)%, after accounting for state preparation and measurement errors. The gate is applied directly to ^{43}Ca^{+} hyperfine "atomic clock" qubits (coherence time T_{2}^{*}≈50 s) using the oscillating magnetic field gradient produced by an integrated microwave electrode.