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Single strontium atom in an ion trap
Credit: David Nadlinger, University of Oxford

David Nadlinger

Early Career Researcher (Merton College)

Research theme

  • Quantum information and computation

Sub department

  • Atomic and Laser Physics

Research groups

  • Ion trap quantum computing
david.nadlinger@physics.ox.ac.uk
Telephone: 01865 (2)72265,01865 (2)72346
Programming blog
College profile
  • About
  • Publications

Experimental quantum key distribution certified by Bell's theorem

Nature Springer Nature 607:7920 (2022) 682-686

Authors:

Dp Nadlinger, P Drmota, Bc Nichol, G Araneda, D Main, R Srinivas, Dm Lucas, Cj Ballance, K Ivanov, Ey-Z Tan, P Sekatski, Rl Urbanke, R Renner, N Sangouard, J-D Bancal

Abstract:

Cryptographic key exchange protocols traditionally rely on computational conjectures such as the hardness of prime factorization<sup>1</sup> to provide security against eavesdropping attacks. Remarkably, quantum key distribution protocols such as the Bennett-Brassard scheme<sup>2</sup> provide information-theoretic security against such attacks, a much stronger form of security unreachable by classical means. However, quantum protocols realized so far are subject to a new class of attacks exploiting a mismatch between the quantum states or measurements implemented and their theoretical modelling, as demonstrated in numerous experiments<sup>3-6</sup>. Here we present the experimental realization of a complete quantum key distribution protocol immune to these vulnerabilities, following Ekert's pioneering proposal<sup>7</sup> to use entanglement to bound an adversary's information from Bell's theorem<sup>8</sup>. By combining theoretical developments with an improved optical fibre link generating entanglement between two trapped-ion qubits, we obtain 95,628 key bits with device-independent security<sup>9-12</sup> from 1.5 million Bell pairs created during eight hours of run time. We take steps to ensure that information on the measurement results is inaccessible to an eavesdropper. These measurements are performed without space-like separation. Our result shows that provably secure cryptography under general assumptions is possible with real-world devices, and paves the way for further quantum information applications based on the device-independence principle.
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High-rate high-fidelity entanglement of qubits across an elementary quantum network

Physical Review Letters American Physical Society 124:11 (2020) 110501

Authors:

Laurent Stephenson, David Nadlinger, Bethan Nichol, Peter Drmota, Timothy Ballance, Keshav Thirumalai, Joseph Goodwin, David Lucas, Christopher Ballance

Abstract:

We demonstrate remote entanglement of trapped-ion qubits via a quantum-optical fiber link with fidelity and rate approaching those of local operations. Two 88Sr+ qubits are entangled via the polarization degree of freedom of two spontaneously emitted 422 nm photons which are coupled by high-numerical-aperture lenses into single-mode optical fibers and interfere on a beam splitter. A novel geometry allows high-efficiency photon collection while maintaining unit fidelity for ion-photon entanglement. We generate heralded Bell pairs with fidelity 94% at an average rate 182 s−1 (success probability 2.18×10−4).

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Benchmarking a high-fidelity mixed-species entangling gate

Physical Review Letters American Physical Society 125:8 (2020) 080504

Authors:

Amy Hughes, Vera Schäfer, Keshav Thirumalai, David Nadlinger, Sarah Woodrow, David Lucas, Christopher Ballance

Abstract:

We implement a two-qubit logic gate between a 43Ca+ hyperfine qubit and a 88Sr+ Zeeman qubit. For this pair of ion species, the S–P optical transitions are close enough that a single laser of wavelength 402 nm can be used to drive the gate but sufficiently well separated to give good spectral isolation and low photon scattering errors. We characterize the gate by full randomized benchmarking, gate set tomography, and Bell state analysis. The latter method gives a fidelity of 99.8(1)%, comparable to that of the best same-species gates and consistent with known sources of error.
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Fast, High-Fidelity Addressed Single-Qubit Gates Using Efficient Composite Pulse Sequences.

Physical review letters 131:12 (2023) 120601

Authors:

AD Leu, MF Gely, MA Weber, MC Smith, DP Nadlinger, DM Lucas

Abstract:

We use electronic microwave control methods to implement addressed single-qubit gates with high speed and fidelity, for ^{43}Ca^{+} hyperfine "atomic clock" qubits in a cryogenic (100 K) surface trap. For a single qubit, we benchmark an error of 1.5×10^{-6} per Clifford gate (implemented using 600 ns π/2 pulses). For 2 qubits in the same trap zone (ion separation 5  μm), 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 3.4×10^{-5} per addressed π/2 gate. The scheme scales theoretically to larger numbers of qubits in a single register.
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Robust quantum memory in a trapped-ion quantum network node

Physical Review Letters American Physical Society 130 (2023) 090803

Authors:

Peter Drmota, Dougal Main, David P Nadlinger, Bethan Nichol, Marius A Weber, Ellis M Ainley, Ayush Agrawal, Raghavendra Srinivas, Gabriel Araneda, Chris J Ballance, David Lucas

Abstract:

We integrate a long-lived memory qubit into a mixed-species trapped-ion quantum network node. Ion-photon entanglement first generated with a network qubit in 88Sr+ is transferred to 43Ca+ with 0.977(7) fidelity, and mapped to a robust memory qubit. We then entangle the network qubit with another photon, which does not affect the memory qubit. We perform quantum state tomography to show that the fidelity of ion-photon entanglement decays ∼ 70 times slower on the memory qubit. Dynamical decoupling further extends the storage time; we measure an ion-photon entanglement fidelity of 0.81(4) after 10 s.
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