University of St Andrews

Quantum mechanics offers a revolutionary approach to how information is processed, with unprecedented levels of security through quantum networking and exponential speed up with quantum computing. Neutral atoms provide an excellent platform for quantum computing, enabling large numbers of identical qubits to be cooled and trapped overcoming major barriers to scaling experienced by competing architectures [1]. A crucial ingredient for quantum computing is the ability to perform two-qubit gate operations, for which the strong, long-range dipole-dipole interaction between Rydberg atoms can be exploited to create a ‘dipole blockade’ which prevents the excitation of more than one Rydberg atom within a radius R < 10 μm [2]. Using this effect we have previously demonstrated ground-state entanglement between a pair of atoms with a fidelity of 81% [4], however when scaling to larger qubit numbers the collective enhancement of the many-body state leads to number dependent pulse areas that reduce gate fidelity. 

We show recent results demonstrating an alternative mesoscopic gate scheme based on electromagnetically induced transparency (EIT), originally proposed by Müller et al. [3]. This protocol provides a scalable approach to performing entanglement of large number of target qubits using a single control atom whilst circumventing challenges of the collective Rabi frequency. The resulting CNOT^N gate protocol is therefore robust against number fluctuations and provides a route to creating useful entangled states for high-precision measurements beyond the standard quantum limit as well as providing a key gate for implementing error correction in neutral atom arrays.

An alternative route to scalable quantum computing involves hybrid quantum systems based on coupling atomic qubits to superconducting microwave cavities via the strong Rydberg atom electric dipole moment. As well as extending the interaction range from microns to centimeters, this provides a versatile quantum interface enabling generation, storage and entanglement of photons in both microwave an optical domains using a scalable on-chip design. We present progress towards a hybrid quantum interface based on superconducting niobium nitride coplanar waveguide resonators, optimized for operation at 4 K to allow strong coupling to Rydberg atom qubits.

[1] C.S. Adams, J.D. Pritchard and J. Shaffer, “Rydberg Atom Quantum Technologies,” J. Phys. B (in press) (2019).

[2] M. Saffman, T.G. Walker and K. Mølmer, “Quantum information with Rydberg atoms,” Rev. Mod. Phys. 82, 2313 (2010).

[3] M. Müller et al., “Mesoscopic Rydberg Gate Based on Electromagnetically Induced Transparency,” Phys. Rev. Lett. 102, 170502 (2009).

[4] C.J. Picken, R. Legaie, K. McDonnell and J.D. Pritchard, “Entanglement of neutral-atom qubits with long ground-Rydberg coherence times,” Quantum Sci. Technol. 4, 015011 (2018).