While spins are excellent quantum bits, their long-range coupling remains a challenge to tackle towards complex quantum devices for quantum computing and quantum simulation [1]. Our research proposes to take up this challenge using a microwave photon as a quantum mediator between distant qubits in silicon or in other word open spin qubits to circuit quantum electrodynamics

Figure 1 : Schematic of two distant spins interacting through a photon.

Practically, we take advantage of the exceptional properties of hole spin-orbit qubits in silicon, namely their strong spin-orbit coupling, to coherently couple them to microwave photons in superconducting cavities made from thin NbN layers [2]. Recently, we demonstrated a strong entanglement between a hole spin and a microwave photon [3].
This crucial demonstration shows that hole spin-orbit qubits can leverage quantum electrodynamics, which holds great promises for quantum computing, quantum simulation and the exploration of light-matter interaction in extreme regimes [4, 5].

Figure 2 : Schematic drawing of a hole spin cQED sample, where a single hole spin (green arrow) interacts coherently with a microwave photon in a cavity symbolized by two yellow mirrors.
(b) An avoided anticrossing – the signature of strong spin-photon coupling – is is observed when the spin transition frequency (dashed line) matches the bare resonator frequency (dashed–dotted line). Frequency linecut at resonance, highlighting a vacuum Rabi-mode splitting 2gs/2π=184 MHz.

Currently we are developing a novel spin-orbit cQED architecture leveraging an unprecedented combination of academic and industrial tools to realize flip-chip devices.
The key idea is to combine one chip with high-quality, high-impedance microwave circuits with a second chip hosting hole spin-orbit qubits in silicon in a flip-chip assembly (see figure below).

Figure 3 (a) Schematic overview of a flip-chip assembly, which contains hole spin qubits on the top chip and high-quality,
high-impedance microwave circuits on the bottom chip. Electrical and mechanical connections between both chips are ensured by micrometer sized indium balls. (b) Picture of a flip-chip sample bonded to a circuit-board.

If successful, our unique flip-chip hardware will allow us to investigate novel research paths. This includes for example long-distance spin-spin entanglement and high-fidelity spin readout on ns time scales, both essential for quantum information processing. Another path we envision is toward analog quantum simulation of many body problems. Indeed, a microwave cavity connected to multiple spins comes naturally with an all-to-all connectivity and hence allows for many-body entanglement and the study of spin models. Similarly, a single hole spin coherently coupled to multiple microwave cavities realizes another prototypical many-body problem, the spin-boson model [6, 7].
The hole spin cQED architecture will hence allow to unlock the physics of many-body interacting systems, a largely unexplored field.

This project involves Romain Maurand, Simon Zihlmann, Etienne Dumur, Léo Noirot, Sébastien Granel, Karl Leuckefeld, Frederic Gustavo, and Jean-luc Thomassin.
It has received funding by the European Research council under the ERC starting grant project LONGPSIN, the French national quantum initiative (Presquile and Miracle_Q)
and the European project QLSI2.

Refs:

[1] Burkard et al. Rev. Mod. Phys. 95, 025003 (2023)

[2] Yu et al. Appl. Phys. Lett. 118, 054001 (2001)

[3] Yu, Zihlmann et al. Nat. Nanotechnol. 18. 741 (2023)

[4] Kockum et al., Nat. Rev. Phys. 1, 19 (2019)

[5] Forn-Diaz et al., Rev. Mod. Phys. 91, 025005 (2019)

[6] Altman Phys. Rev. X Quant. 2, 017003 (2021)

[7] Le Hur Comptes Rendus Physique 17, 808 (2016)

Strong coupling between a photon and a hole spin in silicon

Magnetic field resilient high kinetic inductance superconducting niobium nitride coplanar waveguide resonators