Final Report Summary - EQESD (Exploring Quantum Entanglement using Spins in Diamond)
Project background
Quantum Information Processing (QIP) is a key future technology: it offers superior communication security, improved processing speed and even the ability to solve classes of problems not accessible to current (classical) information technology. Most of the proposed schemes are based on quantum entanglement. For scalable systems, in particular long distance entanglement is a valuable resource. Such entanglement has just recently been demonstrated between two distant atoms, based on the interference of photons that were entangled with the atom spin. Solid-state systems receive large attention because they offer potential for better scalability than atom experiments and promise highly integrated devices. For quantum bits defined in the solid state however, experiments aiming at long-distance entanglement are still in its infancy.
Goals
The EQESD project aims at demonstrating the suitability of Nitrogen Vacancy centres in diamond for the implementation of measurement-based QIP schemes, by combining advanced methods from atomic quantum optics with solid state spin manipulation techniques. The feasibility of creating entangled states of remote spins via quantum interference is studied.
The nitrogen-vacancy (NV) centre in diamonds is an exceptional solid-state system with very attractive properties for these tasks, combining a very long spin coherence time and a strong optical transition that allows spin-photon conversion.
Summary of results
In this work, we developed bright NV centre devices and sophisticated measurement techniques allowing us to demonstrate key elements for QIP in the solid state. The main results are summarised below:
- A crucial ingredient for creation of spin-photon entanglement is the coherent preparation into an optically excited state. We demonstrated this ability by driving optical Rabi oscillations between the orbital states of the NV centre. Furthermore, we demonstrated a technique to achieve high-fidelity of the coherent control even for spectrally unstable emitters, such as the NV centres. This work resulted in a publication in Physical Review Letters [1].
- NV centres have already successfully been used in the fields of quantum information processing, benefitting from optically induced spin polarisation and optical spin readout. However, an understanding of the optically induced spin dynamics of the NV centre was still incomplete. We studied these spin dynamics in detail, using ps-pulsed optical excitation and microwave spin manipulation. This research resulted in publication [2].
- We demonstrated high-fidelity projective readout of the electronic spin associated with a single NV centre in diamond, and exploited this readout to project up to three nearby nuclear spin qubits onto a well-defined state. Conversely, we could distinguish the state of the nuclear spins in a single shot by mapping it onto and subsequently measuring the electronic spin. Finally, we showed the compatibility with qubit control by demonstrating initialisation, coherent manipulation, and single-shot readout in a single experiment on a two-qubit register, using techniques suitable for extension to larger registers. This work is due to be published in Nature [3].
Top left: bright single photon source based on NV centre with nano-fabricated solid immersion lens [3]. Lower left: Optical Rabi oscillation, proving high-fidelity control of the NV centre's optical transition [1]. Right: high-fidelity single shot readout of the NV centre's electronic spin [3].
Conclusions
By combining high-fidelity initialisation, single-shot projective readout and manipulation of solid state spins in a scalable architecture, we demonstrated that NV centres in diamond are excellent candidates for the implementation of large-scale quantum information protocols.
Quantum Information Processing (QIP) is a key future technology: it offers superior communication security, improved processing speed and even the ability to solve classes of problems not accessible to current (classical) information technology. Most of the proposed schemes are based on quantum entanglement. For scalable systems, in particular long distance entanglement is a valuable resource. Such entanglement has just recently been demonstrated between two distant atoms, based on the interference of photons that were entangled with the atom spin. Solid-state systems receive large attention because they offer potential for better scalability than atom experiments and promise highly integrated devices. For quantum bits defined in the solid state however, experiments aiming at long-distance entanglement are still in its infancy.
Goals
The EQESD project aims at demonstrating the suitability of Nitrogen Vacancy centres in diamond for the implementation of measurement-based QIP schemes, by combining advanced methods from atomic quantum optics with solid state spin manipulation techniques. The feasibility of creating entangled states of remote spins via quantum interference is studied.
The nitrogen-vacancy (NV) centre in diamonds is an exceptional solid-state system with very attractive properties for these tasks, combining a very long spin coherence time and a strong optical transition that allows spin-photon conversion.
Summary of results
In this work, we developed bright NV centre devices and sophisticated measurement techniques allowing us to demonstrate key elements for QIP in the solid state. The main results are summarised below:
- A crucial ingredient for creation of spin-photon entanglement is the coherent preparation into an optically excited state. We demonstrated this ability by driving optical Rabi oscillations between the orbital states of the NV centre. Furthermore, we demonstrated a technique to achieve high-fidelity of the coherent control even for spectrally unstable emitters, such as the NV centres. This work resulted in a publication in Physical Review Letters [1].
- NV centres have already successfully been used in the fields of quantum information processing, benefitting from optically induced spin polarisation and optical spin readout. However, an understanding of the optically induced spin dynamics of the NV centre was still incomplete. We studied these spin dynamics in detail, using ps-pulsed optical excitation and microwave spin manipulation. This research resulted in publication [2].
- We demonstrated high-fidelity projective readout of the electronic spin associated with a single NV centre in diamond, and exploited this readout to project up to three nearby nuclear spin qubits onto a well-defined state. Conversely, we could distinguish the state of the nuclear spins in a single shot by mapping it onto and subsequently measuring the electronic spin. Finally, we showed the compatibility with qubit control by demonstrating initialisation, coherent manipulation, and single-shot readout in a single experiment on a two-qubit register, using techniques suitable for extension to larger registers. This work is due to be published in Nature [3].
Top left: bright single photon source based on NV centre with nano-fabricated solid immersion lens [3]. Lower left: Optical Rabi oscillation, proving high-fidelity control of the NV centre's optical transition [1]. Right: high-fidelity single shot readout of the NV centre's electronic spin [3].
Conclusions
By combining high-fidelity initialisation, single-shot projective readout and manipulation of solid state spins in a scalable architecture, we demonstrated that NV centres in diamond are excellent candidates for the implementation of large-scale quantum information protocols.