Demonstration of entanglement-by-measurement of solid state qubits (1206.2031v1)

Abstract: Projective measurements are a powerful tool for manipulating quantum states. In particular, a set of qubits can be entangled by measurement of a joint property such as qubit parity. These joint measurements do not require a direct interaction between qubits and therefore provide a unique resource for quantum information processing with well-isolated qubits. Numerous schemes for entanglement-by-measurement of solid-state qubits have been proposed, but the demanding experimental requirements have so far hindered implementations. Here we realize a two-qubit parity measurement on nuclear spins in diamond by exploiting the electron spin of a nitrogen-vacancy center as readout ancilla. The measurement enables us to project the initially uncorrelated nuclear spins into maximally entangled states. By combining this entanglement with high-fidelity single-shot readout we demonstrate the first violation of Bells inequality with solid-state spins. These results open the door to a new class of experiments in which projective measurements are used to create, protect and manipulate entanglement between solid-state qubits.

Entanglement-by-Measurement of Solid-State Qubits

The paper "Demonstration of entanglement-by-measurement of solid state qubits" presents a significant advancement in the field of quantum information science by achieving entanglement of nuclear spins through projective measurements, without the need for direct interactions between qubits. This method utilizes projective measurements to manipulate quantum states, specifically through qubit parity measurements, to create maximally entangled states essential for quantum error correction and deterministic two-qubit gates.

The authors successfully demonstrate a heralded qubit parity measurement on nuclear spins within diamond using the electron spin of a nitrogen-vacancy (NV) center as a readout ancilla. This technique allows the projection of initially uncorrelated nuclear spins into maximally entangled states and represents the first violation of Bell's inequality with solid-state spins. The experiment capitalizes on recent advances in single-shot electron spin readout and demonstrates the robustness of nuclear spin qubits, which are isolated from the environment and exhibit long dephasing times.

The paper outlines an experimental approach involving the initialization of the ancilla in a specific state, followed by controlled operations based on qubit states, and concludes with ancilla readout to project the nuclear spins. A key challenge addressed is ensuring the non-destructive nature of the parity measurement, preserving the phase of entangled states during ancilla readout. This is achieved with high post-readout fidelity, albeit with a lower success probability due to conditioning on photon detection.

Notably, the implementation of the parity measurement enables the direct projection into Bell states, confirmed through correlation measurements and quantum state tomography. This method demonstrates the universal applicability of entanglement-by-measurement, extending its potential to other hybrid systems like phosphorous donors in silicon and facilitating scalable quantum network creation through coupling with remote entanglement channels.

The strong numerical results of this experiment include a mean fidelity of 90% for the Bell state ${\Phi^+}$ and a violation of the CHSH inequality across all four Bell states by more than four standard deviations. These outcomes firmly establish the efficacy of measurement-induced entanglement in solid-state systems and herald its use in deterministic quantum protocols.

This work represents a pivotal step toward solid-state quantum computing by laying the groundwork for quantum error correction and measurement-based controlled NOT gates due to its innovative use of projective measurements in generating and stabilizing entangled quantum states. Future research and development could focus on enhancing ancilla readout efficiency and further integrating this methodology into broader quantum computing architectures. Overall, the paper provides a robust framework for advancing quantum computation through entanglement manipulation by measurement.