What are the current solutions to generate entangled qubits in the solid state?

Question

Pairs of entangled qubits (or Bell pairs, or EPR pairs) are a fundamental resource for quantum computing, in the sense that any computational platform that cannot generate entanglement will also be unable to provide a computational advantage.[1] In two recent questions, Decoherence of spin-entangled triplet-pair states in the solid state: local vs delocalized vibrations and Entanglement transfer of spin-entangled triplet-pair states between flying qubits and stationary qubits, I asked about a physical scenario with the goal of generating entangled qubits pairs in the solid state. I know of this result of 2013, Heralded entanglement between solid-state qubits separated by 3 meters, which used photons and NV-centers in diamond, so this can be achieved in practice. Hoewever, I am not up-to-date on what is currently the best option.

My question is: What is the current technological status for the generation of entangled qubit pairs in the solid state? In particular, which options are currently fastest and/or most reliable?

[1] Thanks to Niel de Beaudrap who pointed that out in a comment.

Answer 1

Superconducting qubits

There are several possible approaches. In the most popular scheme the qubits are always coupled, but off resonant and thus the energy conservation prevents the exchange of excitation. With external magnetic flux the qubits can be tuned temporarily into a resonance. The qubits will pick up a phase, which will depend on the state of the other qubit.

For first works see Strauch2003 or DiCarlo2009.

These days cPhase gates are done routinely < 300 ns and fidelity >99%. Barends2016, Salathe2015, McKay2016.

As superconducting qubits work in microwave regime and are thus operated at cryogenic temperature it is not straight forward to do a two qubit gate between distant qubits. A gate between qubits on different chips in the same cryostat has been demonstrated (eg. Kurpiers2017a), but in order to conduct a gate between different cryostats either a microwave-to-optical and optical-to-microwave converter is needed or alternatively a low loss cryogenic microwave link needs to be built.

Quantum dots

There are different degrees of freedom in multi-quantum-dot systems which can be used as a qubti. Thus there are different exact mechanisms. Typically either direct exchange interaction or tunable perturbation of a non-computational state is used.

Single-quantum-dot spin qubits: Watson2018

Double-quantum-dot spin qubits: Nichol2017

Between a spin and a photon: Mi2018