Optimism is a layer-two blockchain built on top of Ethereum mainnet that uses optimistic rollups as a scaling solution. With over $250 million in TVL at the time of writing, it is one of Ethereum's most significant scaling solutions. Optimistic rollups outsource the execution of transactions to a layer 2 (L2) solution like Optimism, which rolls up transactions into batches and relays them to the Ethereum mainnet, where only the new chain state is recorded. The validity of these rollups is assumed, making them optimistic. However, a rollup's validity can be challenged for up to seven days, which is why withdrawals from Optimism to Ethereum take seven days.

One of the advantages of Optimism's solution is significantly lower gas costs on the L2, and in the long run, a decrease in gas fees on Ethereum. With the Ethereum mainnet recording less transaction data, Optimism helps to unclog the Ethereum network. Additionally, Optimism operates with the same EVM-compatible smart contracts as Ethereum and is secured by the Ethereum mainnet, making it an ideal solution for new DApps to build on.

However, there are some differences between Optimistic rollups and other L2 solutions like ZK-rollups. Optimistic rollups have longer withdrawal periods due to their security model (one week vs minutes), and they are less technologically complex since they do not employ mathematically complex technology. On the other hand, Optimism has higher gas costs than ZK-based layer-twos, and EVM execution on Optimism is simpler than on ZK-chains. Additionally, off-chain computation costs are higher on Optimism.

Across Protocol is a token bridge that connects Layer 2 and Layer 1 scaling solutions for Ethereum, enabling fast, secure, and inexpensive transfers. The bridge approach utilizes bonded relayers, single-sided liquidity pools, and UMA’s Optimistic Oracle to facilitate distributed, instantaneous transfers. The security of Across Protocol is supported by the Optimistic Oracle and tested smart contracts. Overall, Across Protocol is a hybrid bridge solution that incorporates various technologies to provide a decentralized and efficient asset transfer system between Layer 2 and Ethereum Mainnet.

Synapse is a cross-chain communications protocol that connects decentralized applications across different chains. The protocol consists of a messaging architecture and a robust token design to achieve consensus on inter-chain transactions. Synapse has expandable smart contracts that can be implemented on any blockchain, enabling developers to create native cross-chain applications. The protocol has three main components: Generalized Cross-chain Communication, an Optimistic Security Model, and the Synapse Bridge, which is the primary product. Synapse Bridge currently uses the Synapse network and cross-chain automated market maker (AMM). Synapse plans to release a v2 and a Synapse PoS Chain for developers to build cross-chain DEXs, lending platforms, yield aggregators, and more, but the primary bridge still currently runs on an MPC with a few signers.