Zero Knowledge Rollups, or ZK-Rollups, have generated a lot of optimism that the blockchain industry can finally solve its scalability challenges and deliver seamless interoperability between networks, unifying liquidity in the crypto ecosystem.

Ethereum co-founder Vitalik Buterin, one of the most respected names in the crypto industry, has hailed ZK-Rollups as the "key scalability solution" for the world's most widely used smart contract blockchain. So what is all the excitement about? 

ZK-Rollups are a Layer-2 scaling solution that sits above Layer-1 networks such as Ethereum. They're used to roll up cryptocurrency transactions into bigger batches that can be executed "off-chain." Essentially, they enable hundreds of transactions to be bundled into just one, substantially easing the load on the underlying L1 network. 

The way it works is that the ZK-Rollup operator, known as a "sequencer," simply posts a summary of all the transactions that were bundled together onto the main chain. These summaries deliver the end result together with validity proofs that can demonstrate the correctness of the changes made. 

In other words, ZK-Rollups can be seen as a kind of execution layer that operates independently of the main L1 network, which becomes the final settlement layer. 

A Superior Scaling Solution

ZK-Rollups are an alternative to the popular Optimistic Rollups that have emerged as the main L2 scaling solution on Ethereum and other blockchains. Optimistic Rollups differ from ZK Rollups in some distinct ways, utilizing a fraud-resistant mechanism that enables any network user to dispute the outcome of a batched transaction within a specified timeframe. That's different from ZK-Rollups, which use validity proofs to ensure transactions can be processed in real-time. 

With ZK-Rollups, the sequencer cryptographically verifies the correctness of state changes, whereas Optimistic Rollups simply assume the transactions are valid unless their accuracy is challenged. 

This assumption is a significant disadvantage for Optimistic Rollups, as it means they have to provide a time window during which users are free to challenge the validity of any transaction within a batch once it has been submitted to the L1 base layer. This challenge period is meant to give other users time to check that the transactions within a rollup are valid, and it means transactions cannot be verified until they expire. 

For most Optimistic Rollups, the challenge period extends to seven days, during which time users are unable to withdraw their funds to the base layer network. To circumvent this, some pioneering liquidity providers offer a way for users to access their funds early by paying a small premium, similar to a withdrawal fee. 

zkEVMs: The Next Generation Ff ZK-Rollups

So far, Optimistic Rollups have become the favored L2 solution simply because they're easier to implement. For instance, two of Ethereum's most recognizable L2 scaling networks—Arbitrum and Optimism—both use Optimistic Rollups. 

They use these because many feel that ZK-Rollups are not yet ready for prime time. Although they have more desirable properties, the earliest ZK-Rollups did not support the Ethereum Virtual Machine, which makes it difficult for them to execute smart contracts. That's a big limitation because smart contracts are crucial to Ethereum's DeFi, GameFi, and NFT ecosystems. 

However, the industry has made some rapid advances and appears close to solving this challenge with the emergence of a number of projects building zkEVM L2s, which dramatically simplify the deployment of ZK-Rollups. 

Representing the next generation of ZK-Rollup technology, zkEVMs are implementations of EVM that are customized to adhere to the specifications of ZK-Rollup networks. This allows them to support both ZK-proofs and smart contract execution, giving ZK-Rollups the ability to process the most complex transactions that occur on the underlying L1. 

A number of interesting zkEVMs have emerged, with some choosing to prioritize performance and others favoring compatibility with EVM, and yet others trying to create the perfect balance between the two. Let's take a look at some of the most promising implementations. 

Prom ZkEVM

Prom ZkEVM has created what it calls a "modular" zkEVM L2 that not only supports Ethereum and EVM-compatible networks, but also non-EVM blockchains. The goal is to enable interoperability across every blockchain network while simultaneously accelerating the performance of ZK-Rollups. It builds bridges between ecosystems by submitting its proof of transaction to both the chosen settlement chain and the additional chain it transacts with. 

Powered by the Polygon network, Prom ZkEVM utilizes recursive STARK technology to create zkSNARKS that anonymize transactions, reduce costs, and provide instant finality. In this way, it provides users with an extremely high-performance yet economical scaling solution. Because of the way it seamlessly integrates with Ethereum and other chains, it minimizes the implementation overhead for developers while taking advantage of Ethereum's secure base layer to facilitate secure, immutable transactions. 

Although it's one of the newest zkEVMs, Prom ZkEVM has already proven to be hugely successful, with more than 100,000 active wallets established within two weeks of its testnet launch and a number of high-profile projects building on its platform. Just one week from its testnet launch, it powered more than 235,000 transactions, and it has continued to run flawlessly since. 

Besides its enhanced interoperability to execute transactions seamlessly across the blockchain ecosystem, Prom ZkEVM also features novel account abstraction features that aim to simplify crypto transactions for users and make decentralized applications more accessible. 

Polygon zkEVM

Polygon is perhaps the most prominent flag bearer of zkEVM technology, having acquired what was once one of the most promising implementations of ZK-Rollups, a project called Hermez. Polygon rebranded that project as Polygon zkEVM, and is now seeking to enhance its design with performance customizations. 

The project sacrifices a little of its EVM compatibility to enhance the performance of its rollups, although it's also working on some innovative techniques to improve its EVM equivalence. To improve performance, it utilizes a Proof of Efficiency (PoE) consensus algorithm that's both efficient and resistant to malicious attacks. It has also created a proprietary fast ZK-proof generation capability that relies on recursive STARKs to ensure it can scale. 

To boost EVM compatibility, Polygon zkEVM offers support for opcodes, partial support for pre-compiled smart contracts, and implementation of zk-counters. Polygon says these additions have little to no impact on the developer's experience while improving its ability to scale. What's more, Polygon zkEVM is focused on streamlining ease-of-use, with support for the Web3.js and Ethers.js libraries, as well as gas optimization features and seamless smart contract deployment. 

Another key element of Polygon zkEVM is its zkEVM Bridge, which connects Polygon directly to the Ethereum mainnet and any L2 that sits upon it. 

Scroll Network

As opposed to Polygon zkEVM, which is designed with performance in mind, Scroll Network is more focused on full EVM equivalency. To achieve this, it's built atop one of the most popular Ethereum clients, called GETH, which ensures it is fully compatible with any Ethereum-based dApp. However, the price paid for this enhanced compatibility is that it comes at the expense of rapid prover times. 

To counter this, Scroll has introduced a cutting-edge technology for ZK-proof generation called Halo 2, which is a Plonk-based system for generating scalable proofs that was first designed by the team behind Zcash. 

Scroll's major advantage is the progress it has made thus far, currently in phase three of its planned rollout. It recently launched its Alpha Testnet on Ethereum's public testnet Goerli, the final step before phase four and the rollout of its Mainnet. 

With zkEVM, ZK-Rollups Are Ready for Prime Time

Although they're still seen as being more complex than Optimistic Rollups, ZK-Rollups remain by far the most promising technology for scaling blockchain networks thanks to their superior performance. Not only do they overcome the limitations of Optimistic Rollups, but they also retain the censorship resistance and security of the underlying L1 base layer. 

ZK-Rollups were initially very complicated to deploy, but the recent rise of mature zkEVM L2s such as Prom means that's no longer the case. These days, developers can quickly create and deploy customized, ZK-based computing environments to scale their dApps without any compromise in terms of performance or security. 

Until recently, Web3 developers have always had to think very carefully about the underlying infrastructure that supports their dApps, making trade-offs in terms of performance and useability. But with the rapid advances of zkEVMs, those choices are becoming much easier to make.