> ## Documentation Index
> Fetch the complete documentation index at: https://docs.quantumapi.io/llms.txt
> Use this file to discover all available pages before exploring further.

# Overview

> What Quantum Chain is, how it differs from Ethereum, and how the post-quantum signature scheme works.

**Quantum Chain** is a public, post-quantum blockchain that runs the **Quantum Virtual Machine (QVM)** — a partially EVM-compatible execution layer hardened against the threat of quantum computers. It is a fork of go-ethereum that replaces ECDSA with a **post-quantum signature scheme** for all transactions and account authorization.

| Property         | Value                                                                                                  |
| ---------------- | ------------------------------------------------------------------------------------------------------ |
| Chain ID         | `20803`                                                                                                |
| Consensus        | Clique Proof-of-Authority                                                                              |
| Block period     | 15 seconds                                                                                             |
| Epoch            | 30 000 blocks                                                                                          |
| Address format   | Quantum Chain address (derived from post-quantum public key)                                           |
| Execution layer  | QVM (Quantum Virtual Machine) — partially EVM-compatible. See [QVM vs EVM](/quantum-chain/qvm-vs-evm). |
| Native currency  | Quantum (symbol `QUANTUM`)                                                                             |
| Reference client | `geth` (this repository)                                                                               |

## Why post-quantum

A sufficiently large quantum computer running Shor's algorithm can derive an ECDSA private key from a public key. That is fatal for blockchains: every spent UTXO or transaction-signed account exposes its public key and becomes spendable by an adversary with quantum capability.

Quantum Chain replaces the signature primitive with a **lattice-based post-quantum signature scheme** that has no known efficient quantum attack. Public keys, signatures, and transaction layouts are all updated; the QVM bytecode and opcode set is unchanged.

```mermaid theme={null}
flowchart LR
    User[Wallet] -->|Post-quantum signature| Tx[Signed transaction]
    Tx --> Mempool[Quantum Chain<br/>mempool]
    Mempool --> Sealer[Clique sealer]
    Sealer --> Block[Block]
    Block --> QVM[QVM execution]
```

## What stays the same

If you have built on Ethereum, almost everything you know carries over:

* Solidity, EVM bytecode, and gas accounting are identical.
* JSON-RPC methods (`eth_*`, `net_*`, `web3_*`, `debug_*`) work the same way.
* Block structure, receipts, logs, and events follow the Ethereum schema.
* Tools such as Hardhat, Foundry, and ethers.js work — they just need a Quantum Chain RPC endpoint and an external signer (since the cryptographic primitives differ).

## What changes

| Area                   | Change                                                                                                             |
| ---------------------- | ------------------------------------------------------------------------------------------------------------------ |
| Account creation       | A new account has a post-quantum keypair, not a secp256k1 keypair.                                                 |
| Transaction signing    | Signatures are produced by the post-quantum scheme and are larger than ECDSA signatures.                           |
| Address derivation     | Addresses are derived from the post-quantum public key, but the 20-byte hex format is unchanged.                   |
| Block validation       | After `quantumVerificationBlock`, full nodes verify post-quantum signatures on every transaction.                  |
| Validator extraData    | The Clique extra-data field carries post-quantum sealer signatures.                                                |
| `ecrecover` precompile | Removed. A new precompile at `0x0b` verifies post-quantum signatures. See [QVM vs EVM](/quantum-chain/qvm-vs-evm). |

## Components

```mermaid theme={null}
flowchart TB
    subgraph "Reference client (geth)"
        node[geth node]
        rpc[JSON-RPC server]
        miner[Clique sealer]
    end

    subgraph "Tooling"
        kg[keygenerator]
        ethkey[ethkey]
        clef[clef signer]
    end

    subgraph "Custody platform"
        qustody[Qustody API]
        signer[Post-quantum signer]
    end

    kg --> keystore[Keystore]
    keystore --> node
    node --> rpc
    rpc --> qustody
    qustody --> signer
    signer --> rpc
```

* [`geth`](/quantum-chain/installation) — Quantum Chain reference node, mining, RPC, archive.
* [`keygenerator`](/quantum-chain/accounts-and-keys) — generate post-quantum keypairs and mnemonic recovery phrases.
* [`ethkey`](/quantum-chain/accounts-and-keys) — manipulate keystore files.
* [`clef`](/quantum-chain/signing-transactions) — external signer for hot wallets.
* [Qustody](/qustody/security-model) — multi-tenant institutional custody platform.

## Where to go next

<CardGroup cols={2}>
  <Card title="Run a node" icon="server" href="/quantum-chain/quickstart-node">
    Build geth and connect to mainnet or a local devnet.
  </Card>

  <Card title="Create accounts" icon="key" href="/quantum-chain/accounts-and-keys">
    Generate post-quantum keypairs and mnemonics.
  </Card>

  <Card title="Send transactions" icon="paper-plane" href="/quantum-chain/transactions">
    Build, sign, and broadcast Quantum Chain transactions.
  </Card>

  <Card title="Operate a network" icon="diagram-project" href="/quantum-chain/networks">
    Genesis files, validators, and PoA operations.
  </Card>
</CardGroup>