Smart contract is not yet implemented in cardano, you can check the progress here cardanoroadmap. IOHK is developing a new programming language called PLUTUS for writing smart contracts.
Didn’t they mention it should be easy to port from Solidity too?
Imagine a smart contract that you can leave all your cryptos to your loved ones…
This article gives you insight about support for Solidity in cardano smart contract.
You will be able to write smart contracts in any K defined languages, not just Plutus. It should be shipped around late 2018, early 2019 with Goguen if all goes well