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**No, AES-256 is *not* fully quantum-resistant**, but it is considered **relatively resistant** compared to other symmetric encryption schemes.

### Here's why:

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### 🔒 **Quantum Impact on Symmetric Encryption (like AES):**

Quantum computers can speed up brute-force attacks using **Grover’s algorithm**, which provides a **quadratic speedup** over classical brute-force:

* **Classical brute-force on AES-256**:
  $2^{256}$ operations.
* **Quantum brute-force with Grover’s algorithm**:
  $2^{128}$ operations — still **infeasible with any foreseeable quantum hardware**.

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### 🔐 AES Key Size and Quantum Resistance:

| AES Key Size | Classical Security | Quantum Security (Grover’s) |
| ------------ | ------------------ | --------------------------- |
| AES-128      | 128 bits           | \~64 bits                   |
| AES-256      | 256 bits           | \~128 bits                  |

So, **AES-256 provides \~128 bits of post-quantum security**, which is currently considered **safe** against quantum adversaries.

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### 🧠 Summary:

* **AES-256 is not quantum-proof**, but it is **quantum-resistant** in the sense that even with quantum computers, breaking it would still require **astronomical effort**.
* For **long-term security**, AES-256 is preferred over AES-128.
* AES will likely **remain useful even in a post-quantum world**, particularly if used in combination with quantum-safe key exchange algorithms.

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If you're concerned about **quantum security** in general, the **main focus** should be on replacing **asymmetric cryptography** (RSA, ECC) with **post-quantum algorithms**, since those are far more vulnerable to quantum attacks (e.g., via Shor’s algorithm).