unity - Moving a dynamically spawned object

I would like to create an object dynamically when the player presses Q and use this object in Instantiate and in transform.Translate.

The problem is that the translation must be outside the if that checks for the Q press, but it applies to the cube that I created inside the if.

using System.Collections;
using System.Collections.Generic;

using UnityEngine;

public class instancier_new_object : MonoBehaviour {

    public float moveSpeed=5f;

    void Update () {
        if (Input.GetKeyDown (KeyCode.Q)) {
            GameObject objet = new GameObject ("objet");            
            GameObject cube = new GameObject ("cube");


            cube = Instantiate(
                       objet,
                       new Vector3(1, 2, 3),
                       Quaternion.identity
            );
        } 

        cube.transform.Translate(
                 Time.deltaTime*moveSpeed,

                 0,
                -Time.deltaTime*moveSpeed,
                 Space.Self
        );
    }
}