Moving cube on x by 1

Hello.

I am pretty sure this was solved somewhere but I failed to find it.

I would like to move a cube if someone presses "d" or "a" on x-axis by one.

 public class Player : MonoBehaviour
 {
     public Rigidbody Rb;
     public float sideFroce = 4000f;
     public Transform player;
     public Vector3 positionMovement;
     void FixedUpdate()
     {
         if (Input.GetKey("d"))
         {
         while (player.position.x < (player.position.x + positionMovement.x))
         {
             Rb.AddForce(sideFroce * Time.deltaTime, 0, 0);
         }
         }
         if (Input.GetKey("a"))
         {
             Rb.AddForce(-sideFroce * Time.deltaTime, 0, 0);
         }
 }

} - Rb.AddForce(-sideFroce * Time.deltaTime, 0, 0); works normaly
I tried one solution I found, just to see what it does

  rb.AddForce(transform.forward * sideForce);

But still unity crashed after pressing "d". I also tried using GetKeyDown instead. Same result.

I am pretty sure I created infinite loop somehow, not sure how to fix it. positionMovement.x is set to 1. And unity has huge lad every time I press "d". I know I could just 'teleport' player (cube) to the position I want but this would look more smooth.
Added later (edit):
my try with partially successful. I think I am getting closer to the solution.

 if (Input.GetKeyDown("d"))
 {
     test = player.position + positionMovement;
     Rb.AddForce(sideFroce * Time.deltaTime, 0, 0);
     if (test.x > player.position.x)
     {
     Rb.AddForce(0, 0, 0);
     }
 }