Rotations quaternions vs euler

im trying to rotate a cube like the game bloxorz so im using the local axis, but a want to make a interpolation in quaternion because the euler angle problem but i dont know how to do it.

extends RigidBody
var target_position = Vector3()
var varAlph = 0.0
var basis = Basis()

func _ready():
print($MeshInstance.rotation_degrees)

func _physics_process(delta):

$MeshInstance.rotation = lerp($MeshInstance.rotation, target_position, varAlph)

if varAlph <= 1:
	varAlph += .01

if Input.is_action_just_pressed("ui_left"): 
	$MeshInstance.rotate_object_local(-basis.z, deg2rad(90))
	print($MeshInstance.rotation_degrees)
	varAlph = 0.0
		
	
if Input.is_action_just_pressed("ui_right"):
	$MeshInstance.rotate_object_local(basis.z, deg2rad(90))
	print($MeshInstance.rotation_degrees)
	varAlph = 0.0

if Input.is_action_just_pressed("ui_up"):
	$MeshInstance.rotate_object_local(basis.x, deg2rad(90))
	print($MeshInstance.rotation_degrees)
	varAlph = 0.0
		
if Input.is_action_just_pressed("ui_down"):
	$MeshInstance.rotate_object_local(-basis.x, deg2rad(90))
	print($MeshInstance.rotation_degrees)
	varAlph = 0.0