I am using 50mm potrait lens with my canon 600D. Is there any way to capture low key or high key images without using external flash?
There are many ways of setting up high and low key shots. But for example
Use a dark room. Light your subject with directed light, such as a lamp or light entering through a slightly open door. Flag the light so it lights your subject but doesn't hit the background. You may have to brace the camera or use a tripod, and use a higher ISO to get the correct exposure. The room doesn't have to be super dark, but you need the light source to be strong relative to the ambient light in the room if you want your subject lit while keeping the background relatively dark.
Bright room with light colored walls. Place your subject near a large window (not in direct sun, so north-facing if you're in the northern hemisphere, south facing otherwise). Overexpose if required to suit your tastes.
Low key photography justs asks for a dark place, you can take a room and take all the light away. Then you need a specific light source such as a lamp to shine light onto your model. You can play with this to achieve the result you want. Make sure you have a shutter speed at about 1/250 and you have a 100 or 200 iso.