When can we say that machines are conscious? We have already witnessed complex computing systems winning chess games and IBM Watson beat his human counterparts at a game of 'Jeopardy!'. However, so far we have not seen signs of consciousness amongst these machines.

Is it theoretically possible to create conscious machines? Are there any philosophical bottlenecks that will never allow the development of consciousness in any nonliving carbon-based organism? 

Furthermore, can scientists ever agree on a definition of "consciousness" if a machine claims that it is conscious?
Short answer: We don't know.
Long answer: There are a few major lines of thinking on the subject currently.

Cognitive closure: One common argument is that this question is simply not answerable - at least not by humans.  By this view, it is possible that the creation of an artificial intelligence that even resembles humans sufficiently to suggest consciousness is an intractable engineering problem, or it is possible that understanding consciousness well enough to determine whether or not such an artificially intelligent system might be conscious is beyond us, or it is possible that we will never understand the question well enough to have a meaningful response.

Functionalism: Several interpretations of the implications of monism on consciousness suggest that consciousness is an intrinsic property of whatever it is that our brain does, and therefore, any machine that performs an equivalent function should be equally conscious.  This view potentially allows for a variety of machines to be conscious, as well as having different kinds of consciousness.  In this framework, it may be more meaningful to ask not whether a machine is conscious, but rather, in what way, or how does it compare to our consciousness.

Physicalism: Another interpretation of monism suggests that the function of a machine is not sufficient to determine its consciousness - its physical properties are important as well.  This possibility makes it difficult to ascertain whether a silicon-based computer program of a conscious entity could ever do more than simulate consciousness, or how many other kinds of physical systems like our brain can potentially become conscious.  A theoretical framework under this umbrella that is currently fairly popular and has some empirical support is IIT that suggests that consciousness can be measured in humans using available tools of neuroscience.  How this might be applied to non-human candidates is still unclear.


There are many other possibilities, but hopefully I've captured the most prominent ones.  Currently, there is nothing within our knowledge of physical systems that makes it obvious why (or whether) certain systems might be conscious (and not others).  Put another way: If a race of non-conscious alien robots visited Earth, they might assume that we are non-conscious machines like them - they may have no reason to believe that there is anything more going on.  Similarly, under the assumption of monism, where consciousness appears to somehow supervene on physical systems, if there were other properties aside from consciousness (or different types of consciousness) that supervened on physical systems, then we would have no reason to suspect that they exist either.  So for the moment, we just don't know.