Above: a circuit diagram of a transistor (of the npn type). Normally no current will flow between the collector (c) and
the emitter (e) unless a small current is applied to the base, opening the gate and allowing the main current to pass
from c to e (electron flow from e to c).
Robots need advanced computer systems to carry out the vast array of computations needed to make a robot
perform useful work and conduct itself in a responsible and sensible way. Processing signals from thousands of
sensors and controlling the fine movements of dozens of actuators in real time is a demanding job, and that's without
the higher processing needed, say, to hold a meaningful conversation about philosophy, which is sufficiently adept
to have the potential to generate novel ideas!
The principle type of processor currently in use on Earth is the silicon chip - a solid-state semi-conductor based CPU
in which circuits have been etched or deposited in nanometre detail on a crystal of silicon. The elementary
component of these circuits is the transistor. Transistors have three connection points or 'leads'. One of these, the
base, is used to control the flow of current between the other two: from the collector, c, to the emitter, e,
remembering that electric current is defined to be positive by definition and so opposite to the flow of electrons. In
this way a transistor basically functions as an electronic valve controlling the flow of charge: a small current
applied to the base will open the gate and allow a much larger current to flow from c to e. (See Electro Fogey's
practical electronics lectures on Youtube for an excellent explanation of how transistors work: What is a transistor?)
Silicon-based computers operate using binary logic - a signal in a circuit either flows above a certain threshold or it
doesn't. We indicate this using binary digits or bits. A bit can have only one of two possible values: 1 if current flows
(ON) and 0 if it does not (OFF). See binary operations for examples of how computing makes use of binary logic.
How do transistors make intelligent circuits?
Computers, and robot brains, operating in binary require several elementary operations, which as we shall see allow
more complex computations to take place. These elementary circuits are: NOT, AND, OR and XOR gates.
The NOT gate simply inverts its input, which consists of a binary signal represented by 1 if current
flows, 0 if current does not flow. If current flows into the base of the transistor, from A, then the
gate between c and e is open and current will be diverted to flow from c to e (driven by voltage
source V) reducing current flow to the output (depending on the resistance in the 'out' circuit, with
a resistance much higher than the transistor current flow to out will be small). (In this case this
current is simply drained away to earth by a 'sink'). This is output 0. If, however, no current flows
into A then the gate will not open and all the current will flow to out, giving an output of 1. In other
words, when the input is high the output is low, and when the input is low the output is high. The
NOT gate is a signal inverter.
An AND gate requires two inputs, A and B, and two transistors gated by these inputs. In this case,
current will flow from V to Out if both gates are open, that is when a current flows in BOTH a AND B.
A NAND gate is a 'NOT AND' gate and is the logical representation of an AND gate followed by a
NOT gate and its output is the inverse of an AND gate. In this case, high current will only flow to
OUT when either gate A and/or gate B is closed, that is when there is input in neither A nor B or no
input in both A and B.
An OR gate will send a high signal to Out if there is an input in either A OR B or in both A and B.
This is an inclusive OR, since it includes the situation when BOTH A and B = 1 (i.e. have input
A NOR gate is the logical combination of an OR gate followed by a NOT gate and has the inverted
output of an OR gate. Input from either A OR B, or inputs from both A AND B will divert current away
from out to the sink.
Above: an alternative NOR gate utilising two transistors.
The XOR gate is an alternative type of OR gate which excludes the possibility of both inputs =
1. In other words, the output is 1 if input A OR input B = 1 but not if A = B = 1.
The half-adder circuit will add to bits as follows:
0 + 0 = 0
1 + 0 = 1
0 + 1 = 1
1 + 1 = 0 carry 1
If A=B=1 then the circuit overflows as we need a more significant bit. (E.g. in binary 1 = 2 in
decimal, so 1 + 1 = 1 + 1 in decimal = 2 = 10 in binary). The carry bit, C, carries 1 over to any
subsequent computations, so for a full adder we need an additional input for a carry bit:
In the full adder, A and B represent the bits being added and input C represents the carry bit
coming into the computation, which will be zero unless a carry is left over from a previous
computation. The first XOR and AND gates operate on inputs A and B; The second XOR
operates on C and the output of the first XOR to determine the result of the addition (e.g. 1 + 0
with no carry = 1) whilst the second AND and the second OR gates determine whether or not a
carry remains. E.g. A = 1 + B = 1 + C = 0 gives S = 0, C = 1; whilst: A = 1 + B = 1 + C =1 gives
S = 1 and C = 1.
With binary addition we can also carry out other elementary binary manipulations, such as
subtraction, multiplication and division. For example, to multiply a number by 3, we simply add
that number to itself twice.
Silicon chip design has achieved a remarkable degree of development by miniaturising the
transistor circuits, which also means that they can compute more rapidly and with a lower power
consumption. However, there comes a point when the walls between adjacent charge pathways
or channels ('wires') is so small (on a near atomic scale) that charge randomly leaks from one
channel to the next by quantum tunneling (that is the electron simply jumps or teleports from
one channel to a neighbouring channel).
Of course, we can compensate by making silicon chips larger, or by having multi-core
processors consisting of several silicon chip CPUs (central processing units). This does
complicate software development considerably, however, and programming techniques to
optimise the use of multicore processors are still being developed and are still catching on due
to the necessary learning curve for software developers. Adding in 8 processors instead of 4
does not simply double computation power in of itself! Rather, the software must be able to
make optimum use of the extra processor cores by efficiently dividing work amongst them. This
gives rise to true parallel processing in which more than one task can be computed
side-by-side in real time. Processors with single cores can emulate parallel processing when
more than one program is running by rapidly jumping from one program to another, but actually
only one of the programs is ever being executed at any instant of time. This is the basis of
multi-threading, in which each thread represents one line of code execution and the CPU
rapidly jumps between threads.
Other ways to enhance computational power may be to use DNA computers or quantum
computers, which are able to perform calculations in parallel even on a single core. A quantum
computer uses qubits rather than bits. A qubit (quantum bit) can have the value of 1 or 0, just
like a bit, but it can also have the value of 0 and 1 simultaneously (or neither value depending
how you look at it)!
A qubit can be represented physically by a spin-half particle, such as a silver atom or an
electron, which can have either spin-up (= 1 say) or spin-down (= 0). However, quantum
superposition allows the particle to be in a combined coherent state where it is both spin-up
and spin-down until we actually measure the direction of its spin in which case it's state will
collapse and then it will be either spin-up or spin-down (a strong measurement, see quantum
measurement). This kind of superposition is possible since particles like atoms are also waves
and waves can be superimposed (added on top of one-another) to generate a new wave. In
principle this enables faster parallel computations since we now have more than just two
possible states for each bit whilst calculations are underway, though the final result is still
binary: spin-up or spin-down.
The state of our particle is given by a wavefunction, represented by the Greek letter psi which is
a mixture of up and down spins. For example, we might have twice as much up character as
down character. Since the square of the wavefunction gives us the probability of finding the
particle in a spin-up state or a spin-down state following a measurement, and the sum of the
probabilities must equal 1, we have:
The basis represents the possible pure states, spin-up and spin-down in this case. A quantum
NOT gate must invert the spins: converting spin-up character into spin-down character and
vice-versa. Certain measurements or operations carried out on the particle can achieve this
state inversion. In quantum mechanics spin states are represented by vectors and operations
(measurements) by matrices. For example, a quantum NOT gate can be represented by the
matrix U operating on a spin state by multiplying it to give a new spin state thus:
Things get more interesting when we have two qubits. The two wavefunctions, one for each
qubit, will be coherent and entangled, that is connected if they overlap significantly (an
example of superposition of waves again). Thus, they behave like a single mixed state:
Since the two qubits are entangled, we can change one intentionally, called the control qubit,
and the other, the target qubit, will change in some determined way as a result of a
computation. For example, in a quantum OR gate, the second qubit can be flipped (inverted) if
the first qubit = 1 and left unchanged otherwise.
One potential problem with quantum systems is that entanglement and coherence is that they
tend to be short-lived since thermal fluctuations (thermal noise) can collapse the system and
change the values of the qubits unpredictably. This can be minimised by cooling the quantum
computer to very low temperatures, or by stabilising molecules in the vicinity to prevent thermal
collisions. Interestingly, the latter appears to occur in living cells and has led to speculation that
living cells may be quantum computers. What is essential is some form of error checking and
We can of course measure the spin directions, but we wish to do so in some way which does
not cause the system to collapse into a definite spin-up or spin-down state (we want to
preserve the coefficients alpha and beta which indicate the proportions of 'upness' and
'downness'). we can do this by using three qubits to represent a spin state:
A collective measurement on the first two qubits can determine whether or not they are pointing
in the same direction, without perturbing the system by measuring each individual system
(maintaining the entanglement). If the directions of the first two qubits differ then an error has
occurred. We can then similarly compare the last two spins. We can do each pair-comparison
using two successive NOT (!) gates using an addition 'maid' qubit to help with the calculation. In
this operation we flip (NOT, !) the maid qubit if the comparative qubit in our triple is pointing
upwards and leave it unchanged otherwise. If an error has occurred and the two spins differ
then the maid qubit will be flipped once as follows:
If the two qubits being tested are aligned then the maid qubit gets flipped either twice or not at
all, and so remains unchanged:
Having a powerful computational mind is one thing, but would our robot with its multi-quantum
core processor ever be conscious? Could we ever know? Before we even attempt to answer
such a question, we need to define what consciousness is, if we can.