Bedside tables that detect vital body signs,  bathroom sinks that double as mini-labs, flights that are always on time, weather predictions that never go wrong – all science fiction scenes that could actually be possibilities, thanks to quantum computers.

So, what are quantum computers?

 Quantum computers are computational devices that use quantum mechanics for data manipulation. Quantum particles like atoms, electrons, ions, photons can be used to perform data operations. These quantum particles are called qubits, and are the basic building blocks of a quantum computer.

Quantum computers are different from the classical transistor based computers that are in use now. A transistor based computer uses the bits 0 or 1, (on or off) registers. At a given instant, a bit can either be 0 or 1, on or off, depending on the presence or absence of an electric pulse.

 A qubit, on the other hand, can exist as 0 or 1, or more importantly, in any superposition of the two (that is, it can be 0 and 1 at the same time, with a complex numerical coefficient representing the probability for each state, at the atomic level.) Superposition sounds implausible, but is an actual fact, as demonstrated by Thomas Young’s double slit experiment.

Superposition 101 – Thomas Young’s experiment

Superposition can be explained by Thomas Young’s experiment on wave interference.

A beam of light is aimed at a barrier with two slits, to hit a photographic plate behind the barrier. If one of the slits is closed, the light, as expected, passes through the open slit, and makes a single vertical line of light on the photographic plate, directly behind the open slit. So, if both the slits are open, it would make sense to expect two vertical lines of lights, one behind each slit. However, what results is a series of multiple lines of darkness and light(fringes). This unusual phenomenon is explained by the theory that happens the light waves passing through both the slits interfere with each other, even though they are parallel to each other.

Suppose the wave is drastically slowed down so that individual photons pass through the slits one after another. In this case, we would expect no interference, as each photon could be expected to go through one slit or the other, and ultimately end up in one of the two light lines. But amazingly, the resulting pattern still remains the same, which means that, somehow, the single particles are interfering with themselves. Though this sounds impossible to picturize, each photon goes through both the slits simultaneously, (that is, it takes every possible route to the target), not just in theory, but in fact.

Similarly, superposition works for qubits also. It is practically possible for a qubit to be in all possible states simultaneously. It can be 0 or 1, or 0 and 1 at the same time.

The exponential power of quantum computers

The superpositional behavior of qubits exponentially increases the processing power of the quantum computer. A transistor based 3-bit register can be in one of these 8 states - 000, 001, 010, and so on up to 111. But a 3 bit quantum register can be in all the 8 states at the same time. So if an N-bit transistor register can be in one of the 2^N states, a quantum computer can be in all the 2^N states at the same time. As the number of bits in the register increase, the processing power of the quantum computer increases exponentially. Which boils down to this – a transistor-based classical computer can perform calculations one at a time. Quantum computers can perform many calculations at the same time – like many computers working in parallel.

This is especially useful in big calculations. A calculation like factoring (splitting into factors) of a very large 300 digit prime number, would take a supercomputer billions of years, but a quantum computer could arrive at the answer in short time. This could be of exceptional use in cryptanalysis, where it would be possible to break encryptions.  Quantum computers are useful in situations where:

·         There are n number of solutions to a problem (and n is a very large number)

·         Each solution has the same probability

·         The only way to hit on the right solution would be to try out each one individually.

An example for this would be the brute force password attack algorithms, which would require checking every possible combination of characters. A quantum computer would be able to solve it in seconds, making it very useful in communications.

A quantum computer works by applying a series of quantum logic gates to a sequence of qubits, just like a traditional computer functions by applying Boolean logic gates to bits. These quantum logic gates act as control devices to make the qubits perform specific actions, and can vary depending on what type of particle that particular qubit is (i.e. – ion, photons or  electrons ). These control devices can be

·         electric or magnetic fields that act as ion traps to capture ions,

·         light waves as optical traps to control photons

·         Semiconductor materials to manipulate electrons

·         Superconductor materials to allow electrons to flow at very low resistance levels

The measurement problem, and its solution - Entanglement

One major problem in quantum computing is its inflexibility to measurements. In Thomas Young’s experiment, if attempts are made to study the individual photon in order to determine its path, the interference pattern changes from the alternating bands pattern to the more expected two vertical lines pattern. Apparently, any attempt to measure the photon alters the superposition. Similarly, any attempt to measure qubits in order to figure out if it’s going to be a 0 or 1 apparently bumps the subatomic particles, causing them to assume either 0 or 1 (which effectively changes a quantum computer to a classical one). This makes the idea of sustainable quantum computers quite unfeasible.

In order to make practical quantum computers, it is necessary to be able to make measurements indirectly, so as not to alter the qubit’s state. This can be achieved through entanglement.

Entanglement is a phenomenon explained by Einstein as follows. Applying outside force to a pair of atoms can cause them to be tangled with each other. This inextricable link persists even at arbitrary distances.

If left alone, an atom will spin in all possible directions. If any attempt to measure the spin is made, the atom is disturbed, causing it to choose one spin, or one value. At the same time, the second atom would choose the opposite spin or value, even if it is at a distance from the first one. This can allow indirect measurement of qubits without actually disturbing them.

Practical quantum computers – sci-fi or reality?

So much for the theoretical knowledge of quantum computers. But how feasible are they?

In spite of recent advances, quantum computers remain a far cry from a practically functioning one (which is just as well, as a quantum computer can rip through any encryption in existence today, thereby making any information on the internet unsafe). But there are huge developments being made. In 2000, an IBM team developed a 5 qubit computer from 5 fluorine atoms, which could perform complex mathematical problems. In 2007, a Canadian startup company, D-Wave claimed that it had developed a 16 qubit computer that could solve pattern matching problems like Sudoku. In 2010, a team of Australian scientists have developed an error correcting algorithm that allows the correction of a particular type of error, where the remaining qubits in a set can reconstruct missing information if some qubits in the sequence are lost.

According to quantum engineer Seth Lloyd, “"a quantum computer is to a computer what laser is to a light bulb”. The laser does not replace a light bulb, but it has its own particular use. Similarly, quantum computers would not possibly replace transistor based computers – they’d have their own special uses.

But once quantum computer become practically feasible, the possibilities are limitless. Quantum computers can be amazingly small, as the physical restriction of placing transistors on a silicon chip is overcome. They could be amazingly fast, as they work on subatomic levels, with inherent parallelism. They might not require electricity. Scientists predict quantum computers have the potential to take over lifestyles – they could enable smart cars that steer themselves, embed themselves in revolving doors at airports that could perform security scans in a second. They could even be worn as headbands that would ultrasonically communicate with your brain, feeding it feeding it information from the cyber world.

All these seem to be stuff that science fiction is made on. But electricity was science fiction stuff two hundred years ago. Commercial flights were scoffed at a hundred years ago. Computers for everyday use were a dim possibility fifty years ago. Internet was an unheard and unheralded phenomenon 25 years ago. Youtube and Facebook were unknown ten years ago. But all these are taken as a matter of course now.

And so perhaps somewhere down the future, Iphones and Blackberries would seem as passé as the enormous vacuum tube filled UNIVAC computer seems to us now. For we’d all be wearing headbands that selectively filters our emails and sends them directly to our brains. Who knows?