As we stand today, we have come a long way from creating fire, inventing the wheel, the first of the machines, through to
creating computers and supercomputers. And hats off to humanity, it still isn’t at ease and wants to better upon each of its creations.
We have come through the macroscopic laws of Galilean and Newtonian Physics. The year 1905 saw another revolution with Einstein’s Special Theory of Relativity. And today, Quantum Mechanics is redefining the laws at the microscopic level. So much so that we are set for
another radical change with the setting in of quantum computing.

But why do we need quantum computers? While the current computing needs of the world are more or less being met by the conventional computers, the processing power generated presently has not yet been able to quench the thirst for speed and computing capacity. So we need more and more computer power for our advancing technological needs. This not only includes the needs of an average home user or a working executive, but also the scientific research that involves long and complex calculations.
Try to think of computer with a memory, exponentially larger than its apparent physical size; a computer that can manipulate an exponential set of inputs simultaneously; a computer that computes in the twilight zone of Hilbert space (Hilbert space is an inner product space that is complete with respect to the norm defined by the inner product).
If you can, you are already thinking of a quantum computer. Relatively few and simple concepts from quantum mechanics are needed to make quantum computers a real possibility.
While most research and development in quantum computing is theoretical so far, quantum computers could replace silicon chips, similar to the transistors taking place of vacuum tubes.

The first generation of computers occupied a room or two, with the vacuum tubes hollowing the floors of the room and the processor taking the most of the space. Gradually in the year 1947, transistors brought a revolution when they replaced the bulky and cumbersome vacuum tubes. Since then, following Moore’s law, the processing power of the computer has been growing by 100% every 18 months. If the same has to hold true, a few years hence the microprocessors and circuits will have to be designed on the atomic scale.
But there is another side of the coin!
The increasing density
One of the biggest problems with the current trend of miniaturizing conventional computers with each passing day, is the difficulty of handling dissipated heat. This is because the fight for packing all the miniature components that can generate better computational power in increasingly smaller area generates more heat.
In 1961 Landauer studied the physical limitations placed on computation from dissipation. Surprisingly, he was able to show that almost all operations
required in computation could be performed in a reversible manner, thus dissipating no heat! The first condition for any deterministic device to be reversible is that its input and output be uniquely
retrievable from each other. This is called the principle of ‘logical reversibility.’ If, in addition to being logically reversible, a
device can actually run backwards then it is called physically reversible and the
second law of thermodynamics guarantees that it dissipates no heat.On a quantum computer, programs can be executed by unitary evolution of an input that is given by the state of the system. Since all unitary operators are invertible, we can always ‘uncompute’ (reverse the computation) on a quantum computer.
Future prospects
Is such a computer an inevitability or will it be too difficult to build? What are the prospects for quantum computation? What are the difficulties in building a quantum computer, if any?
Researchers around the globe have devised an algorithm that can individually yield an exponential speedup over conventional methodseffectively the calculation of the period of a long
sequence. To date this is the only algorithm displaying such a speedup. This algorithm was applied to a traditional computerscience problem, factoring, only by recognizing a deeper structure within that problem. How difficult will it be to build a quantum computer? Even within apparently small atomicscale systems, quantum computation runs on the enormous size of Hilbert space.
Quantum computation involves building a trajectory from a standard
initial state to a complex final state. But the main difficulty is keeping to this trajectory. To fail is to be lost in Hilbert space. Another big problem is their hypersensitivity to perturbations. This is due to the Quantum Mechanical principle
according to which the nuclear particles change their state even if the light energy falls on them. These perturbations can cause a shift in the computational trajectory randomly from its path. Such perturbations come from an unintentional coupling to external noise.
Due to their capability of performing complex and long calculations easily, quantum computers seem to be a valuable enhancement in solving factoring problems of large numbers. Consequently, they will be very useful in encoding and decoding secret information.
Therefore, quantum cryptography is another emerging area that applies the fundamental laws of Quantum Mechanics to network communications and computing. It concerns the transport and processing of information using individual particles such as electrons or photons. Using single particles in this way brings unexpected advantages. By sending information encoded upon single photons (particles of light) it is possible to test the secrecy of each communication.
The first movers have already begun to exploit this phenomenon to create a practical system for secure communication over fiber optical cables. This secure method of communication called Quantum Cryptography promises to revolutionize the entire IT industry over the next few decades.
Another possibility! Just think of quantum computers being used to search large databases. This would take them a fraction of the time that it would take a conventional computer to do the same task.
What makes them special?
Today’s computers process and execute all data with the use of only two states1 or 0. But a system working on the principals of Quantum Physics has three quantum stateseither 0 or 1, or a superposition of 0 and 1. Meaning thereby, that a quantum machine isn’t limited to two states. They encode information as quantum bits, or qubits. A qubit can be a 1, or a 0, or it can exist in a superposition that is simultaneously both 1 and 0, or somewhere in between.
Therefore, you can consider the qubits to be atoms that work together to act as computer processor and memory. And since quantum processes are governed by Heisenberg’s Uncertainty Principle, a qubit can be present at many positions at any given time. A quantum computer can contain these multiple states simultaneously, it has the potential to be millions of times more powerful than today’s most powerful supercomputers.
This superpositioning ability of qubits will give the quantum computers a property of parallelism. As a result, a quantum computer can work on millions of computations at once unlike the desktop PC that works on one.
Just to give you the idea of their tremendous power, a 30qubit quantum computer would equal the processing power of a conventional computer that could run at 10 teraflops (trillions of floatingpoint operations per second). On the contrary, today’s typical computers run at speeds measured in gigaflops (billions of floatingpoint operations per second).
The quantum computers will also make use of entanglementanother important property of subatomic particles in the quantum space. This means that the atoms as well as the nuclear particles get entangled when an external force is applied and, thus, can take on the properties of the other particle.
Also when a particle is disturbed it chooses a particular spin, and the other in the pair chooses an opposite spin (value). Thus, you can get to know the value of qubits without actually looking at them, which would collapse them back to 1’s or 0’s (again due to the Heisenberg’s Uncertainty Principle).
So an 10bit quantum computer can exist in all 1k (2 raised to power 10) states at a time unlike a digital computer of same strength that can exist in only one of these 1k states. Thus, such a quantum computer can perform 1k calculations (theory of parallelism) at one time which is a mammoth task for a conventional computer, by any standards.
The hurdles
As of today, the laws that fuel the extraordinary power of quantum computing theory, seem to be troubling their birth in the real world. To begin with, qubitsthe lifeline of quantum computers are very difficult to generate. Also as discussed earlier, even a slightest dope of energybe it light, heat, fieldsdisturbs a quantum state and changes the trajectory of qubits. This would reflect a change in information that you decode. Therefore, we need to isolate a quantum system. It appears, though, that there is no fundamental limit to how well we can isolate a quantum system.
There is another problem that was thought be difficult if solved on quantum computers. This was that of error detection and correction. Earlier it was thought that the property of quantum bits to exist in multiple states would lose them this battle. With the efforts of Peter W Shor and others, it has turned out, however, that the quantum errors can be corrected within the computer without the operator ever having to read the erroneous state.
The birth of a practical quantum computer is still years away. Currently, several implementations are being considered by theoreticians and experimentalists worldwide. One promising scheme involves iontraps —the next generation of atomicclock standards. Over the next few decades conventional computers will approach the atomic scale, but perhaps quantum computers will get there first.
Nonetheless, the most satisfying aspect is that we wouldn’t have to fabricate any tiny circuits for quantum computers as the nature has already done the hardest job for us by assembling the basic components in ordinary molecules.
Rinku Tyagi
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