QUANTUM COMPUTERS

  COFFEE BEAN TO COMPUTERS – IN SEARCH OF                 

QUANTUM COMPUTERS

 B.OBULIRAJ

B.E CSE

ABSTRACT

Strange as it sounds, the computer of tomorrow could be built around a cup of coffee. The caffeine molecule is just one of the possible building blocks of a 'quantum computer', a new type of computer that promises to provide mind boggling performance that can break secret codes in a matter of seconds.

SMALLER. . . smaller . . . smaller. In the semiconductor industry, this mantra translates to faster . . . faster . . . faster. The question is, how small can you go? Every so often a new technology surfaces that enables the bounds of computer performance to be pushed further forwards. From the introduction of valve technology through to the continuing development of VLSI designs, the pace of technological advancement has remained relentless. Lately, the key to improving computer performance has been the reduction of size in the transistors used in modern processors . This continual reduction however, cannot continue for much longer. If the transistors become much smaller, the strange effects of quantum mechanics will begin to hinder their performance. It would therefore seem that these effects present a fundamental limit to our computer technology, or do they? And the logical next step will be to create quantum computers, which will harness the power of atoms . In this paper, you'll learn what a quantum computer is and just what it'll be used for in the next era of computing

1.INTRODUCTION :

Classical  VS Quantum Computers:

Classical computing relies, at its ultimate level, on principles expressed by Boolean algebra, operating with a (usually) 7-mode logic gate principle, though it is possible to exist with only three modes (which are AND, NOT, and COPY). Data must be processed in an exclusive binary state at any point in time - that is, either 0 (off / false) or 1 (on / true). These values are binary digits, or bits. The millions of transistors and capacitors at the heart of computers can only be in one state at any point. While the time that the each transistor or capacitor need be either in 0 or 1 before switching states is now measurable in billionths of a second, there is still a limit as to how quickly these devices can be made to switch state

      As we progress to smaller and faster circuits, we begin to reach the physical limits of materials and the threshold for classical laws of physics to apply. Beyond this, the quantum world takes over, which opens a potential as great as the challenges that are presented.

The Quantum computer, by contrast, can work with a two-mode logic gate: XOR and a mode we'll call QO1 (the ability to change 0 into a superposition of 0 and 1, a logic gate which cannot exist in classical computing). In a quantum computer, a number of elemental particles such as electrons or photons can be used (in practice, success has also been achieved with ions), with either their charge or polarization acting as a representation of 0 and/or 1. Each of these particles is known as a quantum bit, or qubit, the nature and behavior of these particles form the basis of quantum computing. The two most relevant aspects of quantum physics are the principles of superposition and entanglement .

1.1 HISTORY

·         In 1982, the Nobel prize-winning physicist Richard Feynman thought up the idea of a 'quantum computer', a computer that uses the effects of quantum mechanics to its advantage .

·         David Deutsch, of Oxford, publishes a theoretical paper describing a universal quantum computer,

·         Shor’s Algorithm Peter Shor, working for AT&T, proposes a method using entanglement of qubits and superposition to find the prime factors of an integer

·         The National Institute of Standards and Technology and the California Institute of Technology jointly contemplate the problem of shielding a quantum system from environmental influences

·         The algorithm run through the quantum computer was  devised by Lov Grover of Bell Laboratories. 

2.1 QUANTUM COMPUTER BASICS:

Qubit:

In the classical model of a computer, the most fundamental building block, the bit, can only exist in one of two distinct states, a 0 or a 1. In a quantum computer the rules are changed. Not only can a 'quantum bit', usually referred to as a 'qubit', exist in the classical 0 and 1 states, it can also be in a coherent superposition of both. When a qubit is in this state it can be thought of as existing in two universes, as a 0 in one universe and as a 1 in the other.

 An operation on such a qubit effectively acts on both values at the same time. The significant point being that by performing the single operation on the qubit, we have performed the operation on two different values.

Likewise, a two-qubit system would perform the operation on 4 values, and a three-qubit system on eight. Increasing the number of qubits therefore exponentially increases the 'quantum parallelism' we can obtain with the system. With the correct type of algorithm it is possible to use this parallelism to solve certain problems in a fraction of the time taken by a classical computer.

SUPERPOSITION

Think of a qubit as an electron in a magnetic field. The electron's spin may be either in alignment with the field, which is known as a spin-up state, or opposite to the field, which is known as a spin-down state. Changing the electron's spin from one state to another is achieved by using a pulse of energy, such as from a laser - let's say that we use 1 unit of laser energy. But what if we only use half a unit of laser energy and completely isolate the particle from all external influences? According to quantum law, the particle then enters a superposition of states, in which it behaves as if it were in both states simultaneously. Each qubit utilized could take a superposition of both 0 and 1. Thus, the number of computations that a quantum computer could undertake is 2^n, where n is the number of qubits used. A quantum computer comprised of 500 qubits would have a potential to do 2^500 calculations in a single step. This is an awesome number - 2^500 is infinitely more atoms than there are in the known universe (this is true parallel processing - classical computers today, even so called parallel processors, still only truly do one thing at a time: there are just two or more of them doing it). But how will these particles interact with each other? They would do so via quantum entanglement.

ENTANGLEMENT

Particles (such as photons, electrons, or qubits) that have interacted at some point retain a type of connection and can be entangled with each other in pairs, in a process known as correlation . Knowing the spin state of one entangled particle - up or down - allows one to know that the spin of its mate is in the opposite direction. Even more amazing is the knowledge that, due to the phenomenon of superpostition, the measured particle has no single spin direction before being measured, but is simultaneously in both a spin-up and spin-down state. The spin state of the particle being measured is decided at the time of measurement and communicated to the correlated particle, which simultaneously assumes the opposite spin direction to that of the measured particle. This is a real phenomenon (Einstein called it "spooky action at a distance"), the mechanism of which cannot, as yet, be explained by any theory - it simply must be taken as given.

Quantum entanglement allows qubits that are separated by incredible distances to interact with each other instantaneously (not limited to the speed of light). No matter how great the distance between the correlated particles, they will remain entangled as long as they are isolated. Taken together, quantum superposition and entanglement create an enormously enhanced computing power. Where a 2-bit register in an ordinary computer can store only one of four binary configurations (00, 01, 10, or 11) at any given time, a 2-qubit register in a quantum computer can store all four numbers simultaneously, because each qubit represents two values. If more qubits are added, the increased capacity is expanded exponentially.

QUANTUM PROGRAMMING:

Perhaps even more intriguing than the sheer power of quantum computing is the ability that it offers to write programs in a completely new way. For example, a quantum computer could incorporate a programming sequence that would be along the lines of "take all the superpositions of all the prior computations" - something which is meaningless with a classical computer - which would permit extremely fast ways of solving certain mathematical problems, such as factorization of large numbers, one example of which we discuss below.

There have been two notable successes thus far with quantum programming. The first occurred in 1994 by Peter Shor, (now at AT&T Labs) who developed a quantum algorithm that could efficiently factorize large numbers. It centers on a system that uses number theory to estimate the periodicity of a large number sequence. The other major breakthrough happened with Lov Grover of Bell Labs in 1996, with a very fast algorithm that is proven to be the fastest possible for searching through unstructured databases. The algorithm is so efficient that it requires only, on average, roughly N square root (where N is the total number of elements) searches to find the desired result, as opposed to a search in classical computing, which on average needs N/2 searches.

WHY IS IT DIFFICULT TO BUILD A QUANTUM COMPUTER?

As the number of quantum gates in a network increases, we quickly run into some serious practical problems. The more interacting qubits are involved, the harder it tends to be to engineer the interaction that would display the quantum properties. The more components there are, the more likely it is that quantum information will spread outside the quantum computer and be lost into the environment, thus spoiling the computation. This process is called decoherence. Thus our task is to engineer sub-microscopic systems in which qubits affect each other but not the environment.

. Some of the problems with quantum computing are as follows:

• Interference - During the computation phase of a quantum calculation, the slightest disturbance in a quantum system (say a stray photon or wave of EM radiation) causes the quantum computation to collapse, a process known as de-coherence. A quantum computer must be totally isolated from all external interference during the computation phase. Some success has been achieved with the use of qubits in intense magnetic fields, with the use of ions.
• Error correction - Because truly isolating a quantum system has proven so difficult, error correction systems for quantum computations have been developed. Qubits are not digital bits of data, thus they cannot use conventional (and very effective) error correction, such as the triple redundant method. Given the nature of quantum computing, error correction is ultra critical - even a single error in a calculation can cause the validity of the entire computation to collapse. There has been considerable progress in this area, with an error correction algorithm developed that utilizes 9 qubits (1 computational and 8 correctional). More recently, there was a breakthrough by IBM that makes do with a total of 5 qubits (1 computational and 4 correctional).
• Output observance - Closely related to the above two, retrieving output data after a quantum calculation is complete risks corrupting the data. In an example of a quantum computer with 500 qubits, we have a 1 in 2^500 chance of observing the right output if we quantify the output. Thus, what is needed is a method to ensure that, as soon as all calculations are made and the act of observation takes place, the observed value will correspond to the correct answer. How can this be done? It has been achieved by Grover with his database search algorithm, that relies on the special "wave" shape of the probability curve inherent in quantum computers, that ensures, once all calculations are done, the act of measurement will see the quantum state decohere into the correct answer.

3. HOW TO BUILD A QUANTUM COMPUTER?

A quantum computer is nothing like a classical computer in design; you can't for instance build one from transistors and diodes. In order to build one, a new type of technology is needed, a technology that enables 'qubits' to exist as coherent superpositions of 0 and 1 states. The best method of achieving this goal is still unknown, but many methods are being experimented with and are proving to have varying degrees of success.

3.1 QUANTUM DOTS

An example of an implementation of the qubit is the 'quantum dot' which is basically a single electron trapped inside a cage of atoms .When the dot is exposed to a pulse of laser light of precisely the right wavelength and duration, the electron is raised to an excited state: a second burst of laser light causes the electron to fall back to its ground state. The ground and excited states of the electron can be thought of as the 0 and 1 states of the qubit and the application of the laser light can be regarded as a controlled NOT function as it knocks the qubit from 0 to 1 or from ' to 0.

If the pulse of laser light is only half the duration of that required for the NOT function, the electron is placed in a superposition of both ground and excited states simultaneously, this being the equivalent of the coherent state of the qubit. More complex logic functions can be modelled using quantum dots arranged in pairs. It would therefore seem that quantum dots are a suitable candidate for building a quantum computer. Unfortunately there are a number of practical problems that are preventing this from happening:

• The electron only remains in its excited state for about a microsecond before it falls to the ground state. Bearing in mind that the required duration of each laser pulse is around 1 nanosecond, there is a limit to the number of computational steps that can be made before information is lost.
• Constructing quantum dots is a very difficult process because they are so small. A typical quantum dot measures just 10 atoms (1 nanometer) across. The technology needed to build a computer from these dots doesn't yet exist.
• To avoid cramming thousands of lasers into a tiny space, quantum dots could be manufactured so that they respond to different frequencies of light. A laser that could reliably retune itself would thus selectively target different groups of quantum dots with different frequencies of light. This again, is another technology that doesn't yet exist.

3.2 COMPUTING LIQUIDS

Quantum dots are not the only implementation of qubits that have been experimented with. Other techniques have attempted to use individual atoms or the polarisation of laser light as the information medium. The common problem with these techniques is decoherence. Attempts at shielding the experiments from their surroundings, by for instance cooling them to within a thousandth of a degree of absolute zero, have proven to have had limited success at reducing the effects of this problem.

The latest development in quantum computing takes a radical new approach. It drops the assumption that the quantum medium has to be tiny and isolated from its surroundings and instead uses a sea of molecules to store the information. When held in a magnetic field, each nucleus within a molecule spins in a certain direction, which can be used to describe its state; spinning upwards can signify a 1 and spinning down, a 0. Nuclear Magnetic Resonance (NMR) techniques can be used to detect these spin states and bursts of specific radio waves can flip the nuclei from spinning up (1) to spinning down (0) and vice-versa.

The quantum computer in this technique is the molecule itself and its qubits are the nuclei within the molecule. This technique does not however use a single molecule to perform the computations; it instead uses a whole 'mug' of liquid molecules. The advantage of this is that even though the molecules of the liquid bump into one another, the spin states of the nuclei within each molecule remain unchanged. Decoherence is still a problem, but the time before the decoherence sets in is much longer than in any other technique so far. Researchers believe a few thousand primitive logic operations should be possible within time it takes the qubits to decohere.

3.3 TRAPPED  ION :

Trapped ions are among the most attractive systems for scalable quantum information because they can be well isolated from the environment and manipulated easily with lasers.

The current record for qubit manipulation is held by practitioners of the ion-trap technique, which uses oscillating electric fields to hold atomic ions in place, like eggs in a box, chilled to within a few degrees of absolute zero. Information is encoded in each ion's energy state.

 Whereas computing in a conventional processor is done by switching transistor currents on and off, the qubits are manipulated by firing a carefully designed laser pulse at the ions to put them into a particular superposition state.. Taking two ions, they used a series of carefully calibrated laser pulses to perform some simple computations with them and read out the results. They could also move the ions around the processor without losing the information encoded on them, and repeat the process. In other words, their system does everything that a basic conventional computer should do (The team has also managed some, but not yet all, of these feats with arrays of eight or nine trapped ions. There should now be no problem, in principle, with scaling things up to the hundreds or even thousands of qubits that would be necessary to make a useful computer - it is just a question of acquiring more practice in the art of qubit manipulation. "It's hard to imagine that we won't someday be able to control these things to create a useful devic

Over the past couple of years, however, the tried-and-tested ion-trap approach has acquired a competitor. It takes the form of dots of aluminium about one-third of a millimetre across, each of which contains billions of atoms. When chilled to extremely low temperatures, the momentum of these atoms is severely reduced. According to the quantum-mechanical uncertainty principle, the more restricted an atom's momentum is, the more smeared out is its location in space, and it becomes impossible to tell where one atom ends and the next one starts. The result is a dot that behaves as if it were one giant superatom within which electrons flow freely, encountering no electrical resistance.

3.4 FUTURE STEP :

 Scientists have made the world's smallest diamond ring, which could play a role in the future of computing.

At just 5 micrometers across and 300 nanometers thick, the ring is unlikely to fit on anyone's finger, say the Australian researchers who made it. the tiny loop will let them manipulate single photons, the smallest 'packet' of light.

They hope the ring, which was carved from a slither of diamond, will help researchers build powerful computers that use the properties of quantum physics.

"For quantum information processing, diamonds have some truly unique possibilities,"

That's because they offer an ideal way to produce qubits, the quantum equivalent of the "bits" that store information on standard computers. Like normal bits, qubits can have two different values, either 0 or 1. But unlike their standard counterparts, qubits can also exist in a "superposition" of both states at once.

It turns out that tiny impurities in diamonds meet this criterion, and all the other requirements of qubits, extremely well,The diamond offers a fantastic platform in order to make qubits because diamond offers us a gift from nature," he said.

That gift comes about when a single nitrogen atom and a tiny gap disrupt the normal carbon structure of a diamond. Scientists call these nitrogen-vacancy centers, and by shining laser light onto one, researchers can produce single photons of red light in ways that are easy to manipulate and measure.

They can also do this at room temperature, something most other quantum systems can't do. The researchers have already used these properties of diamonds in the field of quantum cryptography, which aims to allow secure information to be sent and received using the properties of quantum physics.

 4. CONCLUSION:

If a quantum computer can be built, says Dr. Austing, "it would not be a universal panacea."

 Even though there are many problems to overcome, the breakthroughs in the last 15 years, and especially in the last 3, have made some form of practical quantum computing not unfeasible, but there is much debate as to whether this is less than a decade away or a hundred years into the future. However, the potential that this technology offers is attracting tremendous interest from both the government and the private sector. The year is 2015. Computers are fast - really fast. But there's a supercharged black box that puts the whole microchip drag race to shame. No one now knows what it'll be called, but this much is certain: The letter Q will be right up front. Q stands for quantum, and it just may replace e and i as the tech prefix of choice. Don't hold out for a qMac anytime soon, but even in its embryonic state, the quantum computer is already turning heads. Indeed enigmatic features of consciousness have already led to proposals for quantum computation in the brain.

But all these takes time…………….

5. REFERENCES:

Cirasella, J. 2008a. "Historical bibliography of quantum computing." Appendix A. In: Yanofsky, N.S. & Mannucci, M.A. Quantum Computing for Computer Scientists. Cambridge: Cambridge UP. p. 319-324. [Online]. Available: http://userhome.brooklyn.cuny.edu/cirasella/Pubs/QChistory.pdf [Accessed February 1, 2009].

Cirasella, J. 2008b. "Keeping abreast of quantum news: Quantum computing on the Web and in the literature." Appendix D. In: Yanofsky, N.S. & Mannucci, M.A. Quantum Computing for Computer Scientists. Cambridge: Cambridge UP. p. 357-359. [Online].

Nielsen, M.A. & Chuang, I.L. 2000. Quantum computation and quantum information. Cambridge: Cambridge UP.