Quantum Computing : The Holy Grail of Modern Science
Computation (Information processing) is one of the vital needs of modern world. Computers have made the process of computing a lot easier. The evolution of computation began with abacus, then the world moved onto gears, valves, relays, transistors and now, we are in the IC age. In the IC age our computers gets twice faster every 18 months (Moore’s Law).If we are able to keep the pace it is expected that by 2020 we’ll hit the quantum level of miniaturization. In other words we start making circuits in the nano scale (1 - 100nm).
The problem with the nano scale is the fact the classical concepts of physics and techniques of electronics will no longer work! Does it sound like the world of computers is coming to a halt…? Not really, where classical physics fails, Quantum Mechanics survives! That’s why we need a radically new technology to replace the existing classical computing, it is – Quantum Computing
Basics
The building blocks of all classical information processing systems are bits .A bit stores two fixed values namely 0 or 1. Therefore any physical system allowing two distinct states can realize a bit, such as charge of a capacitor or polarization of light.
<=========>
The Quantum analog of a bit is called a Qubit. A Qubit can simultaneously store and manipulate 1 and 0 in the form
α|0>+β|1>
, where α and β are complex numbers.
This phenomenon of the existence of a Qubit in linear superposition of 2 or more states is termed as Quantum Superposition. E.g.: spin of an electron, molecule etc could be used to realize a Qubit.
Now let’s push this idea of superposition a bit forward. Classical physics considers everything around us to be ‘universe’ and it’s considered as ultimate. Quantum physics however doesn’t believe that the universe is ultimate but instead it considers universe as a self contained entity within a collection of universes (called a multiverse). There is an interpretation of quantum mechanics called Many Worlds Interpretation (MWI) which uses the concept of Multiverse to explain the phenomenon of quantum superposition. The weirdest part of MWI theory is that it says that the same events occurring in our environment are getting repeated in several universes. Consider the event that you are reading this paper, and then according to MWI this event of reading is happening in several universes with your counterparts reading the same paper. Now consider the case of a qubit – in some universes it may be at state |0> while in some others it will be in the state |1>.Considering the multiversal character of qubit it is now in both |1> and |0> simultaneously. But the theory prohibits the communication between universes at any level other than Quantum level. That’s why such superposition is not observed in the classical world.
Superposition is a quite sensitive state. Any interaction with environment causes a Qubit to lose its superposition and to acquire state 0 or 1 with equal probability. This phenomenon is termed as decoherence. Decoherence makes the direct measurement of a Qubit virtually impossible as the slightest attempt to measure it leads to decoherence. Quantum Entanglement is a viable solution for this problem. In Entanglement we take two quantum systems and then correlate them in such a way that the measurement of the second system helps us predict the value of the first. This allows us to measure the value of a Qubit by allowing it to remain intact. However a Quantum system can never remain in superposition for eternity. After sometime it naturally undergoes decoherence and the time taken for it is called decoherence time.
Making of a Quantum Computer
In principle to make a Quantum computer we start by making simple gates and then connecting them in networks to form a quantum processor. Quantum gates support several new operations exploiting Quantum features in addition to classical operations.
Eg: Hadamard gate, Universal Phase shift gate and Controlled NOT gate
Physical
realization of a quantum computer is even harder! Out of the several
technologies used the most common ones are NMR
based Quantum computing, Quantum dots, ion traps. In Ion Trap technology Ions
are suspended in space in an oscillating electric field and are controlled by LASER.
Quantum Dot technology however traps electrons in a cage of atoms and
manipulate them with electromagnetic waves.NMR Quantum computing uses the
phenomenon of nuclear magnetic resonance (NMR) to manipulate the nuclear spin
of a molecule with magnetic pulses.
Suppose
if you realized a computer out of the any existing technologies, for it to be a
quantum computer it should satisfy certain conditions listed as the DiVincenzo criteria.Firstly
it should use Quantum gates which are fairly accurate and supports quantum
logic. Furthermore it should make use of quantum phenomena like Quantum
Superposition and Quantum Entanglement.
Scalability
is another criterion where it should be possible to interconnect the Qubits to
increase capacity of a processor with satisfactory low decoherence and Quantum
errors. It should be possible to initialize the Qubits to some initial values
and the system should have sufficient means of accurately reading the output.
Above all, a quantum computer should work faster than decoherence time, i.e. it
should initialize Qubits, run algorithm and read the output before decoherence
occurs. This is the essence of the DiVincenzo criteria proposed by David
DiVincenzo.
Pros and cons
Suppose if you really made a computer which could be called as a ‘Quantum computer’ according to DiVincenzo criteria, then what are we going to gain..?Why a quantum computer is called the holy grail of the modern science?
Cardinal advantage of a quantum computer is its inherent Quantum parallelism. A Qubit in superposition can simultaneously store and manipulate two binary digits in the form α|0> + β|1>. Similarly a two-Qubit register can store all possibilities (00, 01, 10, and 11) simultaneously while a two bit register can store only one of the four possibilities at a time. In general a Quantum register of n Qubits is equivalent to 2n classical registers of n-bits operating in parallel. In other words when a classical computer carries out one computation per step, a quantum computer does 2n calculations per step. Hence Quantum computers get exponentially spacious (more capacity) and faster with increase in Qubits. This explains the massive computational capabilities of a quantum computer.
Qubits are realized by quantum systems and hence switching between states will be faster and the theoretical energy consumption will be very low. Furthermore data transfer using Quantum entanglement is secure and instantaneous.
Quantum algorithms can perform many hard computations efficiently by exploiting quantum parallelism and other superposition related advantages. One of the good examples is the Shor’s integer factorization algorithm released in 1994 which enables a quantum computer to factorize a 5000 digit number in just 2 minutes while a classical computer equipped with the best available classical algorithm requires 5 trillion years to do the same. Grover’s Unstructured Search Algorithm is another example of an efficient Quantum algorithm. Such algorithms can help us to solve many so-called hard problems.
Classical computation does not allow true randomness. Randomness is created with the help of mathematical algorithms which make use of some varying quantity (such as system time) to generate a random pattern these days. This randomness is called pseudo randomness because there always some pattern of occurrence associated with this randomness. However Quantum computational methods allow true randomness which could replace the pseudo randomness used nowadays in cryptography to make unbreakable encryption.
Universe itself works more or less like a Quantum computer. It has been theoretically established that even DNA does so. So there is a belief among researchers that quantum computers can lead us to artificial intelligence or the phenomenon of rational thought. Another key finding is that human brain works like a quantum computer with proteins inside it working like quantum gates. The possibility of creation of CYBORGs (Robots that that think like humans) is an area to be investigated upon.
Molecular simulation is an important use of quantum computers. The simulation did on classical computers have lots of limitations. For instance to simulate caffeine molecule 1021 Schrödinger equations are to be solved simultaneously. Quantum simulation can replace classical simulation in almost all fields. The advanced molecular simulation helps in developing effective drugs which are specific in action. Quantum simulation helps in the study of Quantum mechanics about which, we have very little knowledge.
The scaling we have achieved right now makes a quantum computer a machine good for no practical use. It’s still a fancy laboratory idea. Some people indeed call it a theoreticians dream because no one is sure about the realization of such a device. We need technologies for better isolation and scaling of Qubits. The main problem in achieving better scaling is Decoherence; as the no of Qubits increases, so does decoherence. Besides, the quantum errors due to decoherence are threatening the accuracy of Quantum gates and Quantum computation as a whole. Better error corrections algorithms are to be researched to combat quantum noises like decoherence. A present quantum computer takes a whole laboratory room to occupy. So in order to make a compact device like a PC or Laptop, further advancements in nanotechnology is vital. Programming languages and software’s which works in the quantum platform are yet to be developed.
Quantum computing is a futuristic technology. No researcher can claim that quantum computers are possible or impossible at this stage. Only further researches can help us make a judgment regarding its feasibility. One thing the whole world is sure about is the power that a quantum computer provides to the mankind. Quantum computer to classical computer is like Hydrogen bomb to gun powder. Also it is widely believed that the quantum computer is still decades away. Every day new, new findings are happening in this field. If we are able to get as much as a decoherence time of 1s, in super conductor type Quantum computer, then we can beat decoherence. From the decoherence time of 1 nano seconds we have advanced to 500 nano seconds, which is a great advancement. Even theory is developing with better concepts and algorithms. Optimism might do wonders; so believes the scientific world.
Access the research paper @ http://qcresearch.blogspot.com/ with the password compiler
Comments
