Quantum Computing

- Prepared by Vishal Garg
- CSE 5th Semester
- www.linkedin.com/in/vishalgarg4

What is Quantum Computing?

- Quantum computing is the computer technology

based on the principles of quantum theory, which

explains the nature and behaviour of energy and

matter on the quantum (atomic and subatomic)

level. - Quantum computing is essentially harnessing and

exploiting the amazing laws of quantum mechanics

to process information.

What Is Quantum?

- A classical binary bit is always in one of two

states0 or 1while a quantum bit or qubit exists

in both of its possible states at once, a

condition known as a superposition. - An operation on a qubit thus exploits its quantum

weirdness by allowing many computations to be

performed in parallel. - A two-qubit system would perform the operation

on 4 values, a three-qubit system on 8 and so

forth.

What Is Quantum?

What Is Quantum?

- Rather than performing each calculation in turn

on the current single state of its bits, as a

classical computer does, a quantum computer's

sequence of qubits can be in every possible

combination of 1s and 0s at once. - This allows the computer to test every possible

solution simultaneously and to perform certain

complex calculations exponentially faster than a

classical computer. - One curious feature of a qubit is that measuring

it causes it to "collapse" into a single

classical known state0 or 1 againand lose its

quantum properties.

What Is Quantum?

- Many quantum algorithms are non-deterministic

they find many different solutions in parallel,

only one of which can be measured, so they

provide the correct solution with only a certain

known probability. - Running the calculation several times will

increase the chances of finding the correct

answer but also may reduce quantum computing's

speed advantage.

History of Quantum Computing

- Quantum computing tends to trace its roots back

to a 1959 speech by Richard P. Feynman in which

he spoke about the idea of exploiting quantum

effects to create more powerful computers. - This speech is also generally considered the

starting point of nanotechnology. - In 1984, David Deutsch was at a computation

theory conference and began to wonder about the

possibility of designing a computer that was

based exclusively on quantum rules, then

published his breakthrough paper a few months

later. - With this, the race began to exploit his idea.

History of Quantum Computing

- In 1994, ATT's Peter Shor devised an algorithm

that could use only 6 qubits to perform some

basic factorizations ... more cubits the more

complex the numbers requiring factorization

became, of course. - The first, a 2-qubit quantum computer in 1998,

could perform trivial calculations before losing

decoherence after a few nanoseconds. - In 2000, teams successfully built both a 4-qubit

and a 7-qubit quantum computer. - Research on the subject is still very active,

although some physicists and engineers express

concerns over the difficulties involved in

upscaling these experiments to full-scale

computing systems.

How a Quantum Computer Would Work?

- A quantum computer, would store information as

either a 1, 0, or a quantum superposition of the

two states. Such a "quantum bit," called a qubit,

allows for far greater flexibility than the

binary system. - Specifically, a quantum computer would be able to

perform calculations on a far greater order of

magnitude than traditional computers ... a

concept which has serious concerns and

applications in the realm of cryptography

encryption. - Some fear that a successful practical quantum

computer would devastate the world's financial

system by ripping through their computer security

encryptions, which are based on factoring large

numbers that literally cannot be cracked by

traditional computers within the life span of the

universe. - A quantum computer, on the other hand, could

factor the numbers in a reasonable period of time.

How a Quantum Computer Would Work?

- If the qubit is in a superposition of the 1 state

and the 0 state, and it performed an calculation

with another qubit in the same superposition,

then one calculation actually obtains 4 results

a 1/1 result, a 1/0 result, a 0/1 result, and a

0/0 result. - This is a result of the mathematics applied to a

quantum system when in a state of decoherence,

which lasts while it is in a superposition of

states until it collapses down into one state. - The ability of a quantum computer to perform

multiple computations simultaneously (or in

parallel, in computer terms) is called quantum

parallelism).

How a Quantum Computer Would Work?

- if there are n qubits in the supercomputer, then

it will have 2n different states. - So experimentally, it can hold more information

as compared to regular digital bits thereby

increasing the speed of the system exponentially. - The qubits are dynamic and are only the

probabilistic superposition of all of their

states. - So, the accurate measurement is difficult and

requires complex algorithms such as Shors

algorithm

Superposition and Entanglement?

- Superposition is essentially the ability of a

quantum system to be in multiple states at the

same time that is, something can be here and

there, or up and down at the same time. - Entanglement is an extremely strong correlation

that exists between quantum particles so

strong, in fact, that two or more quantum

particles can be inextricably linked in perfect

unison, even if separated by great distances. - The particles are so intrinsically connected,

they can be said to dance in instantaneous,

perfect unison, even when placed at opposite ends

of the universe. - This seemingly impossible connection inspired

Einstein to describe entanglement as spooky

action at a distance.

What can a quantum computer do that a classical

computer cant?

- Factoring large numbers, for starters.
- Multiplying two large numbers is easy for any

computer. - But calculating the factors of a very large (say,

500-digit) number, on the other hand, is

considered impossible for any classical computer. - In 1994, a mathematician from the Massachusetts

Institute of Technology (MIT) Peter Shor, who was

working at ATT at the time, unveiled that if a

fully working quantum computer was available, it

could factor large numbers easily.

I dont want to factor very large numbers

- In fact, the difficulty of factoring big numbers

is the basis for much of our present day

cryptography. - RSA encryption, the method used to encrypt your

credit card number when youre shopping online,

relies completely on the factoring problem. - Since factoring is very hard, no eavesdropper

will be able to access your credit card number

and your bank account is safe. - Unless, that is, somebody has built a quantum

computer and is running Peter Shor's algorithm!

So a quantum computer will be able to hack into

my private data?

- Dont worry classical cryptography is not

completely jeopardized. - This is where quantum mechanics comes in very

handy once again - Quantum Key Distribution (QKD) allows for

the distribution of completely random keys at

a distance.

How can quantum mechanics create these

ultra-secret keys?

- Quantum key distribution relies on another

interesting property of quantum mechanics any

attempt to observe or measure a quantum system

will disturb it. - Photons have a unique measurable property called

polarization.

What is required to build a quantum computer?

- We need qubits that behave the way we want them

to. - These qubits could be made of photons, atoms,

electrons, molecules or perhaps something else. - Scientists at IQC are researching a large array

of them as potential bases for quantum computers.

- But qubits are notoriously tricky to manipulate,

since any disturbance causes them to fall out of

their quantum state (or decohere).

What is required to build a quantum computer?

- The field of quantum error correction examines

how to stave off decoherence and combat other

errors. - Every day, researchers at IQC and around the

world are discovering new ways to make qubits

cooperate.

Challenges To Quantum Computing

- Decoherence
- One of the biggest challenges is to remove

quantum decoherence. - Decoherence in a laymans language could be

understood as the loss of information to the

environment. The decoherence of the qubits occurs

when the system interacts with the surrounding in

a thermodynamically irreversible manner. - So, the system needs to be carefully isolated.

Freezing the qubits is one of the ways to prevent

decoherence.

Challenges To Quantum Computing

- 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.

Challenges To Quantum Computing

- 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.

Challenges To Quantum Computing

- 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 2500 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.

If Its So Complex, Why Is Everyone After Quantum

Computing?

- A fully functional quantum computer would require

around a million atoms. And right now, we are at

a mere thousand. - But, what would happen if we reach that limit or

even its half? - Genome sequencing or Tracking weather patterns
- Second, the modern day encryption systems are

entirely based on the limitations of the regular

computers. - Quantum computing wont be of changing your lives

in day to day operations, but a quantum

communication network would definitely provide a

better and secure network.

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