Quantum computing uses quantum bits (qubits) that can exist in superpositions of states rather than just 1s and 0s. This allows quantum computers to perform exponentially more calculations in parallel than classical computers. Some of the main challenges to building quantum computers are preventing qubit decoherence from environmental interference, developing effective error correction methods, and observing outputs without corrupting data. Quantum computers may one day be able to break current encryption methods and solve optimization problems much faster than classical computers.

1. Quantum Computing
Prepared by Vishal Garg
CSE 5th Semester
www.linkedin.com/in/vishalgarg4

2. 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.

3. What Is “Quantum”?
• A classical binary bit is always in one of two states—0 or 1—while 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.

4. What Is “Quantum”?

5. 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 state—0 or 1 again—and lose its quantum
properties.

6. 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.

7. 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.

8. History of Quantum Computing
• In 1994, AT&T'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.

9. 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.

10. 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).

11. How a Quantum Computer Would Work?
• if there are n qubits in the supercomputer, then it will have 2^n 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 Shor’s algorithm

12. 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.”

13. What can a quantum computer do that a
classical computer can’t?
• 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 AT&T at the time, unveiled that if a
fully working quantum computer was available, it could factor large numbers
easily.

14. I don’t 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
you’re 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!

15. So a quantum computer will be able to
hack into my private data?
• Don’t 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.

16. 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.

17. 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”).

18. 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.

19. Challenges To Quantum Computing
Decoherence
• One of the biggest challenges is to remove quantum decoherence.
• Decoherence in a layman’s 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.

20. 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.

21. 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.

22. 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 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.

23. If It’s 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 won’t 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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