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Quantum Computing


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Title: Quantum Computing

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)
  • 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

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
  • 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
  • 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
  • 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

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
  • So, the accurate measurement is difficult and
    requires complex algorithms such as Shors

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
  • 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
  • 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

What is required to build a quantum computer?
  • We need qubits that behave the way we want them
  • 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
  • Every day, researchers at IQC and around the
    world are discovering new ways to make qubits

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

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
  • 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
  • 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
  • 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
  • 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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