Setting the stage for quantum revolution
Introduction to Quantum Dr. Divyansh Shrimali
Theory and Technologies CDAC Bangalore
Why Quantum? Break from Classical Intuition
❖ Classical physics assumes determinism
and continuous trajectories
❖ But early 20th century experiments
shattered this worldview
❖ QM introduces discreteness, probability
and limits of measurement itself
❖ Not just “smaller-scale physics” - it’s
fundamentally new paradigm
Transition from Classical to Quantum
❖ Blackbody radiation (planck): energy
quantization → E = nhν
❖ Photoelectric effect (Einstein): light as
photons → particles aspect of waves
❖ Atomic spectra (Bohr) : discrete orbits →
quantized energy levels
❖ Wave mechanics (de Boglie, Schroedinger):
matter as waves
Fundamental shift in Philosophy
❖ Determinism → Probabilism
1900 — Planck →
❖ Continuity → Quantization
1905 — Einstein→
❖ Objectivity → Observer - dependence
❖ Locality → Non-Locality 1913 — Bohr →
❖ These transitions make quantum theory both
1926 — Schrödinger
revolutionary and conceptually unsettling
Wave Particle Duality
❖ Light matter exhibit both wave and particle
behaviour
❖ Electrons diffract through slits →
interference pattern builds up particle-by-
particle
❖ Meaning: probability waves are real and
measurable
Uncertainty Principle
❖ Proposed by Heisenberg (1927): ΔxΔp ≥ ℏ/2
❖ Not a measurement aw - nature itself forbids simultaneous precision
❖ Encodes complementarity between conjugate variables (position-momentum, energy-time)
❖ Philosophically: nature limits knowledge, not instruments
fl
Superposition : Heart of Quantum Mechanics
❖ A quantum system can exist in multiple
states simultaneously until measured
❖ Schroedinger’s cat illustrates macroscopic
paradox in superposition
❖ Mathematically : | ψ⟩ = α | 0⟩ + β | 1⟩
❖ Measurement collapses this onto one
outcome probability
Entanglement : Correlations beyond Classical Realms
❖ Einstein called it “spooky action at a
distance”
❖ Two particles share a single quantum state:
measurement on one instantly determines
the other
❖ Bell’s inequality later con rmed that
quantum predictions violate classical
locality assumptions
❖ Foundation of quantum communication
and teleportation
fi
Measurement and Role of Observation
❖ Measurement is not passive - it changes the system
❖ Collapse of wave function = transition from quantum possibility → classical
actuality
❖ Raises interpretational question:
❖ Copenhagen: “reality is created upon measurement”
❖ Many-worlds: “all outcomes occur, we inhabit one branch”
Classical vs Quantum Mechanics: Theoretical Comparison
Concept Classical Quantum
State description De nite (position, momentum) Probabilistic wavefunction
Evolution Deterministic (Newton) Unitary (Schroedinger)
Measurement Non-intrusive Collapse occurs
Correlations Local Non-local (entanglement)
Computation Binary logic Amplitude interference
fi
Quantum systems: Where quantum physics rules
❖ Electrons: quantum wave-packets
orbiting nuclei
❖ Photons: quantum of electromagnetic
radiation with polarization degrees of
freedom
❖ Atoms: discrete energy transitions
explain spectra
❖ Macroscopic quantum systems:
superconductors, BECs
Concept of Quantization
❖ Energy and angular momentum take discrete
values
❖ Transition from continuous classical energy →
quantized levels
❖ Observed in atomic emission spectra,
superconducting qubits, quantum dots
Quantum states and measurement
❖ A state represents all possible measurement
outcomes and their probabilities
❖ Measurements extracts limited information - each
observable has its own basis
❖ The collapse process ties the abstract math to
experimental results
Why Quantum? Scientific and Strategic Drivers
❖ Quantum technologies promise exponential
advantages in computation and sensing
❖ National security: quantum
communication and post quantum
cryptography
❖ Scienti c signi cance: probing limits of
measurement and information
❖ Economic implications: quantum industry
expected to exceed $100 B globally by 2040
fi
fi
Need of Another Paradigm?
Moore’s law is slowing down! Classical scaling has
limitations.
* Many real-world problems grow exponentially
in complexity
For example: simulating large molecules is
infeasible classically
Too complex for classical
computers to handle!
So looking for new paradigm is not just for sake of curiosity but is of necessity!
The Quantum Technology Trend
❖ Quantum Computing: leveraging superposition and entanglement for faster
optimization, simulation
❖ Quantum Communication: exploit no-cloning theorem for unhackable
transmission
❖ Quantum Sensing and Metrology: exploit coherence for ultra-sensitive
measurements
Quantum Computing Overview
❖ Qubits as information carriers; operations as unitary transformations
❖ Quantum parallelism: compute on all superposed states simultaneously
❖ Key algorithms: Shor (factoring), Grover (search), QAOA (optimization)
❖ Current stage: NISQ (noisy intermediate-scale quantum) devices, 50-200 qubits
Quantum Communication
❖ Uses entanglement to distribute secure
keys (QKD: Quantum key distribution)
❖ Quantum teleportation: transmitting
quantum states without physical particle
movement
❖ Satellites now enable intercontinental
quantum links (China’s Micius satellite is
an example)
Quantum Sensing and Metrology
❖ Leverages quantum coherence and entanglement
for ultraprecise measurements
❖ Examples: atomic clocks, SQUID magnetometers,
quantum gravimeters
❖ Quantum sensors already outperform classical
counterparts in speci c domains
fi
Global Quantum Missions
❖ USA: Quantum Initiative Act (2018), NIST
+ DOE + NSF collaboration
❖ EU: Quantum Flagship (1B Euro initiative)
❖ China: Quantum supremacy experiments
and satellite QKD
❖ Others: Japan, Canada, Australia - strong
research ecosystems
India’s National Quantum Mission
❖ Launched 2023-2024; ₹ 6000 cr over 8 years
❖ Four thematic hubs (QComputing,
QCommunications, QMaterials, QSensing)
❖ Goals: build 50-100 qubit hardware,
indigenous simulators, hybrid HPC-QC
integration
❖ CDAC, IISC, IITs and RRI key participants
Strategic Significance of Quantum Technologies
❖ Quantum advantage may rede ne secure
communication, AI acceleration and
logistics
❖ Nations see it as dual-use
(civilian+defense) technology frontier
❖ Ethical and governance implications: post
quantum cryptography and equitable
access
fi
Summary
❖ Quantum theory arose from classical breakdowns → new worldview
❖ Core concepts: superposition, entanglement, uncertainty, measurement
❖ Technologies leverage these principles in computing, sensing and
communication
❖ Global race to harness quantum advantage is already underway
