🖥 Classical vs ⚛ Quantum Computers

1. What is Silicon (Si)?

Silicon is the foundation of modern electronics.

• It is the second most common element on Earth (after oxygen).
• Found mostly in sand and rocks as silicon dioxide (SiO₂).
• Extracted and purified into silicon wafers → used to build transistors and chips.

In nature, it comes in different forms (isotopes):

• Si-28 (92%)
• Si-29 (5%)
• Si-30 (3%)

• Normal chips use all of them.
• Quantum research sometimes uses only Si-28 (the quiet one).

2. Transistors: Tiny Switches

• A transistor is a very tiny switch.
• It controls electricity:
• Billions of transistors are packed onto a chip.

🟦 1. Nature of Silicon

• Pure silicon is a semiconductor → it doesn’t conduct electricity well on its own.
• But if we add tiny amounts of other elements (doping), we can make it either:

🟦 2. The Transistor Structure

• A transistor uses both n-type and p-type silicon.
• In the middle is a channel where electrons might flow.
• A gate (a tiny metal plate above the channel) controls this flow.

🟦 3. Switching Action

• When you apply a voltage to the gate, it changes the electric field in the silicon.
• This field can attract or repel electrons:

So the electric field in silicon controls conductivity, just like flipping a switch.

🟦 4. Why Silicon Specifically?

• Abundant (second most common element on Earth).
• Stable → forms a strong oxide layer (great for insulation).
• Cheap & reliable → perfect for mass production of billions of switches.

How many transistors do real chips have?

• Apple M1 (2020): ~16 billion
• Apple M2 (2022): ~20 billion
• Apple M3 (2023): ~25 billion
• Intel Alder Lake (2021): ~22 billion
• Intel Raptor Lake (2022): ~24–25 billion
• Intel Meteor Lake (2023): ~40–50 billion

3. Quantum Computers (Qubits)

Quantum computers work in a totally new way. They use qubits instead of bits.

• Bit (classical): 0 or 1.
• Qubit (quantum): 0 and 1 at the same time.

👉 Analogy:

• Classical coin: heads (0) or tails (1).
• Quantum coin: spinning in the air = both heads and tails until you catch it.

Superposition & Parallel Power

• 1 qubit = both 0 and 1.
• 2 qubits = 00, 01, 10, 11 (all at once).
• n qubits = 2ⁿ states at once.

Example:

• 10 qubits = 1,024 states at once.
• 50 qubits = ~1 quadrillion states at once!

👉 This is why quantum computers can test many answers at the same time.

Materials for Qubits

Quantum computers are not made only of silicon. They use special compounds:

• Superconductors (aluminum, niobium): for superconducting qubits.
• Silicon carbide (SiC): for spin qubits.
• Diamond: with defects called NV centers.
• Photonics (GaAs, InP): using light particles.
• Trapped ions (Yb, Ca): single atoms as qubits.

👉 Sometimes, pure Si-28 is used because it keeps qubits quiet and stable.

🔹 1. Atoms & electrons in classical computers

• In classical semiconductors (like silicon), electrons sit in energy bands.
• They either flow (current → 1) or they don’t (no current → 0).
• This ON/OFF comes from the energy gap (band gap) between the valence band and conduction band.

This is enough for transistors — but only gives binary 0/1.

🔹 2. What’s different in quantum computing?

In quantum computing, we don’t just care if an electron flows. Instead, we use quantum states of particles (like electrons, ions, or photons).

• An electron in an atom can occupy different energy levels (like shells: n=1,2,3).
• Or, in superconductors, electrons form Cooper pairs that can oscillate between states.
• Or, in spin qubits, an electron’s spin can be “up” or “down”.

Because quantum mechanics allows superposition, the electron can be in a combination of states:

∣ψ⟩=α∣state 0⟩+β∣state 1⟩|\psi\rangle = \alpha | \text{state 0} \rangle + \beta | \text{state 1} \rangle∣ψ⟩=α∣state 0⟩+β∣state 1⟩

So chemically/atomically, the qubit is a particle’s wavefunction overlapping multiple states at once.

🔹 3. Why does superposition happen?

This comes directly from the Schrödinger equation of quantum mechanics:

• If two states are possible solutions (say ∣0⟩|0\rangle∣0⟩ and ∣1⟩|1\rangle∣1⟩),
• Then any linear combination of them is also a valid solution.

👉 That’s why an electron’s wavefunction can spread across multiple states at once, instead of being locked in just one.

Example:

• In an atom, an electron’s orbital can be in a superposition of ground state + excited state.
• In superconducting circuits, a microwave photon can excite a qubit into a mix of energy states.
• In a spin qubit, the spin can be in a mixture of “up” and “down”.

🔹 4. Why do we need to freeze?

Here’s where the chemistry/atomic environment matters:

• At room temperature, atoms vibrate strongly (thermal energy).
• These vibrations disturb the delicate quantum states of electrons or spins.
• Collisions with phonons (lattice vibrations), stray photons, or other particles quickly destroy the superposition → called decoherence.

By cooling close to absolute zero (≈ 0.01 K):

• Atoms in the lattice hardly vibrate.
• Electrons and spins stay in well-defined quantum states without being disturbed.
• Superconductivity also kicks in → electrons form stable Cooper pairs with no resistance.

This frozen, ultra-clean environment is necessary for superposition to last long enough to do computation.

• Classical computers: made of silicon transistors → ON/OFF → bits (0 or 1). Chips like Apple M1–M3 and Intel CPUs have 16–50 billion transistors.
• Quantum computers: use qubits → 0 and 1 at the same time → huge parallel power. Built with special materials (SiC, diamonds, photons, ions, superconductors).
• Cooling: Needed because heat destroys quantum states.
• Isotopes: Classical chips use natural silicon. Quantum chips sometimes use pure Si-28.