00_qct.md

Quantum Communications

University of Bristol, G.T.Kanellos

QCT.md

Classical vs Quantum Comms

Classical:

• -1900
• macroscopic
• deterministic
• intuitive

Quantum

• 1900-
• microscopic
• probabilistic
• observer = central role
• superposition
• not very intuitive

Fundamental Quantum Properties

• a single photon constitutes an elementary quantum system - cannot be split
• semi transparent mirror
  • in classic physics: light enters, 50% reflected, 50% passes through
  • in quantum physics: particle enters, unknown % reflected or through
• a pure qubit is a coherent superposition of the basic states:
  • |0> outside
  • |1> inside
  • |0> + |1> = quantum/unknown state => |ψ> = a|0> + b|1> where a, b complex numbers so that |a|² + |b|² = 1

|-----------------|-------------------------------------------|
| bits            | qubits                                    |
|-----------------|-------------------------------------------|
| b_0             | c_0 |0> + c_1 |1>                         |
| b_0b_1          | c_0 |00> + c_1 |01> + c_2 |10> + c_3 |11> |
| ...             | ...                                       |
| specific n bits | all 2^n possible qubits                   |
|-----------------|-------------------------------------------|

Applications

• 100-200 qubits: chemistry
• 100s-1000s: optimization, ML, material science
• more than 100.000s: unknown problems

Technology Options

• trapped ions (IONQ, Honeywell)
• annealers (DWAVE)
• superconducting qubits (IBM, Google, Intel)
• quantum dots (Intel)
• majorana fermions (Microsoft)
• silicon photonics (PsiQuantum)

Why use photons?

• robust
• easy to manipulate
• fast to measure
• carrier of quantum info
• works at any temperature (not mk)
• is manufacturable

Light Properties

• wavelength
• frequency
• phase
• polarization (time bin encoding)
• direction

Single Photon Source (SPS)

• repetition rate & proability of collection
• P₁ = probability of exactly i photons (per pulse)
• we want P₁ >> P₀, P₂₊
• but temperature is the issue so we try to use laser (photon generation based on Poisson)

Single Photon Detectors (1-1)

1550 nm (photon) = 1.28 × 10^-19 J

ERP Paradox

Einstein, Podolsky, Rsoen: they tried to prove Heisenberg's uncertainty principle wrong & instead proved it right.

Σύζευξη Φωτωνίων

Randomness

Man-mad randomness is fake because it's deterministic.

A true random generator should have something from physics as input (e.g. quantum ones)