Before we can talk about protecting your data from quantum computers, you need a working picture of what they are, and just as importantly, what they are not.
Unit 0 progress
By the end of this lesson you will be able to
Explain the difference between a classical and quantum computer in plain language
Define qubit, superposition, and entanglement without using jargon
State clearly what quantum computers are fast at, and what they are not
Explain why this matters for the security of data you are protecting today
Before the math — why quantum is genuinely strange
Classical computing is built on a principle so intuitive you have never questioned it: a bit is either 0 or 1. Not both. Not something in between. Either. This seems so obvious it barely deserves saying. It is the foundation of every computer you have ever used, every piece of software ever written, every network packet ever transmitted. Zero or one. Off or on.
Quantum mechanics breaks this assumption at the level of physics, not as a clever engineering trick, but as a fundamental property of reality at very small scales. A quantum system can exist in a superposition of states: not 0, not 1, but a weighted combination of both simultaneously. This is not a metaphor or an approximation. It is what actually happens, and it was strange enough that Richard Feynman, one of the greatest physicists of the twentieth century, spent considerable effort helping general audiences understand why classical intuition fails at the quantum scale. His conclusion, delivered in a 1964 lecture that remains one of the clearest explanations ever given, was essentially this: quantum mechanics is not something you can make comfortable by analogy. You can only understand it by accepting that the world at small scales does not work the way the world at human scales works.
This matters for this course because the cryptographic algorithms that protect nearly everything on the internet (RSA, ECC, Diffie-Hellman) derive their security from mathematical problems that are hard for classical computers. Quantum computers, by exploiting superposition and entanglement, can solve specific versions of these problems exponentially faster. You do not need to understand the quantum physics to understand the migration problem. But understanding that quantum is genuinely different, not just faster, is the foundation for understanding why a new category of cryptography is needed, not just a speed upgrade.
Part 1 — The computer you already know
Every computer you have ever used, your laptop, your phone, the server running your company's payroll software, works the same fundamental way. It processes information as bits. A bit is the smallest possible unit of information, and it has exactly two possible states: zero or one. Off or on. False or true.
Everything your computer does, running spreadsheets, encrypting a file, sending an email, is ultimately a very long sequence of zeros and ones being flipped and rearranged at extraordinary speed. Your laptop might process billions of these operations per second. A modern data center might process trillions. But the fundamental unit, the bit, never changes. It is always, at any given moment, either a zero or a one.
This is not a limitation we have worked around. It is the bedrock of how all modern computing and all modern cryptography works. The encryption protecting your bank account, your email, and your company's data is built entirely on the predictable, reliable, either-or nature of classical bits.
Why this matters for security
Every encryption standard in use today, the locks on your VPN, your website, your email, your cloud storage, was designed assuming the attacker has access only to classical computers. That assumption is about to change.
Part 2 — The qubit: a fundamentally different idea
A quantum computer does not use bits. It uses qubits, quantum bits. And a qubit behaves in a way that has no direct equivalent in everyday experience, which is why explaining it requires an analogy rather than a definition.
Think about a coin. When it is sitting flat on a table, it is either heads or tails, that is a classical bit. But when you spin that coin on a table, something different is happening. While it is spinning, it is not heads and it is not tails. It is in a state that contains the possibility of both outcomes simultaneously. Only when it falls, when you observe it, does it commit to one or the other.
A qubit works on the same principle, but using the quantum mechanical properties of a physical particle, an electron, a photon, or an ion, depending on the design. While the qubit is in its "spinning" state, it exists in a condition called superposition: representing zero, one, or any combination of both at the same time. The moment you measure it, the superposition collapses and you get a definite zero or one. But until that moment, the qubit holds both possibilities simultaneously.
Part 3 — Superposition, entanglement, and why they matter
Superposition gives quantum computers something extraordinary. If one qubit can represent zero and one simultaneously, then two qubits can represent four states at once (00, 01, 10, 11). Three qubits can represent eight states. Ten qubits can represent 1,024 states simultaneously. Thirty qubits can represent over a billion states at the same time.
A classical computer with 30 bits can only be in one of those billion states at any given moment. A quantum computer with 30 qubits exists across all of them simultaneously, and can perform a calculation across all of them in a single operation. This exponential scaling is the source of quantum computing's power for certain types of problems.
The second key property is entanglement. When two qubits become entangled, they form a linked pair, the state of one instantly affects the state of the other, regardless of physical distance. Measuring one entangled qubit immediately tells you something definite about its partner. Einstein famously called this "spooky action at a distance" and was deeply uncomfortable with it. But it is real, it has been experimentally verified thousands of times, and it allows quantum computers to perform coordinated calculations across many qubits simultaneously.
Classical computer
Quantum computer
The security problem
The library search Opens one book at a time, reads it, puts it back, opens the next. Exhaustively checks every option in sequence.
The parallel reader Opens every book in the library simultaneously, reads them all at once, and amplifies the one with the right answer.
Finding the key Today's encryption relies on the fact that trying every possible key takes billions of years classically. Quantum computers change that calculation for certain key types.
Part 4 — What quantum computers are actually fast at
This is where most explanations go wrong. Quantum computers are not faster than classical computers at everything. They are not going to make your laptop load web pages faster or run spreadsheets more quickly. They are specialized machines that excel at a narrow set of mathematical problems, specifically, problems that involve exploring a very large number of possibilities simultaneously.
The three categories that matter most for our purposes are: factoring very large numbers (which breaks RSA encryption), solving the discrete logarithm problem (which breaks elliptic curve cryptography, the same system used to protect TLS, VPN connections, and blockchain wallets), and searching through unsorted data (which weakens but does not fully break symmetric encryption like AES).
The algorithm that does the factoring and discrete logarithm work is called Shor's algorithm, developed by mathematician Peter Shor in 1994. It requires a large, stable quantum computer to run effectively. We do not yet have that machine. But the consensus among cryptographers is that we will, most likely within this decade, and the consequences for current encryption are severe enough that preparation must begin now, not when the machine arrives.
The key sentence to remember
RSA and elliptic curve cryptography, the mathematical foundations of almost every digital certificate, secure website, VPN, and encrypted email in existence today, are broken by Shor's algorithm running on a sufficiently powerful quantum computer. That computer does not exist yet. But your data does. And some of it will still need to be secret when that computer arrives.
Part 5 — Where quantum computers are today
We are currently in what researchers call the NISQ era: Noisy Intermediate-Scale Quantum computing. "Noisy" means current quantum computers make errors frequently because qubits are extraordinarily fragile. Heat, vibration, electromagnetic interference, any of these can cause a qubit to lose its quantum state, a problem called decoherence. "Intermediate-scale" means current machines have hundreds to a few thousand physical qubits, when running Shor's algorithm effectively against real-world encryption would require millions of reliable, error-corrected qubits.
The leading companies building quantum computers include IBM, Google, IonQ, Quantinuum, and several others. Google made significant news in 2023 with error correction results demonstrating that larger quantum systems could be more reliable, not less, a key threshold. IBM has published roadmaps targeting fault-tolerant quantum computers within this decade. These are not theoretical claims. They are engineering programs with published milestones and significant investment behind them.
The honest summary: the threat is not here yet. But the preparation window is now, because migrating the world's cryptographic infrastructure takes years, and some of the data being encrypted today will still need protection when the threat arrives.
Persona notes
A
The motivated learner. Understanding the NISQ era context matters for your professional conversations. When someone asks "are quantum computers a threat right now?" the accurate answer is: not yet to encryption specifically, but the preparation window is open and closing. That nuance is what separates an informed practitioner from someone repeating headlines.
B
The SMB decision-maker. You do not need to understand the physics. What you need to understand is this: the encryption protecting your customer data, your financial records, and your business communications was designed before this threat existed. The question is not whether you will eventually need to update it. The question is whether you will do it in an orderly, planned way or in a crisis.
C
The IT professional. The systems on your migration checklist, TLS certificates, VPN tunnels, code signing, email encryption, all use either RSA or elliptic curve cryptography. Both are in Shor's algorithm's crosshairs. The NIST standards that replace them (FIPS 203, 204, and 205) were finalized in August 2024. The migration clock is running.
Recommended video — A beginner's guide to quantum computing
Shohini Ghose · TED Talk · 11 minutes · Highly recommended before the next lesson. This talk covers the same concepts from this lesson using visual animations and a different framing. Watch it after completing the comprehension check below, it will reinforce what you have just read and present the ideas from a slightly different angle, which helps retention.
The Feynman connection
In 1981, Richard Feynman published a paper titled "Simulating Physics with Computers" in which he argued that classical computers are fundamentally incapable of efficiently simulating quantum mechanical systems, and proposed that a computer built on quantum mechanical principles could do so. This is widely regarded as the founding document of quantum computing as a field.
What makes this historically interesting for a PQC course is the timeline: Feynman proposed quantum computing in 1981. RSA had been published in 1977. The vulnerability of RSA to quantum computers (via what would become Shor's algorithm) would not be formally described until 1994. The window between "quantum computing proposed" and "classical cryptography shown to be vulnerable" was thirteen years. The window between Shor's algorithm and the first NIST post-quantum standards was thirty years. Migration from classical to post-quantum cryptography is a slow, deliberate, civilization-scale undertaking, and it begins with understanding why Feynman's 1981 insight changed the game.
Source: Feynman, R.P. (1982). "Simulating Physics with Computers." International Journal of Theoretical Physics, 21(6/7), 467–488.
Before you answer the comprehension questions below, sit with this for a moment: what would it mean for a computer to exist in two states simultaneously, to be working on the answer to "is this number prime?" while also working on "what is its prime factor?" at the same time, in the same physical hardware? The discomfort or confusion you feel right now is the correct response. It means you are taking the question seriously. Feynman felt it too. Everyone who has thought carefully about quantum mechanics has felt it. Hold that feeling. It is the intellectual honesty that will make you a better practitioner.
Comprehension check
Question 1 of 7
A classical bit and a qubit both represent binary information. What is the fundamental difference between them?
Question 2 of 7
Shor's algorithm threatens RSA encryption. Which property of a quantum computer makes this possible?
Question 3 of 7
Which of the following best describes why quantum computers are not a general replacement for classical computers?
Question 4 of 7
Richard Feynman proposed quantum computing in 1981. What was his original motivation, and why does it matter for this course?
Question 5 of 7
When a qubit is "measured," what happens?
Question 6 of 7
A colleague says: "Quantum computers will break all encryption." How would you correct this statement?
Question 7 of 7
What is the most accurate reason that post-quantum cryptography (PQC) uses mathematical problems rather than quantum physics to achieve quantum resistance?
Going deeper — optional but recommended
These resources are not required to complete the course. They are here because the learners who go deepest into PQC are usually the ones who first went deep into understanding why quantum mechanics is strange. Follow your curiosity.
Feynman's sixth Messenger Lecture at Cornell (1964), restored to HD video with a searchable scrolling transcript. This is Feynman explaining superposition and quantum uncertainty to a general audience, the same concepts that underpin why quantum computers can threaten RSA. Free to watch with no account required. Note: This is lecture 6 of 7. If you want the full series, start at lecture 1. The series is titled "The Character of Physical Law."
IBM's accessible explainer on qubits and quantum superposition, written for a technical but non-physicist audience. A useful modern complement to Feynman's foundational perspective.
Scott Aaronson is one of the clearest writers on quantum computing theory. The opening chapter of his lecture series is freely available and covers the question "what is computation?" in a way that makes quantum computing feel like a natural extension rather than a discontinuity.
The paper that started the field. You do not need to read the full paper, but reading the abstract and first page is worth doing once to understand that quantum computing originated as a physics question, not a cryptography one.