When we assemble multiple qubits into a single system, they often form a special relationship called entanglement. This is one of the most fascinating and counterintuitive concepts in quantum mechanics, famously described by Albert Einstein as ‘spooky action at a distance.’ While a system of multiple qubits can be thought of as a single state, entanglement means that the individual qubits no longer have their own distinct states. Instead, they are inextricably linked in a shared superposition, so closely connected that observing one instantly tells you about the state of the other, no matter how far apart they are. This connection is a core component of many quantum algorithms, including teleportation.
🤝 Creating Entangled Pairs
We can easily create an entangled pair, or a Bell state, using simple quantum gates. Starting with two qubits in the state $|00\rangle$, we apply a Hadamard gate (H) to the first qubit and then a Controlled-X (CX) gate. This creates a state known as $|\beta_{s+}\rangle = \frac{1}{\sqrt{2}}(|00\rangle + |11\rangle)$. This state is not just a combination of the two qubits; it’s a new, single state that cannot be broken down into a simple tensor product of two separate qubits. This inseparability is the hallmark of entanglement.
❓ So, what does this mean?
Imagine you have an entangled pair of qubits. Before you measure them, each qubit is in a superposition, so neither has a definite state. But the instant you measure one and get a result (say, a 0), you immediately know what the result would be if you were to measure the other one (in this case, a 0). The other qubit, no matter if it is a few centimeters or a few light-years away, has instantly collapsed to the same state. This connection is not a secret that you are discovering; it’s the very act of measurement that forces both qubits to choose a definite state from their shared superposition.
🚫 Entanglement Isn’t Faster-than-Light Communication
This instantaneous collapse might sound like a way to send information faster than the speed of light. However, this is not the case. While the collapse happens instantly, you can’t use it to transmit any information. To confirm what state the other qubit collapsed to, you would still need to communicate with your partner through classical means, like a radio signal. And since classical signals are limited by the speed of light, you can’t use entanglement for superluminal communication. Despite this limitation, entanglement is a powerful resource that plays a vital role in many quantum algorithms, from quantum cryptography to teleportation.
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Glassner, Andrew. Quantum Computing: From Concepts to Code. No Starch Press, 2025.
More Topics
- The Quantum Threat: How Shor’s Algorithm Puts Modern Encryption at Risk
- Cracking the Code: How Quantum Parallelism Solves Deutsch’s Problem in One Step
- The Deutsch-Jozsa Algorithm: An Exponential Leap in Quantum Problem Solving
- The Bernstein-Vazirani Algorithm: Unmasking a Secret With a Single Query
- Simon’s Algorithm: The First Quantum Speedup That Left Classical Computers in the Dust
- The Quantum Enigma: Why Measurement is the Most Unpredictable Step
- Can You Teleport a Qubit? The Amazing Alice & Bob Experiment
