Why Do Quantum Computers Make So Many Mistakes?
Mikhail Lukin on Quantum Error Correction
This week on the 632nm podcast, we speak with Mikhail Lukin, Harvard physicist and co-director of the Harvard Quantum Initiative. Lukin’s lab is pioneering the use of neutral atoms held in optical tweezers to build scalable, programmable quantum computers. In this conversation, he unpacks why quantum evolution is fundamentally analog, how error correction works despite the no-cloning theorem, and what it means to test the very limits of quantum mechanics in the lab.
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I. The Paradox of Quantum Error Correction
Lukin begins with the contradiction at the heart of quantum computing — you can’t copy or measure quantum information, and yet it can somehow be protected:
“You cannot copy quantum information. Number two, in quantum mechanics, if you measure the state, you destroy it. Nevertheless, it turns out that you can use this redundancy to actually encode and protect quantum information. But actually it requires you to prepare these entangled states, these big superpositions that sometimes are called Schrödinger cat states.”
What makes this possible, he explains, is redundancy built not from copies, but from entanglement — a subtle kind of correlation that spreads information across many qubits without ever making it directly observable.
🎧 Listen at 00:02:00
II. All Evolution in Quantum Mechanics Is Analog
Every quantum operation is continuous — a smooth rotation in Hilbert space. That’s what makes controlling qubits so demanding:
“All evolution in quantum mechanics is analog evolution. There is no such thing as a digital operation that just snaps something from zero to one. Every evolution is continuous, so any small imperfection will make your state drift away continuously.”
In classical computing, tiny voltage errors are erased by digital thresholds. In quantum computing, even a microscopic phase error accumulates — a challenge that makes error correction not just useful, but essential.
🎧 Listen at 00:09:00
III. Ancilla Qubits: Measuring Without Collapsing
To detect errors without destroying information, Lukin explains, physicists use special “helper” qubits that indirectly sense when something has gone wrong:
“You can measure some auxiliary qubits—ancilla qubits—which don’t contain information themselves but are entangled with the data. They tell you something about whether an error has occurred, but not what the encoded quantum state actually is.”
This approach allows researchers to extract classical error signals from a quantum system, preserving the delicate superpositions that store computational information.
🎧 Listen at 00:15:00
IV. Entangling Logical Qubits Through Rydberg Interactions
Lukin’s group builds these ideas into hardware using neutral atoms excited into Rydberg states, whose strong interactions allow for controlled entanglement across entire arrays:
“You take one group of atoms and bring it so that the atoms sit nearly on top of each other. Then you shine one pulse of light which entangles them all in parallel. That’s a transversal operation between two logical qubits.”
These transversal gates perform the same operation across many atom pairs at once — a key ingredient for fault-tolerant quantum computation.
🎧 Listen at 00:28:00
V. Testing the Limits of Quantum Mechanics
Ultimately, Lukin views each advance in quantum error correction not just as an engineering step, but as a test of physics itself:
“When we make these larger and larger superpositions and keep them stable, we are really testing whether the laws of quantum mechanics still hold for systems of this size. Every experiment is a test of the theory itself.”
The pursuit of stable superpositions isn’t only about computation — it’s about seeing how far quantum mechanics can stretch before giving way to new physics.
🎧 Listen at 00:41:00
Full episode out now on YouTube.
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