Counting Light to Ten Thousand express
You're pouring water into a glass, but you want exactly one million drops — not approximately, but exactly. An ordinary faucet pours a stream where the number of drops always varies. In the quantum world, photons have the same problem: a laser emits light where the number of particles fluctuates like a crowd in a square. But for many tasks, from ultra-precise measurements to quantum computers, you need states with a perfectly defined number of photons, the so-called Fock states.
Previously, they were obtained in two ways. The first is to wait for a random event where the beam happens to contain the right number of photons. The second is to cleverly control the system, constantly tweaking parameters. Both paths are painful: either the success probability is minuscule, or the control is monstrously complex. The authors of the new work proposed a third path — an adaptive measurement protocol. The quantum system (light in a cavity) is coupled to a tiny sensor — an auxiliary qubit. A series of measurements is performed on the qubit, but not identical ones; instead, the interval between measurements depends on the previous result. This is like playing "hot and cold": after each step, we learn whether we're closer to the goal and adjust the next step.
In calculations on the Jaynes-Cummings model (light interacting with a qubit), a large coherent pulse (roughly speaking, a laser beam) almost certainly turns into a Fock state with ten thousand photons in just ten rounds of measurements. The average fidelity is about 87%. If you need fidelity above 99%, then with a probability of 35% you can obtain a state with a photon number from 20000 − √20000 to 20000 + √20000. This is a huge step forward compared to previous attempts.
The practical value is great. Large Fock states are a resource for quantum metrology: they can measure distances, fields, and forces with precision unattainable in classical physics. In quantum computing, such states serve as carriers of error-protected information. The new protocol doesn't require exotic equipment, just a cavity, a qubit, and standard measurements. This brings closer the day when quantum devices with macroscopic Fock states become a common tool.
So a light beam, like a disorderly crowd, turns into a handful of sand where every grain has been counted individually.
To feel the difference: in a bright laser beam, the number of photons per second is about $10^{16}$, but it is never exactly known. A Fock state with 10,000 photons is like a handful of sand where every grain has been counted individually.
In Arthur Koestler's novel "The Sleepwalkers," the protagonists strive for absolute knowledge but encounter uncertainty. Here, scientists do the opposite: they tame uncertainty, turning the probabilistic world of photons into a realm of exact numbers.