Quantum Jump in Sound: Real-Time Observation Explained

Yes, researchers have observed a quantum jump in sound in real time, confirming that the same fundamental quantum rules governing light and matter also apply to acoustic phonons. This breakthrough, achieved by trapping a tiny mechanical resonator in a superconducting circuit, provides direct evidence that sound energy can exist in discrete quantum states and transition between them abruptly. The observation opens new avenues for quantum computing and ultra-sensitive sensing using sound waves.

Formula

The probability of observing exactly n phonons in a thermal state is given by the Bose-Einstein distribution:

P(n) = (1 / (1 + ⟨n⟩)) * (⟨n⟩ / (1 + ⟨n⟩))^n

where P(n) is the probability, n is the number of phonons (dimensionless), and ⟨n⟩ is the mean phonon number. For a coherent state (like a laser for sound), the probability follows a Poisson distribution: P(n) = (⟨n⟩^n * e^(-⟨n⟩) / n!). The calculator above uses the Poisson formula to compute the probability of observing a specific phonon number given a mean phonon number, which is the relevant distribution for a coherent sound source.

Worked Example

Using the default values: mean phonon number ⟨n⟩ = 1, and we want the probability of observing exactly n = 1 phonon.

1. Plug into the Poisson formula: P(1) = (1^1 * e^(-1)) / 1! = e^(-1) ≈ 0.3679
2. Convert to percentage: 0.3679 * 100 = 36.79%
3. The calculator shows 36.79%, matching the manual calculation.

Background: What is a Quantum Jump?

Quantum jumps are sudden, discrete transitions between energy levels in a quantum system. First theorized by Niels Bohr in 1913 to explain atomic spectra, they were long considered instantaneous and unobservable. In the 1980s, experiments with trapped ions showed that jumps could be detected, but only indirectly. For sound, the challenge was that phonons (quantized vibrations) are difficult to isolate from their environment, and their energy levels are extremely closely spaced.

Sound quanta, called phonons, are the elementary particles of vibration. In a crystal lattice, they represent collective atomic vibrations. In a macroscopic resonator, a phonon is a single quantum of mechanical vibration. Observing a quantum jump in sound means detecting the system’s transition from, say, 0 to 1 phonon, or from 1 to 2, in real time.

How the Experiment Works

The breakthrough experiment, published in 2023 by researchers at ETH Zurich, used a superconducting circuit called a transmon qubit coupled to a mechanical resonator—a tiny drumhead about 10 micrometers in diameter. The setup works as follows:

• Cooling: The entire device is cooled to near absolute zero (about 10 millikelvin) to minimize thermal noise.
• Coupling: The qubit interacts with the mechanical resonator via piezoelectric coupling, meaning the qubit’s electric field can excite or detect vibrations in the drumhead.
• Measurement: The qubit acts as a sensitive detector. By measuring the qubit’s state, researchers can infer the phonon number without destroying it (quantum non-demolition measurement).
• Real-time tracking: By repeatedly measuring the qubit, they can track the phonon number as it jumps between 0, 1, 2, etc., in real time.

The key innovation was using the qubit to make weak, repeated measurements that do not collapse the system completely, allowing continuous monitoring of the phonon number.

What Was Observed

The researchers observed clear, discrete jumps in the phonon number, exactly as quantum theory predicts. They saw the system remain in one phonon state for a while, then abruptly jump to another. The jumps were not smooth transitions but instantaneous (on the measurement timescale). They also observed that the jumps occurred randomly, following the statistical rules of quantum mechanics.

This was the first direct, real-time observation of quantum jumps in a mechanical system. Previous experiments had observed phonon number states but not the dynamic jumps themselves. The team also demonstrated that they could stabilize the system in a certain phonon state by applying feedback, effectively preventing jumps—a technique called quantum error correction for sound.

Why It Matters

Observing quantum jumps in sound is not just a curiosity. It has practical implications:

• Quantum computing: Mechanical resonators could serve as long-lived quantum memories or transducers between different types of qubits (e.g., superconducting and optical).
• Quantum sensing: Phonons are extremely sensitive to mass and force, so quantum control could lead to ultra-precise sensors for detecting tiny masses or forces.
• Fundamental physics: It confirms that quantum mechanics applies to macroscopic objects, helping to explore the boundary between quantum and classical worlds.

The ability to monitor and control phonon jumps in real time is a step toward building hybrid quantum systems that combine the best of different platforms.

Common Misconceptions

One common misconception is that a quantum jump means the sound wave disappears and reappears. In reality, the energy of the sound is always present, but it can only take discrete values. The jump is a change in the number of phonons, not a change in the existence of sound.

Another misconception is that this experiment allows us to see the jumps with our eyes. The observation is done through electronic measurements, not visual. The term “real time” means that the measurement is fast enough to track the jumps as they happen, not that we see them directly.

Some might think that this violates the energy conservation principle. It does not: the energy change is accounted for by the exchange with the environment or the measurement apparatus.

Frequently Asked Questions

What exactly is a phonon?

A phonon is a quantum of vibrational energy, analogous to a photon for light. In a solid, it represents a collective vibration of atoms. In a mechanical resonator, it represents a single quantum of the drumhead’s motion.

How do they measure phonons without destroying them?

They use a quantum non-demolition (QND) measurement. The qubit is coupled to the resonator in such a way that the measurement of the qubit’s state does not change the phonon number, only reveals it. This allows repeated measurements of the same phonon state.

Can we hear a quantum jump?

No. The phonon energies involved are far above the audible range (the resonator vibrates at gigahertz frequencies), and the amplitude is incredibly small. Even if amplified, the quantum nature would not be perceptible to the human ear.

Does this mean sound is quantized in everyday life?

Yes, in principle, but the quantum steps are so tiny that they are completely unobservable at macroscopic scales. For a normal sound wave, the phonon number is astronomically large, and the discreteness is smoothed out.

What are the next steps for this research?

Researchers aim to create entangled states of multiple phonons, use phonons as quantum memory, and explore the limits of quantum mechanics with larger mechanical objects. They also want to improve the coherence time of the mechanical resonator.

How to Use the Calculator

This calculator helps you compute the probability of observing a specific phonon number in a coherent sound field, which is the typical state produced by a laser-like source. Enter the mean phonon number (average number of phonons) and the specific phonon number you want to know about. The result is the probability of that exact number occurring, based on the Poisson distribution. This is useful for understanding the quantum statistics of sound in experiments.

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in t | Measurement time | s | 1 | 0.1
in n | Phonon number | - | 1 | 0
out prob | Probability of n phonons | % | (exp(-n)*n^n)/fact(n)*100 | 2
out mean | Mean phonon number | - | n | 2