A laser beam is trapped between two mirrors, one of which trembles under an invisible force. Each vibration of the mirror changes the phase of the light, and from these changes you can infer the force. But the light itself pushes on the mirror, which is quantum backaction, and it imposes a fundamental limit on measurement accuracy. This limit is called the standard quantum limit (the minimum noise due to the quantum nature of light).
Instead of a single light wave in a Fabry–Perot cavity (two mirrors reflecting light many times), a trio is excited: a central pump mode and two sideband modes (optical oscillations with satellite frequencies), shifted exactly by the mechanical oscillator frequency. This "optical triplet" resembles a musical triad: the central note is the pump, and the two sidebands are the response to the external force. The secret is that the sidebands are asymmetric: they have different decay rates and different coupling strengths to the mechanics. This asymmetry, like unequal string tensions in an instrument, allows the noise to be canceled.
By measuring the quadratures (amplitude components, like cosine and sine) of the two sidebands independently, one can construct a linear combination such that the contributions of backaction and shot noise (quantum noise due to the discreteness of photons) cancel completely. When the condition g₊ = g₋ is met, backaction disappears, and when G = γₘ (the difference in coupling coefficients equals the mechanical decay rate), the measured quantity becomes an amplified version of the original signal without added noise. In the long-measurement limit, the output signal contains neither shot noise nor backaction.
This result breaks the usual picture: previously it was thought that quantum noise is unavoidable and sensitivity is limited by the standard quantum limit. Now it is shown that asymmetry is not a defect but a resource. In practice, such a scheme can be implemented using higher-order modes in the cavity, and mode sorters with efficiency above 80% can separate the modes at the output. Such noiseless amplifiers could boost the sensitivity of gravitational wave detectors, atomic force microscopes, and quantum sensors, and also find use in quantum communication.
The triad in which one string is tuned slightly tighter produces a pure tone without background hum. So here: a slight asymmetry of the optical modes turns quantum noise into silence, leaving only the signal.
The scheme uses three optical modes whose frequencies are spaced exactly by the mechanical oscillator frequency. This "optical triplet" resembles a musical triad, where the central note is the pump and the two sidebands are the response to the external force.
The idea of noiseless amplification of weak signals resonates with the concept of "quantum nondemolition measurement" from Greg Egan's novel "Quarantine": there, the protagonists use quantum effects to observe without disturbing the state. In the paper, a similar principle is applied to a classical force acting on a quantum oscillator.