Observation of quantum noise reduction in a Raman amplifier via quantum correlation between atom and light

Any amplifier requires coupling to its internal degrees of freedom for energy gain. This coupling introduces extra quantum noise to the output [1]. On the other hand, if the internal degree of the amplifier can be accessed and manipulated, we can manage and even reduce the quantum noise of the amplifier’s output [2,3]. In this investigation, we study the quantum noise in an atomic Raman amplifier. The Stokes output of the Raman amplifier is related to both input Stokes field and internal atomic spin wave. So, placing atomic spin wave (internal degree of freedom) in a quantum correlated state with the input Stokes can reduce the quantum noise of the Raman amplifier [4]. We report an observation of quantum noise reduction of more than 3.5 dB in the atomic Raman amplification process by placing atoms and input Stokes in a quantum correlated state through another Raman process. Conversely, study of the output noise, especially in the high gain regime of the Raman amplifier, can also reveal the quantum correlation between input Stokes and atomic spin wave, where the second Raman amplifier acts as a part of the measurement device [5]. This provides the first measurement of atom-light quantum correlation. Our work is of great interest to a broad readership, for communities from quantum sensor and quantum-enhanced measurement to quantum information, where quantum entanglement between matter and light is explored.

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[3] Z.Y. Ou, S.F. Pereira, and H.J. Kimble, “Quantum noise reduction in optical amplification,” Phys. Rev. Lett. 70, 3239 (1993).

[4] Z. Y. Ou, “Quantum amplification with correlated quantum fields,” Phys. Rev. A 48, R1761 (1993).

[5] J. Li et al, “Measuring continuous-variable quantum entanglement with parametric amplifier assisted homodyne detection”, Phys. Rev. A 101, 053801 (2020).