Mechanical oscillators have key technological applications, but extending their use for quantum technologies requires creating well defined quantum states. We demonstrate that we can create the n=1 phonon Fock state in diamond by using a combination of Raman scattering from ultra-fast laser pulses and time-correlated single photon counting.
Mechanical oscillators have key technological applications, but extending their use for quantum technologies requires creating well defined quantum states. We demonstrate that we can create the n=1 phonon Fock state in diamond by using a combination of Raman scattering from ultra-fast laser pulses and time-correlated single photon counting.
A single quantum of excitation of a mechanical oscillator is a textbook example of the principles of quantum physics. Mechanical oscillators, despite their pervasive presence in nature and modern technology, do not generically exist in an excited Fock state. In the past few years, careful isolation of GHz-frequency nano-scale oscillators has allowed experimenters to prepare such states at milli-Kelvin temperatures. These developments illustrate the tension between the basic predictions of quantum mechanics that should apply to all mechanical oscillators existing even at ambient conditions, and the complex experiments in extreme conditions required to observe those predictions. We resolve the tension by creating a single Fock state of a vibration mode of a crystal at room temperature using a technique that can be applied to any Raman-active system. After exciting a bulk diamond with a femtosecond laser pulse and detecting a Stokes-shifted photon, the 40~THz Raman-active internal vibrational mode is prepared in the Fock state ∣1> with 98.5% probability. The vibrational state is read out by a subsequent pulse, which when subjected to a Hanbury-Brown-Twiss intensity correlation measurement reveals the sub-Poisson number statistics of the vibrational mode. By controlling the delay between the two pulses we are able to witness the decay of the vibrational Fock state over its 3.9 ps lifetime at room temperature. Our technique is agnostic to specific selection rules, and should thus be applicable to any Raman-active medium, opening a new generic approach to the experimental study of quantum effects related to vibrational degrees of freedom in molecules and solid-state systems.
Recognized as a potential resource for quantum technologies and a possible test bed for fundamental physics, the control and preparation of nonclassical states of mechanical oscillators have been explored extensively. Within optomechanics, quantum state synthesis can be realized by entangling photonic and phononic degrees of freedom followed by optical detection. Single-photon detection enables one of the most powerful forms of such heralded quantum state preparation, permitting the creation of single-phonon states when applied to conventional cavity optomechanical systems. As the complexity of optomechanical systems increases, single-photon detection may provide heralded access to a larger class of exotic quantum states. Here, we examine the quantum dynamics of optomechanical systems that permit forward Brillouin scattering, where a single-phonon mode mediates transitions between a collection of equally spaced optical resonances. Solving both the Schrodinger equation and the Lindblad master equation for this system, we find that initial states comprised of single photons or weak laser pulses evolve into complex quantum states where the frequency of single-photon states and the phonon occupation number are entangled. Physically, these interactions permit a single photon to scatter to lower frequencies, where phonon excitation occurs for each scattering event. Combining this result with frequency filtering, we show how single-photon detection can herald selected multiphonon Fock states, even in the presence of optical losses. We also present an approach for quantum tomography of the heralded phonon states, which in the form of a state swap from the phononic to photonic domain enables on-demand generation of multiphoton Fock states.
With the development of quantum optics, photon correlations acquired a prominent role as a tool to test our understanding of physics, and played a key role in verifying the validity of quantum mechanics. The spatial and temporal correlations in a light field also reveal information about its origin, and allow us to probe the nature of the physical systems interacting with it. Additionally, with the advent of quantum technologies, they have acquired technological relevance, as they are expected to play an important role in quantum communication and quantum information processing. This thesis develops techniques that combine spontaneous Raman scattering with Time Correlated Single Photon Counting, and uses them to study the quantum mechanical nature of high frequency vibrations in crystals and molecules. We demonstrate photon bunching in the Stokes and anti-Stokes fields scattered from two ultrafast laser pulses, and use their cross-correlation to measure the 3.9 ps decay time of the optical phonon in diamond. We then employ this method to measure molecular vibrations in CS2, where we are able to excite the respective vibrational modes of the two isotopic species present in the sample in a coherent superposition, and observe quantum beating between the two signals. Stokes scattering, when combined with a projective measurement, leads to a well defined quantum state. We demonstrate this by measuring the second order correlation function of the anti-Stokes field conditional on detecting one or more photons in the Stokes field, which allows us to observe a phonon modeâ s transition form a thermal state into the first excited Fock state, and measure its decay over the characteristic phonon lifetime. Finally, we use this technique to prepare a highly entangled photon-phonon state, which violates a Bell-type inequality. We measure S = 2.360 ± 0.025, violating the CHSH inequality, compatible with the non-locality of the state. The techniques we developed open the door to the study of a broad range of physical systems, where spectroscopic information is obtained with the preparation of specific quantum states. They also hold potential for future technological use, and promote vibrational Raman scattering to a resource in nonlinear quantum optics -- where it used to be considered as a source of noise instead.
We present a new experiment, in which we measure quantum correlations between single photons that are mediated by the exchange of a single phonon. We create and annihilate a single optical phonon in bulk diamond using two ultrashort laser pulses at two different wavelengths, generated by a Ti:Sapph oscillator and a frequency-doubled optical parametric oscillator (APE Berlin). During Stokes Raman scattering, the first pulse creates a photon-phonon pair, while the second pulse convert the same phonon into an anti-Stokes photon. Using spectral filtering and photon counting, we measure the cross-correlations between the Stokes and anti-Stokes photons with a few hundred femtoseconds time resolution. As expected, the non-classical correlation (g(2) much larger than the classical bound) decays within a few picosecond, following to the dynamics of the phonon mode. Our results demonstrate a new source of broadly tunable quantum correlated photons, and can be extended to provide a new way of measuring non-classical dynamics in nanoscale systems — down to individual nanostructures.
We show how to use the radiation pressure optomechanical coupling between a mechanical oscillator and an optical cavity field to generate in a heralded way a single quantum of mechanical motion (a Fock state). Starting with the oscillator close to its ground state, a laser pumping the upper motional sideband produces correlated photon-phonon pairs via optomechanical parametric down-conversion. Subsequent detection of a single scattered Stokes photon projects the macroscopic oscillator into a single-phonon Fock state. The nonclassical nature of this mechanical state can be demonstrated by applying a readout laser on the lower sideband to map the phononic state to a photonic mode and performing an autocorrelation measurement. Our approach proves the relevance of cavity optomechanics as an enabling quantum technology.
Tunable single-frequency lasers are essential for high-resolution spectroscopy and quantum technologies, yet achieving narrow-linewidth performance in compact, scalable systems across the UV-visible spectrum remains a key challenge. Recent work has revealed that Raman scattering in diamond can act as a natural spectral squeezer, where phonon dynamics help suppress frequency noise and concentrates optical power into a single, narrow spectral mode. Experiments at CERN and elsewhere have demonstrated linewidth compression by orders of magnitude, frequency-noise suppression exceeding 10³-10⁴, and pathways toward Schawlow–Townes-limited performance in CW regimes. These results establish Raman phonon damping as a universal mechanism for generating ultra-coherent, widely tunable lasers, opening new opportunities in quantum metrology, optical clocks, and high-resolution spectroscopy.