Research


Diamond Phononics

Despite the great success of quantum mechanics, we still only have limited understanding of how the microscopic, quantum domain gives way to the more familiar realm of classical, “macroscopic” physics. Quantum coherence in a system tends to decay rapidly due to thermalization by interaction with the system’s environment, making it difficult to observe large-scale quantum behaviour except in extremely cold environments (e.g. Bose-Einstein condensates). One of the key interests of the quantum technologies group is how to extend the realm of non-classical phenomena to larger-scale systems at room temperature. This is of interest as both (a) a pure physics problem: can non-classical effects be observed and utilized in large scale objects?; and (b) as an applied physics problem: can interesting structural/physical information be discerned from a system by observing non-classical physics?

The Quantum Technologies group has created and detected non-classical physics in the optical phonon (vibrational) modes of bulk diamond crystals [1]. Using ultrafast Raman scattering, an optical phonon is created via a spontaneous Stokes scattering event. Despite being a single quantized excitation, the coherent phonon is a highly non-classical state distributed over the whole focal volume of the ultrafast pulse in the diamond. A subsequent ultrafast “read” pulse reads out the non-classical state by completing an anti-Stokes scattering event, annihilating the optical phonon, and creating a blue-shifted photon. Measurement of Stokes—anti-Stokes correlations confirmed the existence of non-classical correlations between the two pulses. In a recent collaboration with Oxford, the unusual aspects of non-classical physics were further emphasized by creating and measuring an entangled quantum state in which a single vibrational excitation was distributed between two spatially separated, millimeter-sized diamonds at room temperature [2].

The creation and measurement of optical phonons in a single diamond is also useful from a material science perspective: by repeated measurement of the Stokes—anti-Stokes correlations as a function of delay between the write and read pulses, the phonon population decay time T1 can be extracted and compared with that of other diamond samples. This method is called stimulated anti-Stokes ultrafast correlated Excitation Raman Spectroscopy (SAUCERS).

Stimulated anti-Stokes ultrafast correlated Excitation Raman Spectroscopy (SAUCERS). 1. Creation of an optical phonon by spontaneous Raman scattering from the write pulse is indicated by detection of a Stokes photon. 2. Detection of an anti-Stokes photon from the read pulse indicates that phonon has been read-out. 3. Measurement of Stokes/anti-Stokes coincidences as a function of write/read delay allows extraction of phonon lifetime T1.

Stimulated anti-Stokes ultrafast correlated Excitation Raman Spectroscopy (SAUCERS). 1. Creation of an optical phonon by spontaneous Raman scattering from the write pulse is indicated by detection of a Stokes photon. 2. Detection of an anti-Stokes photon from the read pulse indicates that phonon has been read-out. 3. Measurement of Stokes/anti-Stokes coincidences as a function of write/read delay allows extraction of phonon lifetime T1.

A closely-related experiment allows extraction of information about the coherence lifetime T2 of phonons in diamond, using a spectral technique called Transient Coherent Ultrafast Phonon Spectroscopy (TCUPS) [3,4]. This is effectively a time-frequency version of Young’s famous two-slit experiment. In TCUPS, two weak femtosecond “pump” pulses separated by a short delay propagate through the diamond lattice, generating Stokes light by spontaneous Raman scattering. The Stokes light is detected using an imaging spectrometer. At low power, the scattering is purely spontaneous so that on average less than one Stokes photon is generated per pulse pair. The production of a Stokes photon from the first pulse corresponds to one slit/pathway, and the production of a Stokes photon in the second pulse corresponds to the other. For very short delays between the two pump pulses, the two possible pathways are strongly phase-correlated and so spectral fringes are observed in the Stokes spectrum. As the delay is increased, decoherence increases as vibrational information leaks into the environment and the “which-way” information becomes available. The visibility of the spectral fringes therefore decreases as a function of delay, with the loss of phase coherence. Measurement of the visibility can thus be used as a measure of decoherence.

Transient coherent ultrafast phonon spectroscopy. By spontaneous Raman scattering in diamond, two ultrafast pump pulses generate on average one Stokes photon between them. Within the lifetime of the diamond optical phonon, the pathways 1 and 2 are phase coherent, resulting in spectral fringes when the Stokes light is detected. The fringe visibility decays as the delay T is increased, with information leaking into the environment by decoherence.

Transient coherent ultrafast phonon spectroscopy. By spontaneous Raman scattering in diamond, two ultrafast pump pulses generate on average one Stokes photon between them. Within the lifetime of the diamond optical phonon, the pathways 1 and 2 are phase coherent, resulting in spectral fringes when the Stokes light is detected. The fringe visibility decays as the delay T is increased, with information leaking into the environment by decoherence.

1. Nature Photonics, 6, 41 (2011) pdf
2. Science, 334, 1253 (2011) pdf
3. Physical Review B, 78, 155201 (2008) pdf
4. Diamond and Related Materials, 19, 1289 (2010) pdf