Non-classical photon-phonon correlations at room temperature
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2021-01-01 |
| Journal | Infoscience (Ecole Polytechnique Fédérale de Lausanne) |
| Authors | Santiago Tarrago Velez |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive Summaryâ- Core Innovation: Demonstration of non-classical photon-phonon correlations and quantum state preparation in macroscopic materials (diamond, liquid CS2) operating at ambient (room) temperature.
- Quantum State Preparation: Successfully prepared the 40 THz vibrational mode of diamond into the non-classical n=1 Fock state with 98.5% probability via Stokes photon heralding.
- Bell Correlation Violation: Achieved a maximum CHSH parameter value of S = 2.360 ± 0.025, significantly violating the classical limit (S †2) and confirming strong hybrid photon-phonon entanglement.
- Coherence Time: The Bell correlations persisted for over 5 ps, corresponding to more than 200 oscillation periods of the phonon, confirming the high quality factor (Q-frequency product ~4 x 1016 Hz) of the diamond phonon memory.
- Spectroscopic Advancement: Developed a versatile two-color pump-probe technique using spectral multiplexing, circumventing the polarization restrictions of previous quantum memory schemes, thus broadening applicability to diverse Raman-active materials.
- Molecular Coherence: Observed long-lived quantum beats in liquid CS2, demonstrating the spontaneous generation of a collective vibrational excitation shared between two isotopic sub-ensembles.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Phonon Frequency (Diamond) | 39.9 (or 40) | THz | Optical phonon mode |
| Phonon Lifetime (Diamond) | 3.9 ± 0.7 | ps | Measured via S-aS correlation decay |
| CHSH Parameter (Max) | 2.360 ± 0.025 | N/A | Violation of Bell inequality (Classical limit †2) |
| Vibrational Q-Product | ~4 x 1016 | Hz | Room temperature diamond phonon |
| Fock State Purity (n=1) | 98.5 | % | Probability conditioned on Stokes detection |
| Heralded Anti-Bunching (α(0)) | ~0.11 | N/A | Sub-Poissonian statistics of the prepared Fock state |
| Write Pulse Wavelength | 695 | nm | Used for Stokes scattering (heralding) |
| Read Pulse Wavelength | 800 | nm | Used for anti-Stokes scattering (readout) |
| Laser Repetition Rate | 80.7 | MHz | System clock frequency |
| Time-Bin Separation (âTbin) | ~3 | ns | Used for time-bin entanglement encoding |
| CS2 Isotope Separation | 8.6 (258) | cm-1 (GHz) | Symmetric stretch mode (CS322 vs CS32S34) |
| CS2 Phonon Lifetime (Ïph1) | 8.4 ± 1.3 | ps | Faster decaying mode |
| CS2 Phonon Lifetime (Ïph2) | 1.7 ± 0.2 | ps | Slower decaying mode |
| Thermal Occupancy (nth) | 1.5 x 10-3 | N/A | Diamond vibrational mode at 295 K |
| Stokes Detection Efficiency (ηA) | ~10 | % | Overall efficiency including collection and detector loss |
Key Methodologies
Section titled âKey Methodologiesâ- Two-Color Pump-Probe Scheme: Utilized two synchronized femtosecond laser pulses (Write pulse at Ï1, Read pulse at Ï2) with different center frequencies. This spectral multiplexing allows distinguishing the Stokes and anti-Stokes signals without relying on the materialâs polarization selection rules.
- Spontaneous Raman Scattering (SRS): The Write pulse induces Stokes scattering (Ï1 â Ï1 - Ωm), creating a phonon (vibrational quantum, Ωm) in the material.
- Quantum State Heralding (Write Step): Detection of the Stokes photon (S) acts as a projective measurement, probabilistically preparing the vibrational mode into the desired quantum state (e.g., the n=1 Fock state).
- Phonon Readout (Read Step): After a controlled time delay (ât), the Read pulse induces anti-Stokes scattering (Ï2 â Ï2 + Ωm), annihilating the stored phonon and converting it back into a detectable anti-Stokes photon (aS).
- Time-Correlated Single Photon Counting (TCSPC): Used single-photon avalanche photodiodes (SPADs) and a coincidence counter to measure the second-order cross-correlation function (g(2)S,aS(ât)) between the heralded Stokes and anti-Stokes photons.
- Time-Bin Entanglement Encoding: For Bell tests, both Write and Read pulses were split into âearlyâ and âlateâ time bins (separated by ~3 ns) using an unbalanced Mach-Zehnder interferometer.
- Temporal Information Erasure: A second, identical unbalanced interferometer was used on the scattered photons to erase the âwhich-timeâ information, mapping the time-bin qubits onto polarization qubits (H/V basis) and generating the entangled state.
- CHSH Measurement: Variable retarders and polarizing beam splitters were used to perform local rotations (Alice and Bob settings) on the polarization qubits, allowing measurement of the correlation parameter E and calculation of the CHSH value S.
Commercial Applications
Section titled âCommercial ApplicationsâThe demonstrated techniques elevate vibrational Raman scattering from a source of noise to a resource for nonlinear quantum optics, enabling several technological advances:
- Quantum Information Processing (QIP):
- Room-Temperature Quantum Memory: High-coherence phonons in bulk materials (like diamond) can serve as robust, ambient-condition buffer memories for photonic qubits.
- Hybrid Quantum Systems: Creation of entangled photon-phonon states, crucial for interfacing light (communication) and matter (storage/processing) qubits.
- Quantum Sensing and Metrology:
- Ultrafast Quantum Spectroscopy: Probing vibrational dynamics and coherence times (e.g., pure dephasing rates) with sub-picosecond resolution, far exceeding classical limits.
- Device-Independent Certification: The Bell violation provides the most stringent test of non-locality, useful for certifying the security and performance of quantum network links and devices.
- Advanced Materials Characterization:
- Molecular and Solid-State Physics: Study of quantum effects in vibrational dynamics of complex systems (e.g., organic molecules, low-dimensional structures, nanocavities) at ambient conditions.
- Phononic Engineering: Techniques can be used to optimize materials for longer phonon lifetimes (e.g., by creating phononic band gaps in isotope superlattices).
- Nonlinear Quantum Optics:
- High-Purity Single Photon Sources: Vibrational modes can be used to produce high-purity, on-demand anti-Stokes photons with arbitrary wavelength and bandwidth choices.
View Original Abstract
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 modeĂą 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 ñ 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 â where it used to be considered as a source of noise instead.