Non-classical correlations between single photons and phonons from a mechanical oscillator (1512.05360v2)

Abstract: Interfacing a single photon with another quantum system is a key capability in modern quantum information science. It allows quantum states of matter, such as spin states of atoms, atomic ensembles or solids, to be prepared and manipulated by photon counting and, in particular, to be distributed over long distances. Such light-matter interfaces have become crucial to fundamental tests of quantum physics and realizations of quantum networks. Here we report non-classical correlations between single photons and phonons -- the quanta of mechanical motion -- from a nanomechanical resonator. We implement a full quantum protocol involving initialization of the resonator in its quantum ground state of motion and subsequent generation and read-out of correlated photonphonon pairs. The observed violation of a Cauchy-Schwarz inequality is clear evidence for the non-classical nature of the mechanical state generated. Our results demonstrate the availability of on-chip solid-state mechanical resonators as light-matter quantum interfaces. The performance we achieved will enable studies of macroscopic quantum phenomena as well as applications in quantum communication, as quantum memories and as quantum transducers.

• The paper demonstrates the experimental generation and verification of non-classical photon-phonon correlations using a DLCZ-based protocol in a cryogenically cooled silicon resonator.
• It reports a photon-phonon correlation value up to 19.6 and a phonon-to-photon conversion efficiency of approximately 3.7%, marking a significant achievement in state preparation.
• The study highlights the engineered optomechanical coupling in nanobeam devices, paving the way for advanced quantum state transfer and integrated quantum networks.

Non-Classical Correlations in Optomechanics: Photon-Phonon Interactions

The interaction between single photons and phonons at the quantum level presents potential applications in quantum information technologies. The paper, authored by Ralf Riedinger et al., explores the generation and characterization of non-classical correlations between photons and phonons using a nanomechanical resonator. By demonstrating the ability to generate and manipulate photon-phonon pairs, this research marks a step towards integrating optomechanics in quantum computing and communication systems.

Summary of Results

The authors report the experimental observation of non-classical correlations between single photons and phonons, the quantized modes of vibration, in a silicon optomechanical system. This was achieved via a protocol based on the DLCZ (Duan-Lukin-Cirac-Zoller) scheme, generally utilized for atomic systems. The experiment involves initializing the mechanical system in its ground state using cryogenic cooling, followed by generating photon-phonon pairs using blue-detuned optical pulses. These pairs are subsequently read out using red-detuned pulses.

Central to the experiment is the violation of the Cauchy-Schwarz inequality, which confirms the non-classical nature of the generated mechanics state. The measured cross-correlation of the photon and phonon streams substantially exceeded this classical limit, demonstrating a joint quantum state between the two. Specifically, the authors recorded a photon-phonon correlation value of up to 19.6, which implies a significant efficiency in single-phonon state preparation.

Numerical and Technical Insights

The experiment achieved a phonon-to-photon conversion efficiency of about 3.7%, and the initial state was prepared with a phonon occupancy of 0.025. The mechanical resonator employed had an impressive quality factor of 1.1 million at cryogenic temperatures.

One notable aspect of the setup is the device’s engineered optical and mechanical modes, designed to maximize interaction rates. The use of a photonic crystal nanobeam permitted the achievement of significant optomechanical coupling rates, which is pivotal for efficient quantum state transfer. The coupling coefficient was calculated at 825 kHz, a testament to the device's design quality and fabrication precision.

Theoretical and Practical Implications

The practical implications of this research are expansive. Solid-state optomechanical devices with photon-phonon interfaces could be instrumental in quantum networks, where mechanical systems act as quantum memory or transducers, bridging disparate quantum systems. Moreover, such interfaces extend the potential for non-local quantum computing where processing is assisted by the transfer of quanta between distant nodes.

Theoretically, the paper enhances the understanding of macroscopic quantum phenomena, positioning mechanical systems as versatile players in the domain of hybrid quantum systems. The usage of BPA-free (Biphoton Annihilation-free) heralding schemes in optomechanics grants alternative avenues to photon-mediated state transfer without reliance on continuous strong optical drives, which often induce decoherence.

Future Directions

Future research may explore enhanced system integrations, with improved coupling rates and purity of phonon state preparation. Engineering advancements are required to achieve regimes where single-photon cooperativity is significantly improved. Moreover, further reduction in heating from optical sources will be essential in extending storage times and scaling the system for multi-node quantum networks.

In conclusion, the demonstration of non-classical correlations between photons and phonons enriches the field of optomechanics, promising a framework for reliable quantum state engineering and transfer in next-generation quantum systems. The research by Riedinger et al. underscores an essential step towards realizing complex hybrid quantum systems that may underpin future advancements in quantum communication and computation technologies.