Nano-structured optomechanical crystals (OMC) form an interface between mechanical modes with long coherence times and telecom optical photons, ideal for long-distance distribution of quantum information. However, the implementation of scalable quantum networks based on OMCs has been inhibited by thermal mechanical noise. Here, we overcome this limitation using a quasi-two-dimensional OMC and generate single photons via single phonon-photon conversion. In this work, we verify the low thermal noise and high purity of the generated single photons through a Hanbury Brown-Twiss experiment with g(2)(0)=0.35−0.08+0.10. We perform Hong-Ou-Mandel interference of the emitted photons showcasing the indistinguishability and coherence with visibility V = 0.52 ± 0.15 after 1.43 km fiber delay. Lastly, we use two-photon interference to measure the temporal wavepackets of optomechanically generated single photons demonstrating narrow bandwidths as low as 10 MHz. Our results pave the way for multinode quantum networks of mechanical oscillators and hybrid entanglement generation between mechanical oscillators and telecom quantum emitters.

Superfluid helium is a prototypical quantum liquid. As such, it has been a prominent platform for the study of quantum many body physics. More recently, the outstanding mechanical and optical properties of superfluid helium, such as low mechanical dissipation and low optical absorption, have positioned superfluid helium as a promising material platform in applications ranging from dark matter and gravitational wave detection to quantum computation. However, experiments with superfluid helium incur a high barrier to entry, as they require the incorporation of complex optical and electrical setups within a hermetically sealed cryogenic chamber to confine the superfluid. Here, we report on the design and construction of a helium chamber setup for operation inside a dilution refrigerator at millikelvin temperatures, featuring electrical and optical fiber access. By incorporating an automated gas handling system, we can precisely control the amount of helium gas inserted into the chamber, rendering our setup particularly promising for experiments with superfluid helium thin films, such as superfluid thin film optomechanics. Using silicon nanophotonic resonators, we demonstrate precise control and in situ tuning of the thickness of a superfluid helium film on the sub-nanometer level. By making use of the exceptional tunability of the superfluid film thickness, we demonstrate optomechanically induced phonon lasing of phononic crystal cavity third sound modes in the superfluid film and show that the lasing threshold crucially depends on the film thickness. The large internal volume of our chamber (V_chamber ≈ 1 l) is adaptable for the integration of various optical and electrical measurement and control techniques. Therefore, our setup provides a versatile platform for a variety of experiments in fundamental and applied superfluid helium research.

High-frequency phonons hold great promise as carriers of quantum information on chip and as quantum memories. Because of their coherent interaction with several systems, their compact mode volume, and slow group velocity, multiple experiments have recently demonstrated coherent transport of information on chip using phonon modes, interconnecting distinct quantum devices. Strongly confined phonons in waveguidelike geometries are particularly interesting because of their long lifetime. However, spectral diffusion has been observed to substantially limit their coherence times [S. M. Meenehan et al., Silicon optomechanical crystal resonator at millikelvin temperatures, Phys. Rev. A 90, 011803(R) (2014), A. Wallucks et al., A quantum memory at telecom wavelengths, Nat. Phys. 16, 772 (2020), and G. S. MacCabe et al., Nano-acoustic resonator with ultralong phonon lifetime, Science 370, 840 (2020)]. Coupling to two-level systems is suspected to be a major contributor to the diffusion; however, to date, the origin and underlying mechanisms are still not fully understood. Here, we perform a time-domain study on two adjacent mechanical modes (separated by around 5 MHz) and show that the frequency positions of the two modes are not correlated in time, in agreement with our theoretical model and Monte Carlo simulations. This result is an important step in fully understanding the microscopic mechanisms of dephasing in mechanical quantum buses and memories.

In recent years, nanomechanical oscillators in thin films of superfluid helium have attracted attention in the field of optomechanics due to their exceptionally low mechanical dissipation and optical scattering. Mechanical excitations in superfluid thin films - so-called third sound waves - can interact with the optical mode of an optical microresonator by modulation of its effective refractive index enabling optomechanical coupling. Strong confinement of third sound modes enhances their intrinsic mechanical nonlinearity paving the way for strong phonon-phonon interactions with applications in quantum optomechanics. Here, we realize a phononic crystal cavity confining third sound modes in a superfluid helium film to length scales close to the third sound wavelength. A few-nanometer-thick superfluid film is self-assembled on top of a silicon nanobeam optical resonator. The periodic patterning of the silicon material creates a periodic modulation of the superfluid film leading to the formation of a phononic band gap. By engineering the geometry of the silicon nanobeam, the phononic band gap allows the confinement of a localized phononic mode.