Search

We describe a helium source cell for use in cryogenic experiments that is hermetically sealed in situ on the cold plate of a cryostat. The source cell is filled with helium gas at room temperature and, subsequently, sealed using a cold weld crimping tool before the cryostat is closed and cooled down. At low temperatures, the helium condenses and collects in a connected experimental volume, as monitored via the frequency response of a planar superconducting resonator device sensitive to small amounts of liquid helium. This on-cryostat helium source negates the use of a filling tube between the cryogenic volumes and room temperature, thereby preventing unwanted effects such as temperature instabilities that arise from the thermomechanical motion of helium within the system. This helium source can be used in experiments investigating the properties of quantum fluids or to better thermalize quantum devices. [ABSTRACT FROM AUTHOR]

Transverse bulk phonons in a multimode integrated quantum acoustic device are excited and characterized via their free-space coupling to a three-dimensional (3D) microwave cavity. These bulk acoustic modes are defined by the geometry of the Y-cut lithium niobate substrate in which they reside and couple to the cavity electric field via a large dipole antenna, with an interaction strength on the order of the cavity linewidth. Using finite element modeling, we determine that the bulk phonons excited by the cavity field have a transverse polarization with a shear velocity matching previously reported values. We demonstrate how the coupling between these transverse acoustic modes and the electric field of the 3D cavity depends on the relative orientation of the device dipole, with a coupling persisting to room temperature. Our study demonstrates the versatility of 3D microwave cavities for mediating contact-less coupling to quantum, and classical, piezoacoustic devices. [ABSTRACT FROM AUTHOR]

Phonons, the ubiquitous quanta of vibrational energy, play a vital role in the performance of quantum technologies. Conversely, unintended coupling to phonons degrades qubit performance and can lead to correlated errors in superconducting qubit systems. Regardless of whether phonons play an enabling or deleterious role, they do not typically admit control over their spectral properties, nor the possibility of engineering their dissipation to be used as a resource. Here we show that coupling a superconducting qubit to a bath of piezoelectric surface acoustic wave phonons enables a novel platform for investigating open quantum systems. By shaping the loss spectrum of the qubit via the bath of lossy surface phonons, we demonstrate preparation and dynamical stabilization of superposition states through the combined effects of drive and dissipation. These experiments highlight the versatility of engineered phononic dissipation and advance the understanding of mechanical losses in superconducting qubit systems. Hybrid quantum acoustic systems integrating qubits with phonons offer a novel platform for investigating open quantum systems. Kitzman et al. report control of superposition states of a transmon qubit under the effect of drive and dissipation by engineering its coupling to a bath of surface acoustic wave phonons. [ABSTRACT FROM AUTHOR]

We demonstrate a flip-chip device for performing low-temperature acoustoelectric measurements on exfoliated two-dimensional materials. With this device, we study gate-tunable acoustoelectric transport in an exfoliated monolayer graphene device, measuring the voltage created as high-frequency surface acoustic waves dynamically drive the graphene charge carriers, the density of which we simultaneously control with a silicon back-gate. We demonstrate ambipolar dependence of the acoustoelectric signal, as expected from the sign of the graphene charge carriers. We observe a marked reduction in the magnitude of the acoustoelectric signal over a well-defined range of density in the vicinity of charge neutrality, which we attribute to a spatially heterogeneous charge-disorder landscape not directly revealed by conventional transport measurements. [ABSTRACT FROM AUTHOR]

Recent experiments on the coupling of the in-plane motional state of electrons floating on the surface of liquid helium to a microwave resonator have revealed the importance of helium surface fluctuations to the coherence of this motion. Here we investigate these surface fluctuations by studying the resonance properties of a superconducting coplanar waveguide (CPW) resonator filled with superfluid helium, where a significant fraction of the resonator's electromagnetic mode volume is coupled to the surface dynamics of the liquid. We present preliminary results on real-time CPW resonator frequency shifts driven by helium fluctuations, which are quantified via their power spectral density and compared with measurements using a commercial accelerometer. We find that a considerable contribution to the CPW resonator noise originates from the mechanical vibrations of the helium surface generated by the pulse tube (PT) cryocooler on the cryostat on which the experiments were performed. [ABSTRACT FROM AUTHOR]

Circuit quantum electrodynamics (cQED) experiments on superconducting qubit systems typically employ radiation shields coated in photon absorbing materials to achieve high qubit coherence and low microwave resonator losses. In this work, we present preliminary results on the performance of Vantablack as a novel infrared (IR) shielding material for cQED systems. We compare the coherence properties and residual excited state population (or effective qubit temperature) of a single-junction transmon qubit housed in a shield coated with a standard epoxy-based IR absorbing material, i.e. Berkeley Black, to the coherence and effective temperature of the same qubit in a shield coated in Vantablack. Based on a statistical analysis of multiple qubit coherence measurements, we find that the performance of the radiation shield coated with Vantablack is comparable in performance to the standard coating. However, we find that in the Vantablack coated shield the qubit has a higher effective temperature. These results indicate that improvements are likely required to optimize the performance of Vantablack as an IR shielding material for superconducting qubit experiments and we discuss possible routes for such improvements. Finally, we describe possible future experiments to more precisely quantify the performance of Vantablack to improve the coherences of more complex cQED systems. [ABSTRACT FROM AUTHOR]

We report on low-temperature noise measurements of a single electron transistor (SET) immersed in superfluid 4He. The device acts as a charge sensitive electrometer able to detect the fluctuations of charged defects in close proximity to the SET. In particular, we measure telegraph switching of the electric current through the device originating from a strongly coupled individual two-level fluctuator. By embedding the device in a superfluid helium immersion cell, we are able to systematically control the thermalizing environment surrounding the SET and investigate the effect of the superfluid on the SET noise performance. We find that the presence of superfluid 4He can strongly suppress the switching rate of the defect by cooling the surrounding phonon bath. [ABSTRACT FROM AUTHOR]

Piezoelectric surface acoustic waves (SAWs) are powerful for investigating and controlling elementary and collective excitations in condensed matter. In semiconductor two-dimensional electron systems SAWs have been used to reveal the spatial and temporal structure of electronic states, produce quantized charge pumping, and transfer quantum information. In contrast to semiconductors, electrons trapped above the surface of superfluid helium form an ultra-high mobility, two-dimensional electron system home to strongly-interacting Coulomb liquid and solid states, which exhibit non-trivial spatial structure and temporal dynamics prime for SAW-based experiments. Here we report on the coupling of electrons on helium to an evanescent piezoelectric SAW. We demonstrate precision acoustoelectric transport of as little as ~0.01% of the electrons, opening the door to future quantized charge pumping experiments. We also show SAWs are a route to investigating the high-frequency dynamical response, and relaxational processes, of collective excitations of the electronic liquid and solid phases of electrons on helium. Hybrid devices based on electrons on helium may find application in quantum devices. Here the authors demonstrate surface acoustic wave driven acoustoelectric transport of electrons on superfluid helium. [ABSTRACT FROM AUTHOR]

We report on the observation of an anomalously high attenuation of high-frequency surface acoustic waves by thin films of liquid 4 He. The piezoelectric acoustic waves propagate along the surface of a lithium niobate substrate, which is coated with varying amounts of liquid helium. When the thickness of the helium layer is much larger than the wavelength of the surface acoustic wave on the substrate, its attenuation is dominated by the excitation of compressional waves into the liquid, in good agreement with theory and previous measurements. However, for sufficiently thin helium coverage, we find that the acoustic wave attenuation is significantly increased beyond that measured with the substrate submerged in bulk liquid. Possible mechanisms for this enhanced attenuation are discussed. [ABSTRACT FROM AUTHOR]

Superfluid He-A in a fully characterized stretched aerogel, used in previous work by Pollanen et al., has been studied for parallel and perpendicular orientations of the magnetic field relative to the anisotropy axis of the aerogel. Consistently, we find that an equal spin pairing state (ESP) is stabilized down to the lowest temperature. From our pulsed NMR frequency shifts as a function of temperature and tip angle, the orientation of the orbital angular momentum $\hat{l}$ has been determined. The aerogel anisotropy introduced by uniaxial stretching tends to align $\hat{l}$ in the axial state parallel to the strain axis, consistent with the theory proposed by Sauls and contrary to Volovik's prediction based on an impurity calculation of Rainer and Vuorio. [ABSTRACT FROM AUTHOR]

It is established theoretically that an ordered state with continuous symmetry is inherently unstable to arbitrarily small amounts of disorder. This principle is of central importance in a wide variety of condensed systems including superconducting vortices, Ising spin models and their dynamics, and liquid crystals in porous media, where some degree of disorder is ubiquitous, although its experimental observation has been elusive. On the basis of these ideas, it was predicted that 3He in high-porosity aerogel would become a superfluid glass. We report here our nuclear magnetic resonance measurements on 3He in aerogel demonstrating destruction of long-range orientational order of the intrinsic superfluid orbital angular momentum, confirming the existence of a superfluid glass. In contrast, 3He-A generated by warming from superfluid 3He-B has perfect long-range orientational order providing a mechanism for switching offthis effect. [ABSTRACT FROM AUTHOR]

We have performed optical birefringence measurements on isotropic aerogel samples grown with varying amounts of ammonia catalyst to determine the relationship between stress, strain, and transmitted light intensity, focusing on the regime of relatively small axial strain. We find that our samples exhibit a delayed onset of birefringence at ∼3 % strain, beyond which the transmitted intensity increases linearly, with a faster rate of increase for samples prepared with larger catalyst concentrations. We have also calculated the sound velocity from the slope of the stress-strain curve, and found an approximately linear increase with catalyst solution pH for samples of similar density. Samples of similar density also appear to exhibit a universal relationship between applied stress and transmitted intensity. We are not able to identify the mechanism for this unique behavior. [ABSTRACT FROM AUTHOR]

We have studied the acoustic Faraday effect in superfluid He up to significantly larger magnetic fields than in previous experiments achieving rotations of the polarization of transverse sound as large as 1710°. We report nonlinear field effects, and use the linear results to determine the Zeeman splitting of the imaginary squashing mode (ISQ) frequency in He- B. [ABSTRACT FROM AUTHOR]

A rich variety of Fermi systems condense by forming bound pairs, including high-temperature and heavy-fermion superconductors, Sr2RuO4 (ref. ), cold atomic gases and superfluid 3He (ref. ). Some of these form exotic quantum states with non-zero orbital angular momentum. We have discovered, in the case of 3He, that anisotropic disorder, engineered from highly porous silica aerogel, stabilizes a chiral superfluid state that otherwise would not exist. Furthermore, we find that the chiral axis of this state can be uniquely oriented with the application of a magnetic field perpendicular to the aerogel anisotropy axis. At sufficiently low temperature we observe a sharp transition from a uniformly oriented chiral state to a disordered structure consistent with locally ordered domains, contrary to expectations for a superfluid glass phase. [ABSTRACT FROM AUTHOR]

We present results of the Q and period shift, Δ P, for He confined in a 98% nominal open aerogel on a torsion pendulum. The aerogel is compressed uniaxially by 10% along a direction aligned to the torsion pendulum axis and was grown within a 400 μm tall pancake (after compression) similar to an Andronikashvili geometry. The result is a high Q pendulum able to resolve Q and mass coupling of the impurity-limited He over the whole temperature range. After measuring the empty cell background, we filled the cell above the critical point and observe a temperature dependent period shift, Δ P, between 100 mK and 3 mK that is 2.9% of the period shift (after filling) at 100 mK. The Q due to the He decreases by an order of magnitude between 100 mK and 3 mK at a pressure of 0.14±0.03 bar. We compare the observable quantities to the corresponding calculated Q and period shift for bulk He. [ABSTRACT FROM AUTHOR]

Collective modes are the fingerprint of a condensed phase. The spectroscopy of these modes in superfluid 3He and unconventional superconductors can provide key information on the symmetry of the condensate as well as the microscopic pairing mechanism responsible for the ground state and excitation energies. Here, we use interferometry within an acoustic cavity—which is very sensitive to changes in the velocity of transverse sound—to reveal a new collective mode in the B phase of superfluid 3He (3He-B). We identify the mode as an excited bound state of Cooper pairs, which is weakly bound with an excitation energy within 1% of the pair-breaking edge. On the basis of the selection rules for coupling of transverse sound to a collective mode in 3He-B, combined with the observation of acoustic birefringence near the collective mode frequency, we infer that the new mode is most likely a spin-triplet (S=1), f-wave pair exciton (orbital momentum L=3) with total angular momentum, J=4. The existence of a pair exciton with J=4 suggests an attractive, subdominant, f-wave pairing interaction in liquid 3He. [ABSTRACT FROM AUTHOR]

In order to study antiferromagnetic order in the unconventional superconductor UPt3, we have measured the magnetoresistance of high-quality bulk single crystals over a broad temperature range. We observe a linear magnetoresistance when the magnetic field is applied in the basal plane for temperatures below ≈5K where magnetic Bragg scattering has been observed. However, when we apply a magnetic field along the c-axis, we find a negative magnetoresistance that appears below a characteristic field, H0 ≈ 2 T, which we associate with suppression of spin-dependent scattering and which changes abruptly near 5 K. [ABSTRACT FROM AUTHOR]