Surface acoustic wave unidirectional transducers for quantum applications

Abstract

The conversion efficiency of electric microwave signals into surface acoustic waves in different types of superconducting transducers is studied with the aim of quantum applications. We compare delay lines containing either conventional symmetric transducers (IDTs) or unidirectional transducers (UDTs) at 2.3 GHz and 10 mK. The UDT delay lines improve the insertion loss with 4.7 dB and a directivity of 22 dB is found for each UDT, indicating that 99.4 % of the acoustic power goes in the desired direction. The power lost in the undesired direction accounts for more than 90 % of the total loss in IDT delay lines, but only ∼3 % percent of the total loss in the FEUDT delay lines.Surface acoustic waves (SAWs) are Rayleigh waves propagating on the surface of a solid [1]. It has recently been suggested [2] and shown [3] that SAWs can interact with artificial atoms at the quantum level. This is fundamentally interesting because the artificial atoms can be made much larger than the wavelength of the SAW, which is not possible in other systems [4]. There are extensive new possibilities for quantum devices utilizing SAW; such as resonators [5,6], absorption in double quantum dots [7], transport of quantum information [8][9][10] and phonon assisted tunneling [11].When SAWs are used to carry quantum information, it is important to have low losses. The purpose here is to lower the conversion loss between electric signals (photons) and SAWs (phonons). In all studies about quantum SAW applications, SAWs are converted to and from electric microwave signals using conventional symmetric interdigital transducers (IDTs). The IDT can be described by a three port scattering matrix, where port 1 and 2 are acoustic and port 3 is electric [12]. It has the same electric to SAW conversion in both ports, i.e. S 13 = S 23 , and hence 50 % of the power is converted in the wrong direction. This means that IDTs are limited by a theoretical minimum insertion loss of -3 dB and because of reciprocity delay lines with two IDTs are theoretically limited to -6 dB.Unlike the symmetric IDT, a unidirectional transducer (UDT) [13,14] can be optimized to release most of its SAW energy in one preferred direction, by maximizing the scattering element S 13 while minimizing S 23 . In this way UDTs can exceed the -3 dB loss, and therefore UDTs are interesting to study for quantum SAW applications.

Cavity-free vacuum-Rabi splitting in circuit quantum acoustodynamics

“…However, it may be advantageous to use a strongly piezoelectric material, such as lithium niobate (LiNbO 3 ), in order to increase the coupling strength between the atom and the substrate. Additionally, any auxiliary IDTs used to send or receive SAWs would benefit from the increased conversion efficiency [20]. Under the assumption that the IDTs should be impedance matched (typically to 50 Ω), they would also have a larger bandwidth on a stronger piezoelectric.…”

Giant acoustic atom: A single quantum system with a deterministic time delay

“…However, the recent work in Ref. [58] demonstrated unidirectional transducers (UDTs), which can increase the conversion efficiency up to 99.4% at GHz frequencies and millikelvin temperatures. From a technical perspective, it is of interest whether the diagrammatic Lippmann-Schwinger approach can be extended to scattering with more than two phonons.…”

Towards phonon routing: controlling propagating acoustic waves in the quantum regime

“…In transmission, a continuous signal was sent to IDT A where it was partly electrically reflected due to impedance mismatch and partly transduced into SAWs in both directions and the right propagating waves were measured at IDT B. The measured signal in the frequency domain has been Fourier-transformed into the time domain, where the different parts of the signal could be distinguished due to the slow speed of the SAWs and the sufficient distances between the IDTs and the transmon [35,36]. For instance in the reflection measurements, the signal reflected from the transmon is separated from the purely electric reflection due to impedance mismatch and from triple transits by about 200ns (two times the distance between IDT A and the transmon).…”