Material-based two-level systems (TLSs), appearing as defects in low-temperature devices including superconducting qubits and photon detectors, are difficult to characterize. In this study we apply a uniform dc-electric field across a film to tune the energies of TLSs within. The film is embedded in a superconducting resonator such that it forms a circuit quantum electrodynamical (cQED) system. The energy of individual TLSs is observed as a function of the known tuning field. By studying TLSs for which we can determine the tunneling energy, the actual pz, dipole moments projected along the uniform field direction, are individually obtained. A distribution is created with 60 pz. We describe the distribution using a model with two dipole moment magnitudes, and a fit yields the corresponding values p = p1 = 2.8 ± 0.2 Debye and p = p2 = 8.3 ± 0.4 Debye. For a strong-coupled TLS the vacuum-Rabi splitting can be obtained with pz and tunneling energy. This allows a measurement of the circuit's zero-point electric field fluctuations, in a method that does not need the electric-field volume.PACS numbers: 03.67.-a, 03.67. Pp, 33.15.Kr, 66.35.+a Dielectric two-level systems (TLSs) have attracted the attention of the quantum computing community ever since they were identified as a major source of decoherence in superconducting qubits [1]. Subsequent studies found that TLSs were also a performance-limiting source of noise in photon detectors used for astronomy [2,3]. This motivation has led to quantum characterization of TLSs in the tunneling barrier of superconducting qubits [1,4] and both noise and loss characterization in high quality superconducting resonator circuits. Thus TLSs are found as defects in various dielectric structures: deposited insulating films [1,5,6], Josephson Junction (JJ) tunneling barriers [1,7,8], imperfect interfaces between superconducting films with crystalline substrates [9], and the native oxides on materials [10]. Recent modeling has predicted possible structures and values for the TLS dipole moment [11][12][13]. While these TLSs are generally known to be charged atomic configurations that spatially tunnel, their microscopic structure and elemental composition are generally unknown.In a qubit made from an anharmonic oscillator the interaction energy is observed as a spectroscopic splitting in the qubit state. However, this quantity is not only dependent on circuit element parameters, but also on the thickness of the tunneling barrier [1]. The barrier has a thickness variation of 1-2 nm [14], such that this lack of accuracy is also present in the electric field amplitude and the measured TLS dipole moment. In general, individual measurements of TLSs within JJs find the transition dipole moment p tr = p z ∆ 0 /E, where p z ≡ |p z | is the absolute value of the dipole moment projected in the field direction, and the TLS tunneling energy over the total energy ∆ 0 /E is generally unknown. As a result only a lower bound of p z is determined from those measurements (∆ 0 /E ≤ 1). However, in studie...

In the quasi two-dimensional GaAs/AlGaAs system, we investigate the effect of rotating in-situ the electric field of linearly polarized microwaves relative to the current, on the microwave-radiationinduced magneto-resistance oscillations. We find that the frequency and the phase of the photoexcited magneto-resistance oscillations are insensitive to the polarization. On the other hand, the amplitude of the resistance oscillations are remarkably responsive to the relative orientation between the microwave antenna and the current-axis in the specimen. The results suggest a striking linear polarization sensitivity in the radiation-induced magnetoresistance oscillations.

Light-matter coupling is investigated by rotating, by an angle θ, the polarization of linearly polarized microwaves with respect to the long-axis of GaAs/AlGaAs Hall-bar electron devices. At low microwave power, P , experiments show a strong sinusoidal variation in the diagonal resistance Rxx vs. θ at the oscillatory extrema, indicating a linear polarization sensitivity in the microwave radiation-induced magnetoresistance oscillations. Surprisingly, the phase shift θ0 for maximal oscillatory Rxx response under photoexcitation appears dependent upon the radiation-frequency f , the extremum in question, and the magnetic field orientation or sgn(B).

We examine and compare the diagonal magnetoresistance, R xx , and the photo-voltage induced by microwave (42 ≤ f < 300GHz) and terahertz (f ≥ 300GHz) photoexcitation in the high mobility quasi two-dimensional GaAs/AlGaAs system. The data demonstrate strong radiation-induced magneto-resistance oscillations in R xx to 360GHz. In addition, cyclotron resonance is observed in the photo-voltage to 725GHz. These results show that our high mobility GaAs/AlGaAs 2DES specimens remain photo-active in magnetotransport into the terahertz band.

Random tunneling two-level systems (TLSs) in dielectrics have been of interest recently because they adversely affect the performance of superconducting qubits. The coupling of TLSs to qubits has allowed individual TLS characterization, which has previously been limited to TLSs within (thin) Josephson tunneling barriers made from aluminum oxide. Here we report on the measurement of an individual TLS within the capacitor of a lumped-element LC microwave resonator, which forms a cavity quantum electrodynamics (CQED) system and allows for individual TLS characterization in a different structure and material than demonstrated with qubits. Due to the reduced volume of the dielectric (80 µm 3 ), even with a moderate dielectric thickness (250 nm), we achieve the strong coupling regime as evidenced by the vacuum Rabi splitting observed in the cavity spectrum. A TLS with a coherence time of 3.2 µs was observed in a film of silicon nitride as analyzed with a Jaynes-Cummings spectral model, which is larger than seen from superconducting qubits. As the drive power is increased, we observe an unusual but explicable set of continuous and discrete crossovers from the vacuum Rabi split transitions to the Glauber (coherent) state.