Cryogenic detectors are extremely sensitive and have a wide variety of applications (particularly in astronomy), but are difficult to integrate into large arrays like a modern CCD (charge-coupled device) camera. As current detectors of the cosmic microwave background (CMB) already have sensitivities comparable to the noise arising from the random arrival of CMB photons, the further gains in sensitivity needed to probe the very early Universe will have to arise from large arrays. A similar situation is encountered at other wavelengths. Single-pixel X-ray detectors now have a resolving power of DeltaE < 5 eV for single 6-keV photons, and future X-ray astronomy missions anticipate the need for 1,000-pixel arrays. Here we report the demonstration of a superconducting detector that is easily fabricated and can readily be incorporated into such an array. Its sensitivity is already within an order of magnitude of that needed for CMB observations, and its energy resolution is similarly close to the targets required for future X-ray astronomy missions.

The authors have measured noise in thin-film superconducting coplanar waveguide resonators. This noise appears entirely as phase noise, equivalent to a jitter of the resonance frequency. In contrast, amplitude fluctuations are not observed at the sensitivity of their measurement. The ratio between the noise power in the phase and amplitude directions is large, in excess of 30 dB. These results have important implications for resonant readouts of various devices such as detectors, amplifiers, and qubits. They suggest that the phase noise is due to two-level systems in dielectric materials. © 2007 American Institute of Physics. ͓DOI: 10.1063/1.2711770͔ Thin-film superconducting microwave resonators are of interest for a number of applications, including the multiplexed readout of single electron transistors, 1 microwave kinetic inductance detectors ͑MKIDs͒, 2,3 normal metalinsulator-superconductor tunnel junction detectors, 4 superconducting quantum interference devices, 5,6 and qubits. 7,8 The device to be measured presents a variable dissipative or reactive load to the resonator, influencing the resonator quality factor Q r or frequency f r , respectively. Changes to both Q r and f r may be determined simultaneously by sensing the amplitude and phase of a microwave probe signal. 2 While several early demonstrations used hand-assembled lumpedelement circuits, 1,4,5 frequency-domain multiplexing of large arrays generally will require compact microlithographed high-Q r resonators. 1 Such resonators are also needed for strong coupling to charge qubits. 7 Noise in microlithographed resonators has been observed 2,3 and can be a limiting factor for device performance but is not well understood. In this letter, we report measurements of resonator noise, show how the noise spectra separate into amplitude and phase components, and discuss the physical origin of the noise.We studied quarter-wavelength coplanar waveguide ͑CPW͒ resonators 2 ͓Fig. 1͑a͔͒ with center strip widths w of 0.6-6 m and gaps g between the center strip and ground planes of 0.4-4 m, and with impedances Z 0 Ϸ 50 ⍀. Resonator lengths of 3 -7 mm produce resonance frequencies f r between 4 and 10 GHz. Frequency multiplexed arrays of up to 100 resonators are coupled to a single CPW feedline. The CPW circuits are patterned from a film of either Al ͑T c = 1.2 K͒ or Nb ͑T c = 9.2 K͒ on a crystalline substrate, either sapphire, Si, or Ge. The surfaces of the semiconductor substrates are not intentionally oxidized, although a native oxide due to air exposure is expected to be present.A microwave synthesizer at frequency f is used to excite a resonator. The transmitted signal is amplified with a cryogenic high electron mobility transistor ͑HEMT͒ amplifier and is compared to the original signal using an IQ mixer, whose output voltages I and Q are proportional to the in-phase and quadrature amplitudes of the transmitted signal 2,3 ͑see Fig. 2 inset͒. As f is varied, the output = ͓I , Q͔ T ͑the superscript T represents the transpose͒ traces out a resonance circle ͓Fi...

We present measurements of the temperature-dependent frequency shift of five niobium superconducting coplanar waveguide microresonators with center strip widths ranging from 3 to 50 m, taken at temperatures in the range of 100-800 mK, far below the 9.2 K transition temperature of niobium. These data agree well with the two-level system ͑TLS͒ theory. Fits to this theory provide information on the number of TLSs that interact with each resonator geometry. The geometrical scaling indicates a surface distribution of TLSs and the data are consistent with a TLS surface layer thickness of the order of a few nanometers, as might be expected for a native oxide layer. © 2008 American Institute of Physics. ͓DOI: 10.1063/1.2906373͔Superconducting microresonators have attracted substantial interest for low temperature detector applications due to the possibility of large-scale microwave frequency multiplexing.1-7 Such resonators are also being used in quantum computing experiments [8][9][10] and for sensing nanomechanical motion. 11 We previously reported that excess frequency noise is universally observed in these resonators and suggested that two-level systems ͑TLSs͒ in dielectric materials 14,15 may be responsible for this noise.12 TLS effects are also observed in superconducting qubits.9 The TLS hypothesis is strongly supported by the observed temperature dependence of the noise and also by the observation of temperature-dependent resonance frequency shifts that closely agree with the TLS theory. 13 To make further progress, it is essential to constrain the location of the TLSs, to determine whether they exist in the bulk substrate or in surface layers, perhaps oxides on the exposed metal or substrate surfaces, or in the interface layers between the metal films and the substrate. In this paper, we provide direct experimental evidence for a surface distribution of TLSs.TLSs are abundant in amorphous materials 14,15 and have electric dipole moments that couple to the electric field E ជ of our resonators. For microwave frequencies and at temperatures T between 100 mK and 1 K, the resonant interaction dominates over relaxation, which leads to a temperaturedependent variation of the dielectric constant given bywhere is the frequency, ⌿ is the complex digamma function, and ␦ = Pd 2 / 3⑀ represents the TLS-induced dielectric loss tangent at T = 0 for weak nonsaturating fields. Here, P and d are the two-level density of states and dipole moment, as introduced by Phillips. 16 Equation ͑1͒ has been extensively used to derive values of Pd 2 in amorphous materials. If TLSs are present in superconducting microresonators, their contribution to the dielectric constant described by Eq. ͑1͒ could be observable as a temperature-dependent shift in the resonance frequency. Indeed, it has recently been suggested that the small anomalous low-temperature frequency shifts often observed in superconducting microresonators may be due to TLS effects, 17,18 and, in fact, excellent fits to the TLS theory can be obtained. 13 Assuming that the TLSs ar...

Amplifiers are ubiquitous in electronics and play a fundamental role in a wide range of scientific measurements. From a user's perspective, an ideal amplifier has very low noise, operates over a broad frequency range, and has a high dynamic range -it is capable of handling strong signals with little distortion.Unfortunately, it is difficult to obtain all of these characteristics simultaneously.For example, modern transistor amplifiers offer multi-octave bandwidths and excellent dynamic range. However, their noise remains far above the fundamental limit set by the uncertainty principle of quantum mechanics.[1] Parametric amplifiers, which predate transistor amplifiers and are widely used in optics, exploit a nonlinear response to transfer power from a strong pump tone to a weak signal.If the nonlinearity is purely reactive, i.e. nondissipative, in theory the amplifier noise can reach the quantum-mechanical limit.[2] Indeed, microwave frequency superconducting Josephson parametric amplifiers [3, 4] do approach the quantum limit, but generally are narrow band and have very limited dynamic range. In this paper, we describe a superconducting parametric amplifier that overcomes these limitations. The amplifier is very simple, consisting only of a patterned metal film on a dielectric substrate, and relies on the nonlinear kinetic inductance of a superconducting transmission line. We measure gain extending over 2 GHz on either side of an 11.56 GHz pump tone, and we place an upper limit 1 arXiv:1201.2392v1 [cond-mat.supr-con] 11 Jan 2012 on the added noise of the amplifier of 3.4 photons at 9.4 GHz. Furthermore, the dynamic range is very large, comparable to microwave transistor amplifiers, and the concept can be applied throughout the microwave, millimeter-wave and submillimeter-wave bands.Over the past decade, the combination of high-performance superconducting microresonators and low-noise, microwave frequency cryogenic transistor amplifier readouts has proven to be particularly powerful for a wide range of applications including photon detection and quantum information experiments. [5][6][7] These developments have generated strong renewed interest in superconducting amplifiers that achieve even lower readout noise. [8][9][10][11][12].Most of these devices are parametric amplifiers that make use of the nonlinear inductance of the Josephson junction, which is almost ideally reactive with little dissipation below the critical current I c . As a result, Josephson paramps can be exquisitely sensitive, approaching the standard quantum limit of half a photon ω/2 of added noise power per unit bandwidth in the standard case when both quadratures of a signal at frequency ω are amplified equally.Here is Planck's constant divided by 2π. Even less noise is possible in situations when only one quadrature is amplified. [1]. In comparison, the added noise of cryogenic transistor amplifiers is typically 10-20 times the quantum limit.[13] However, the dynamic range of Josephson paramps is regulated by the Josephson energy E J = I c ...

Titanium nitride (TiN x ) films are ideal for use in superconducting microresonator detectors because: a) the critical temperature varies with composition (0 < T c < 5 K); b) the normal-state resistivity is large, ρ n ∼ 100 µΩ cm, facilitating efficient photon absorption and providing a large kinetic inductance and detector responsivity; and c) TiN films are very hard and mechanically robust. Resonators using reactively sputtered TiN films show remarkably low loss (Q i > 10 7 ) and have noise properties similar to resonators made using other materials, while the quasiparticle lifetimes are reasonably long, 10−200 µs. TiN microresonators should therefore reach sensitivities well below 10 −19 W Hz −1/2 .