Wide-band parametric amplifier readout and resolution of optical microwave kinetic inductance detectors

Abstract

The energy resolution of a single photon counting Microwave Kinetic Inductance Detector (MKID) can be degraded by noise coming from the primary low temperature amplifier in the detector's readout system. Until recently, quantum limited amplifiers have been incompatible with these detectors due to dynamic range, power, and bandwidth constraints. However, we show that a kinetic inductance based traveling wave parametric amplifier can be used for this application and reaches the quantum limit. The total system noise for this readout scheme was equal to ∼2.1 in units of quanta. For incident photons in the 800 to 1300 nm range, the amplifier increased the average resolving power of the detector from ∼6.7 to 9.3 at which point the resolution becomes limited by noise on the pulse height of the signal. Noise measurements suggest that a resolving power of up to 25 is possible if redesigned detectors can remove this additional noise source.Optical MKIDs 1 are superconducting, single photon counting, energy resolving sensors which are sensitive to radiation in the ultraviolet to near infrared range. Advantages over semiconductor detectors in this wavelength band include the absence of false counts (read noise, dark current, and cosmic rays), intrinsic spectral resolution, high speed, and radiation hardness. Other superconducting detectors have shown promise at these wavelengths, 2,3 but they are difficult to chain together into large arrays. Optical MKIDs, however, are naturally frequency domain multiplexed, which has enabled full-scale instruments at the Palomar observatory 4,5 and Subaru telescope. 6 In the future, these detectors will be included in a balloon borne mission. 7 Photon counting MKIDs operate differently than MKIDs designed for longer wavelength detection in the bolometric regime. Instead of measuring a constant flux of photons, they record individual photon events similarly to an X-ray calorimeter. To achieve a measurable detector response for a single photon event they tend to be smaller and able to handle less signal power than their longer wavelength bolometric counterparts. In these conditions, amplifier noise can be comparable in magnitude to the detector phase noise that originates from microscopic two-level system (TLS) states on the surface or between layers of the device. 8 The TLS noise can be mitigated through careful sample preparation, 9 fabrication, 10,11 and device design 12-14 while the effect of amplifier noise can be addressed by designing detectors that can handle higher signal powers. 15,16 These routes are actively pursued, but, for optical MKIDs, improving the main readout amplifier's noise floor offers an additional path to lowering the total system noise.Quantum mechanics imposes an uncertainty relationship between the two quadratures of an electromagnetic signal. 17 This relationship results in a lower limit to the noise that a high gain, phase-preserving, linear amplifier adds to its input signal, equal to that of the electromagnetic zero-point fluctuations (A = 1 /2). To readout a...

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Second generation readout for large format photon counting microwave kinetic inductance detectors

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