Aims. Future astrophysics and cosmic microwave background space missions operating in the far-infrared to millimetre part of the spectrum will require very large arrays of ultra-sensitive detectors in combination with high multiplexing factors and efficient lownoise and low-power readout systems. We have developed a demonstrator system suitable for such applications. Methods. The system combines a 961 pixel imaging array based upon Microwave Kinetic Inductance Detectors (MKIDs) with a readout system capable of reading out all pixels simultaneously with only one readout cable pair and a single cryogenic amplifier. We evaluate, in a representative environment, the system performance in terms of sensitivity, dynamic range, optical efficiency, cosmic ray rejection, pixel-pixel crosstalk and overall yield at an observation centre frequency of 850 GHz and 20% fractional bandwidth. Results. The overall system has an excellent sensitivity, with an average detector sensitivity NEP det = 3 × 10 −19 W/ √ Hz measured using a thermal calibration source. At a loading power per pixel of 50 fW we demonstrate white, photon noise limited detector noise down to 300 mHz. The dynamic range would allow the detection of ∼1 Jy bright sources within the field of view without tuning the readout of the detectors. The expected dead time due to cosmic ray interactions, when operated in an L2 or a similar far-Earth orbit, is found to be <4%. Additionally, the achieved pixel yield is 83% and the crosstalk between the pixels is <−30 dB. Conclusions. This demonstrates that MKID technology can provide multiplexing ratios on the order of a 1000 with state-of-the-art single pixel performance, and that the technology is now mature enough to be considered for future space based observatories and experiments.

We demonstrate high-contrast state detection of a superconducting flux qubit. Detection is realized by probing the microwave transmission of a nonlinear resonator, based on a SQUID. Depending on the driving strength of the resonator, the detector can be operated in the monostable or the bistable mode. The bistable operation combines high-sensitivity with intrinsic latching. The measured contrast of Rabi oscillations is as high as 87%; of the missing 13%, only 3% of the loss of contrast is unaccounted for. Experiments involving two consecutive detection pulses are consistent with preparation of the qubit state by the first measurement. DOI: 10.1103/PhysRevLett.96.127003 PACS numbers: 85.25.Dq, 03.67.Lx, 85.25.Cp Superconducting qubits [1] have been established as promising candidates for the implementation of a quantum information processor [2]. Remarkable achievements in this field include the realization of complex single-qubit manipulation schemes [3] and the generation of entangled two-qubit states [4,5].For qubit readout, several detectors have been investigated experimentally. In general, their efficiency is relatively poor, for reasons which are currently not well understood. Most detectors to date rely on irreversible processes in mesoscopic Josephson circuits [6 -10]. For these schemes, energy is dissipated on the chip where the qubit is placed. Therefore, long waiting times are necessary to bring the qubit, readout, and control circuits to their proper initial state. In particular, the qubit state is strongly disturbed. More recently, dispersive measurement schemes are being investigated, which overcome these drawbacks [11][12][13][14][15]. They are based on the measurement of the impedance of a resonator coupled to the qubit. The energy used to probe this resonator is mostly dissipated at a place remote from the qubit chip. Also, the backaction on the qubit is low. However, the qubit relaxation times are typically comparable to the time necessary for a reliable measurement of the impedance. This limits the detection efficiency when a linear resonator is used [12,13]. With a nonlinear resonator, this limitation can be removed, by using a bifurcation transition [16]. In this case, the result of a qubit measurement is either of two possible oscillation states of the driven resonator. These oscillation states can be latched and reliably discriminated irrespective of subsequent relaxation of the qubit. A dispersive method using latching was successfully demonstrated in Ref. [14], where it was used for the readout of a quantronium.In this Letter we present experimental results on the readout of a superconducting flux qubit using a dispersive method. We observed coherent oscillations in two distinct operation modes of the detector: monostable for weak driving and bistable for strong driving of our nonlinear resonator. In the bistable regime the measurement contrast is very large, 87%, which is a significant improvement over previous measurements [6 -10,13,14]. We also performed consecutive measurements of...

We probe the effects of strong disorder (2.4

We observed the dynamics of a superconducting flux qubit coupled to an extrinsic quantum system ͑EQS͒. The presence of the EQS is revealed by an anticrossing in the spectroscopy of the qubit. The excitation of a two-photon transition to the third excited state of the qubit-EQS system allows us to extract detailed information about the energy-level structure and the coupling of the EQS. We deduce that the EQS is a two-level system, with a transverse coupling to the qubit. The transition frequency and the coupling of the EQS changed during experiments, which supports the idea that the EQS is a two-level system of microscopic origin. DOI: 10.1103/PhysRevB.80.172506 PACS number͑s͒: 85.25.Cp, 03.67.Lx, 03.65.Yz Superconducting qubits are artificial quantum systems that consist of microfabricated circuits including Josephson junctions. Research on these systems is motivated both by the perspective of quantum computing 1,2 and by the fact that they are model systems for fundamental studies in quantum mechanics. [3][4][5][6][7] Decoherence of superconducting qubits is an example of such a topic, relevant both for quantum computing and for understanding the dynamics of open quantum systems.We report experiments on a superconducting flux qubit, where spectroscopic measurements show that the qubit is coupled to an extrinsic quantum system ͑EQS͒. Similar observations have been reported for superconducting phase qubits, [8][9][10] where EQSs have been identified as two-level systems ͑TLS͒ and showed to cause decoherence of qubits. We study the dynamics of the coupled qubit-EQS system using one-photon spectroscopy, as in, [8][9][10] and in addition two-photon spectroscopy. In this Brief Report, we show that two-photon spectroscopy provides important information on the energy-level structure of the EQS and on its coupling to the qubit. This tool can be used to distinguish a resonance due to a microscopic defect from a spurious resonance in the control or readout circuit, the latter of which can be eliminated by an improved design of these circuits.The origin of decoherence of superconducting qubits is still not well understood. A few studies have been reported up to date for different types of superconducting qubits. 5,8,[10][11][12][13][14][15][16][17] Decoherence properties are characterized by two different time scales: the energy relaxation time, T 1 , and the dephasing time, T 2 . Systematic studies often show a strong and sample-dependent variation of T 1 with qubit control parameters. 14 It is not clear whether relaxation has a microscopic origin or is due to a poorly controlled electromagnetic environment of the qubit. The dephasing time is partly limited by energy relaxation ͑T 2 Յ 2T 1 ͒ and is further reduced by slow fluctuations of the qubit parameters, arising from charge, flux, and junction critical-current noise. 16 This noise very often has a 1 / f power spectrum.Microscopic two-level systems are highly relevant for understanding the decoherence of superconducting qubits. A first reason is that 1 / f noise ...

Context. Millimetre-wave continuum astronomy is today an indispensable tool for both general astrophysics studies (e.g. star formation, nearby galaxies) and cosmology (e.g. cosmic microwave background and high-redshift galaxies). General purpose, large-fieldof-view instruments are needed to map the sky at intermediate angular scales not accessible by the high-resolution interferometers (e.g. ALMA in Chile, NOEMA in the French Alps) and by the coarse angular resolution space-borne or ground-based surveys (e.g. Planck, ACT, SPT). These instruments have to be installed at the focal plane of the largest single-dish telescopes, which are placed at high altitude on selected dry observing sites. In this context, we have constructed and deployed a three-thousand-pixel dual-band (150 GHz and 260 GHz, respectively 2 mm and 1.15 mm wavelengths) camera to image an instantaneous circular field-of-view of 6.5 arcmin in diameter, and configurable to map the linear polarisation at 260 GHz. Aims. First, we are providing a detailed description of this instrument, named NIKA2 (New IRAM KID Arrays 2), in particular focussing on the cryogenics, optics, focal plane arrays based on Kinetic Inductance Detectors, and the readout electronics. The focal planes and part of the optics are cooled down to the nominal 150 mK operating temperature by means of an adhoc dilution refrigerator. Secondly, we are presenting the performance measured on the sky during the commissioning runs that took place between October 2015 and April 2017 at the 30-m IRAM telescope at Pico Veleta, near Granada (Spain). Methods. We have targeted a number of astronomical sources. Starting from beam-maps on primary and secondary calibrators we have then gone to extended sources and faint objects. Both internal (electronic) and on-the-sky calibrations are applied. The general methods are described in the present paper. Results. NIKA2 has been successfully deployed and commissioned, performing in-line with expectations. In particular, NIKA2 exhibits full width at half maximum angular resolutions of around 11 and 17.5 arcsec at respectively 260 and 150 GHz. The noise equivalent flux densities are, at these two respective frequencies, 33 ± 2 and 8 ± 1 mJy s 1/2 . A first successful science verification run was achieved in April 2017. The instrument is currently offered to the astronomy community and will remain available for at least the following ten years.Key words. instrumentation: detectors -instrumentation: photometers -instrumentation: polarimeters -submillimeter: ISMsubmillimeter: galaxies -cosmic background radiation Article published by EDP Sciences A115, page 1 of 15 A&A 609, A115 (2018)