The shift of energy levels owing to broadband electromagnetic vacuum fluctuations-the Lamb shift-has been pivotal in the development of quantum electrodynamics and in understanding atomic spectra 1-6 . Currently, small energy shifts in engineered quantum systems are of paramount importance owing to the extreme precision requirements in applications such as quantum computing 7,8 . However, without a tunable environment it is challenging to resolve the Lamb shift in its original broadband case. Consequently, the observations in other than atomic systems [1][2][3][4][5]9 are limited to environments comprised of narrowband modes 10-12 . Here, we observe a broadband Lamb shift in high-quality superconducting resonators, a scenario also accessing any static shift inaccessible in Lamb's experiment 1,2 . We measure a continuous change of several megahertz in the fundamental resonator frequency by externally tuning the coupling strength of the engineered broadband environment which is based on hybrid normal-metal-superconductor tunnel junctions [13][14][15] . Our results may lead to improved control of dissipation in high-quality engineered quantum systems and open new possibilities for studying synthetic open quantum matter 16-18 using this hybrid experimental platform.Physical quantum systems are always open. Thus, exchange of energy and information with an environment eventually leads to relaxation and degradation of quantum coherence. Interestingly, the environment can be in a vacuum state and yet cause significant perturbation to the original quantum system. The quantum vacuum can be modelled as broadband fluctuations which may absorb energy from the coupled quantum systems. These fluctuations also lead to an energy level renormalizationthe Lamb shift-of the system, such as that observed in atomic systems [1][2][3][4][5]9 . Despite of its fundamental nature, the Lamb shift arising from broadband fluctuations is often overlooked outside the field of atomic physics as a small constant shift that is challenging to distinguish 20 . Due to the emergence of modern engineered quantum systems, in which the desired precision of the energy levels is comparable to the Lamb shift, it has, however, become important to predict accurately the perturbation as a function of external control parameters. Neglecting energy shifts can potentially take the engineered quantum systems outside the region of efficient operation 21,22 and may even lead to undesired level crossings between subsystems. These issues are pronounced in applications requiring strong dissipation. Examples include reservoir engineering for autonomous quantum error correction 23,24 , or rapid on-demand entropy and heat evacuation 14,15,25,26 . Furthermore, the role of dissipation in phase transitions of open many-body quantum systems has attracted great interest through the recent progress in studying synthetic quantum matter 16,17 .In our experimental setup, the system exhibiting the Lamb shift is a superconducting coplanar waveguide resonator with the resonance frequ...

Superconducting microwave circuits show great potential for practical quantum technological applications such as quantum information processing. However, fast and on-demand initialization of the quantum degrees of freedom in these devices remains a challenge. Here, we experimentally implement a tunable heat sink that is potentially suitable for the initialization of superconducting qubits. Our device consists of two coupled resonators. The first resonator has a high quality factor and a fixed frequency whereas the second resonator is designed to have a low quality factor and a tunable resonance frequency. We engineer the low quality factor using an on-chip resistor and the frequency tunability using a superconducting quantum interference device. When the two resonators are in resonance, the photons in the high-quality resonator can be efficiently dissipated. We show that the corresponding loaded quality factor can be tuned from above 105 down to a few thousand at 10 GHz in good quantitative agreement with our theoretical model.

We report on fast tunability of an electromagnetic environment coupled to a superconducting coplanar waveguide resonator. Namely, we utilize a recently-developed quantum-circuit refrigerator (QCR) to experimentally demonstrate a dynamic tunability in the total damping rate of the resonator up to almost two orders of magnitude. Based on the theory it corresponds to a change in the internal damping rate by nearly four orders of magnitude. The control of the QCR is fully electrical, with the shortest implemented operation times in the range of 10 ns. This experiment constitutes a fast active reset of a superconducting quantum circuit. In the future, a similar scheme can potentially be used to initialize superconducting quantum bits.Tunable dissipative environments for circuit quantum electrodynamics (cQED) are pursued intensively in experiments due to the unique opportunities to study non-Hermitian physics 1,2 , such as phase transitions related to parity-time symmetry 3 , decoherence and quantum noise 4 . Interesting effects can be observed in experiments on exceptional points 5-7 , which also gives possibilities to use such systems as models in nonlinear photonics, for example, for metamaterials 8 and for photonic crystals 9 .From the practical point of view, tunable environments are utilized to protect and process quantum information 10-12 and to implement qubit reset 13,14 . The latter application calls for fast tunability of the environment due to the aim to increase the rate of the operations on a quantum computer. Recent advances in the field of cQED for quantum information processing 14-17 render this topic highly interesting.There are different ways for resetting superconducting qubits. Firstly, one may tune the qubit frequency to reduce its life time 18 . The disadvantages of this method include the broad frequency band reserved by the qubit and the required fast frequency sweep which may lead to an increased amount of initialization error 19 . Conventionally, it is beneficial to maintain the qubits at the optimal parameter points during all operations. Secondly, it is possible to use microwave pulses to actively drive the qubit to the ground state 20-23 . Such methods are popular because no additional components or new control steps are needed. However, to achieve high fidelity one usually needs to increase the reset time to the microsecond range. Thirdly, one can engineer a tunable environment for the qubits 13,24-26 . This approach demand changes in the chip design, but may lead to high fidelity for a fast reset without compromises on the other properties.Here, we focus on a single-parameter-controlled tunable environment implemented by a quantum-circuit refrigerator (QCR) 5,27-31 . The refrigerator is based on photonassisted electron tunneling through two identical normalmetal-insulator-superconductor junctions (NIS). It has been used to cool down a photon mode of the resonator 27 , and to observe a Lamb shift in a cQED system 31 . Furthermore, QCR can be used as a cryogenic photon source 29 , which ma...

A silicon electron pump operating at the temperature of liquid helium has demonstrated repeatable operation with sub-ppm accuracy. The pump current, approximately 168 pA, is measured by three laboratories, and the measurements agree with the expected current ef within the uncertainties which range from 0.2 ppm to 1.3 ppm. All the measurements are carried out in zero applied magnetic field, and the pump drive signal is a sine wave. The combination of simple operating conditions with high accuracy demonstrates the possibility that an electron pump can operate as a current standard in a National Measurement Institute. We also discuss other practical aspects of using the electron pump as a current standard, such as testing its robustness to changes in the control parameters, and using a rapid tuning procedure to locate the optimal operation point.

Various applications of quantum devices call for an accurate calibration of cryogenic amplification chains. To this end, we present a convenient calibration scheme and use it to accurately measure the total gain and noise temperature of an amplification chain by employing normal-metal-insulator-superconductor (NIS) junctions. Our method is based on the radiation emitted by inelastic electron tunneling across voltage-biased NIS junctions. We derive an analytical equation that relates the generated power to the applied bias voltage which is the only control parameter of the device. After the setup has been characterized using a standard voltage reflection measurement, the total gain and the noise temperature are extracted by fitting the analytical equation to the microwave power measured at the output of the amplification chain. The 1σ uncertainty of the total gain of 51.84 dB appears to be of the order of 0.1 dB.Superconducting circuits provide a promising approach to implement a variety of quantum devices and to explore fundamental physical phenomena, such as the lightmatter interaction 1 in the ultrastrong coupling regime 2 . In addition, superconducting circuits are potential candidates for building a large-scale quantum computer 3,4 : superconducting qubits can be coupled in a scalable way 5-12 , and both the gate and the measurement fidelity of qubits exceed the threshold required for quantum error correction 10,13-15 .Since superconducting quantum circuits typically operate in the single-photon regime, signals are amplified substantially for readout 3,16-21 using a chain of amplifiers, which is distributed over several temperature stages 16,17 . In the first stage, a near-quantumlimited amplifier 22 , such as a Josephson parametric amplifier 23-26 , is often used to lower the noise temperature of the amplification chain 27 . As a result of cascading several amplifiers, the uncertainty in the total gain of the amplification chain becomes significant and may complicate, for example, the estimation of the photon number in the superconducting circuit. Therefore, accurate, fast, and simple methods for measuring the total gain of an amplification chain are desirable in the investigation of quantum electric devices.The gain and the noise temperature of cryogenic amplifiers can be measured, for example, using superconducting qubits 22,28 , Planck spectroscopy of a sub-kelvin thermal noise source 29 , and the Y -factor method 30,31 which utilizes the Johnson-Nyquist noise emitted at different temperatures. In addition to these methods, shot noise 32,33 sources, such as normal-metal-insulatornormal-metal junctions, can be used to determine the gain and noise temperature of cryogenic amplifiers 34,35 . However, this method typically requires a calibration measurement of the setup due to impedance mismatch 34 .In this paper, we present an accurate alternative calibration scheme for the total gain and noise temperature of an amplification chain by utilizing photonassisted electron tunneling in normal-metal-insulatorsuper...