Magnetic flux noise is a dominant source of dephasing and energy relaxation in superconducting qubits. The noise power spectral density varies with frequency as 1/f α with α ∼ < 1 and spans 13 orders of magnitude. Recent work indicates that the noise is from unpaired magnetic defects on the surfaces of the superconducting devices. Here, we demonstrate that adsorbed molecular O2 is the dominant contributor to magnetism in superconducting thin films. We show that this magnetism can be suppressed by appropriate surface treatment or improvement in the sample vacuum environment. We observe a suppression of static spin susceptibility by more than an order of magnitude and a suppression of 1/f magnetic flux noise power spectral density by more than a factor of 5. These advances open the door to realization of superconducting qubits with improved quantum coherence.A quantum computer will allow efficient solutions for certain problems that are intractable on conventional, classical computers, including factoring and quantum simulation. Superconducting quantum bits ("qubits") based on Josephson junctions are a leading candidate for scalable quantum information processing in the solid state [1, 2]. Gate and measurement operations have attained a level of fidelity that should enable quantum error correction [3, 4], and there is interest in scaling to larger systems [5, 6]. However, qubit performance is limited by dephasing [7,8]. The dominant source of dephasing is low-frequency 1/f magnetic flux noise [9][10][11]. Uncontrolled variation of the flux bias of the qubit leads to the accumulation of spurious phase during periods of free evolution, resulting in a rapid decay of qubit coherence. Magnetic flux noise was first identified in the 1980s [12,13]. Efforts to avoid flux noise include operation at a "sweet spot" where the device is insensitive to first order to magnetic flux fluctuations [14], or elimination of superconducting loops that allow the frequency of the qubit to be tuned in situ [15]. However, restriction to fixedfrequency qubits results in longer gate times, and static disorder in the junction critical currents makes it difficult to target specific frequencies, leading to frequency clashes in larger multiqubit circuits. In the context of a quantum annealer [16,17], flux noise degrades performance by limiting the number of qubits that can tunnel coherently. For these reasons, there is strong motivation to understand and eliminate the flux noise.Recent experiments indicate that there is a high density of unpaired surface spins in superconducting integrated circuits [18] and it is believed that fluctuations of * Present address: Northrop Grumman Corporation, Linthicum, Maryland 21203, USA † Electronic address: rfmcdermott@wisc.edu these spins give rise to the 1/f flux noise [19][20][21]. There is experimental evidence that interactions between the surface spins are significant [22]. To date, however, there has been no experimental data pointing toward the microscopic nature of the surface magnetic defects, althou...

We propose a hybrid quantum gate between an atom and a microwave photon in a superconducting coplanar waveguide cavity by exploiting the strong resonant microwave coupling between adjacent Rydberg states. Using experimentally achievable parameters gate fidelities >0.99 are possible on submicrosecond time scales for waveguide temperatures below 40 mK. This provides a mechanism for generating entanglement between two disparate quantum systems and represents an important step in the creation of a hybrid quantum interface applicable for both quantum simulation and quantum information processing

High-fidelity gate operations are essential to the realization of a fault-tolerant quantum computer. In addition, the physical resources required to implement gates must scale efficiently with system size. A longstanding goal of the superconducting qubit community is the tight integration of a superconducting quantum circuit with a proximal classical cryogenic control system. Here we implement coherent control of a superconducting transmon qubit using a Single Flux Quantum (SFQ) pulse driver cofabricated on the qubit chip. The pulse driver delivers trains of quantized flux pulses to the qubit through a weak capacitive coupling; coherent rotations of the qubit state are realized when the pulse-to-pulse timing is matched to a multiple of the qubit oscillation period. We measure the fidelity of SFQ-based gates to be ∼95% using interleaved randomized benchmarking. Gate fidelities are limited by quasiparticle generation in the dissipative SFQ driver. We characterize the dissipative and dispersive contributions of the quasiparticle admittance and discuss mitigation strategies to suppress quasiparticle poisoning. These results open the door to integration of large-scale superconducting qubit arrays with SFQ control elements for low-latency feedback and stabilization.

We present measurements of a hybrid system consisting of a microwave transmission-line resonator and a lateral quantum dot defined on a GaAs heterostructure. The two subsystems are separately characterized and their interaction is studied by monitoring the electrical conductance through the quantum dot. The presence of a strong microwave field in the resonator is found to reduce the resonant conductance through the quantum dot, and is attributed to electron heating and modulation of the dot potential. We use this interaction to demonstrate a measurement of the resonator transmission spectrum using the quantum dot.The interaction of light and matter is one of the most fundamental processes in physics. One way to explore this area is to use artificial atoms such as quantum dots which offer e.g. the possibility to tune the energy spacing of the individual electronic states. Using this possibility the resonant absorption of photons by electrons in a quantum dot has been investigated in transport measurements of photon assisted tunneling 1,2 . Cavity quantum electrodynamics (QED), the study of the coupling of matter to light confined in a cavity 3 , is traditionally studied with atoms but also with solid state systems such as self-assembled quantum dots 4,5 . Furthermore the realization of circuit QED 6 , in which a single microwave photon is trapped in an on-chip cavity and coherently coupled to a quantum two-level system, has led to significant progress in control and coupling of microwave photons and superconducting qubits. The study of the interaction between the electromagnetic field of such a resonator and a semiconductor quantum dot marks an important step toward realizing a hybrid quantum information processor 7 , in which the advantages of different systems, such as a long relaxation time of the individual qubit 8 and interaction between distant qubits 9 , could be exploited in one device.The sample, shown in Fig. 1 (a), consists of a laterally defined quantum dot positioned at an antinode of the electric field of a microwave transmission-line resonator. The dot is realized on an Al x Ga 1−x As heterostructure with a two-dimensional electron gas (2DEG) residing at the heterointerface about 35 nm below the surface. The device is fabricated by three stages of optical lithography followed by local anodic oxidation (LAO) 10 with an atomic force microscope (AFM) to define the quantum dot. In the first of the three lithography steps the mesa for the quantum dot (dark gray parts, labeled M in Fig. 1 (a)) is wet etched. Ohmic contacts (labeled C in Fig. 1 (a)) are then used to contact the 2DEG. Finally, the microwave resonator and its ground plane (labeled R and GND in Fig. 1 (a)) are defined in a lift off process by a) Electronic mail: freytob@phys.ethz.ch FIG. 1. (Color online): (a) Optical micrograph of a microwave resonator (R) with an integrated quantum dot, (GND): ground plane of the resonator, (C): ohmic contact, (M): 2DEG mesa. (b) Magnified view of a coupling capacitor, location on the chip marked with rec...

Fast, high-fidelity measurement is a key ingredient for quantum error correction. Conventional approaches to the measurement of superconducting qubits, involving linear amplification of a microwave probe tone followed by heterodyne detection at room temperature, do not scale well to large system sizes. Here we introduce an alternative approach to measurement based on a microwave photon counter. We demonstrate raw single-shot measurement fidelity of 92%. Moreover, we exploit the intrinsic damping of the counter to extract the energy released by the measurement process, allowing repeated high-fidelity quantum non-demolition measurements. Crucially, our scheme provides access to the classical outcome of projective quantum measurement at the millikelvin stage. In a future system, counter-based measurement could form the basis for a scalable quantum-to-classical interface.