Physical systems usually exhibit quantum behavior, such as superpositions and entanglement, only when they are sufficiently decoupled from a lossy environment. Paradoxically, a specially engineered interaction with the environment can become a resource for the generation and protection of quantum states. This notion can be generalized to the confinement of a system into a manifold of quantum states, consisting of all coherent superpositions of multiple stable steady states. We have confined the state of a superconducting resonator to the quantum manifold spanned by two coherent states of opposite phases and have observed a Schrödinger cat state spontaneously squeeze out of vacuum before decaying into a classical mixture. This experiment points toward robustly encoding quantum information in multidimensional steady-state manifolds.

Owing to the low-loss propagation of electromagnetic signals in superconductors, Josephson junctions constitute ideal building blocks for quantum memories, amplifiers, detectors and high-speed processing units, operating over a wide band of microwave frequencies. Nevertheless, although transport in superconducting wires is perfectly lossless for direct current, transport of radio-frequency signals can be dissipative in the presence of quasiparticle excitations above the superconducting gap. Moreover, the exact mechanism of this dissipation in Josephson junctions has never been fully resolved experimentally. In particular, Josephson's key theoretical prediction that quasiparticle dissipation should vanish in transport through a junction when the phase difference across the junction is π (ref. 2) has never been observed. This subtle effect can be understood as resulting from the destructive interference of two separate dissipative channels involving electron-like and hole-like quasiparticles. Here we report the experimental observation of this quantum coherent suppression of quasiparticle dissipation across a Josephson junction. As the average phase bias across the junction is swept through π, we measure an increase of more than one order of magnitude in the energy relaxation time of a superconducting artificial atom. This striking suppression of dissipation, despite the presence of lossy quasiparticle excitations above the superconducting gap, provides a powerful tool for minimizing decoherence in quantum electronic systems and could be directly exploited in quantum information experiments with superconducting quantum bits.

Superconducting circuits have attracted growing interest in recent years as a promising candidate for fault-tolerant quantum information processing. Extensive efforts have always been taken to completely shield these circuits from external magnetic fields to protect the integrity of the superconductivity. Here we show vortices can improve the performance of superconducting qubits by reducing the lifetimes of detrimental single-electron-like excitations known as quasiparticles. Using a contactless injection technique with unprecedented dynamic range, we quantitatively distinguish between recombination and trapping mechanisms in controlling the dynamics of residual quasiparticle, and show quantized changes in quasiparticle trapping rate because of individual vortices. These results highlight the prominent role of quasiparticle trapping in future development of superconducting qubits, and provide a powerful characterization tool along the way.

We have measured the plasma resonances of an array of Josephson junctions in the regime E(J)>>E(C), up to the ninth harmonic by incorporating it as part of a resonator capacitively coupled to a coplanar waveguide. From the characteristics of the resonances, we infer the successful implementation of a superinductance, an electrical element with a nondissipative impedance greater than the resistance quantum [R(Q)=h/(2e)(2) is approximately equal to 6.5 kΩ] at microwave frequencies. Such an element is crucial for preserving the quantum coherence in circuits exploiting large fluctuations of the superconducting phase. Our results show internal losses less than 20 ppm, self-resonant frequencies greater than 10 GHz, and phase-slip rates less than 1 mHz, enabling direct application of such arrays for quantum information and metrology. Arrays with a loop geometry also demonstrate a new manifestation of flux quantization in a dispersive analog of the Little-Parks effect.

Three-dimensional microwave cavities have recently been combined with superconducting qubits in the circuit quantum electrodynamics (cQED) architecture. These cavities should have less sensitivity to dielectric and conductor losses at surfaces and interfaces, which currently limit the performance of planar resonators. We expect that significantly (>103 ) higher quality factors and longer lifetimes should be achievable for 3D structures. Motivated by this principle, we have reached internal quality factors greater than 0.5×10 9 and intrinsic lifetimes of 0.01 seconds for multiple aluminum superconducting cavity resonators at single photon energies and millikelvin temperatures. These improvements could enable long lived quantum memories with submicrosecond access times when strongly coupled to superconducting qubits.In circuit quantum electrodynamics (cQED), microwave resonators protect superconducting qubits from decoherence, suppress spontaneous emission 1 , allow for quantum non-demolition measurements 2,3 , and serve as quantum memories 4 . Single photon lifetimes between 10-50 µs (Q≈10 6 ) have been achieved in thin film resonators with careful surface preparation and geometrical optimization 5-7 . The route toward an optimal geometry also sheds light on the physical mechanisms responsible for damping. Planar resonators with larger features are generally found to be higher quality, which is interpreted as loss dominated by surface elements 5-9 , as the relative energy stored in surface defects is inversely proportional to the size of the resonator.Three dimensional, macroscopic cavity resonators are at the extreme limit of this trend and historically exhibit remarkable lifetimes 10 . Progress with superconducting niobium cavities for particle acceleration has led to dwell times of seconds for RF field strengths of 10 MeV/m at 2 K bath temperatures 11 . At the much lower drive powers corresponding to single-photon excitations, or fields of ∼1 µV/m, storage time in excess of 100 ms has been achieved in three dimensional, niobium Fabry Perot resonators at 51 GHz and 0.8 K 12 , and also in 3D, niobium micromaser cavities at 22 GHz and 0.15 K 13 . The coupling of superconducting qubits to 3D microwave cavities 14 could lead to cQED-type experiments with coherence on these timescales.We have set out to construct very high quality microwave cavities (Q≫10 6 ) in superconducting aluminum while focusing on geometries that may be compatible a) Electronic mail: robert.schoelkopf@yale.edu with single-photon cQED experiments at ∼10 GHz and 20 mK. We study two types of waveguide cavities (rectangular and cylindrical) that support a diversity of modes to test the effects of material purity and surface treatment on cavity lifetimes in the quantum regime. We find that pure, chemically etched aluminum produces the best results, with rectangular resonators reaching lifetimes, τ int =Q int /ω of 1.2 ms (Q int =6.9×10 7 ) and cylindrical resonators as long as 10.4 ms (Q int =7.4×10 8 ). Realizing these timescales in cQED experime...

Qubit reset is crucial at the start of and during quantum information algorithms. We present the experimental demonstration of a practical method to force qubits into their ground state, based on driving appropriate qubit and cavity transitions. Our protocol, called the double drive reset of population, is tested on a superconducting transmon qubit in a three-dimensional cavity. Using a new method for measuring population, we show that we can prepare the ground state with a fidelity of at least 99.5% in less than 3 μs; faster times and higher fidelity are predicted upon parameter optimization.

As the energy relaxation time of superconducting qubits steadily improves, non-equilibrium quasiparticle excitations above the superconducting gap emerge as an increasingly relevant limit for qubit coherence. We measure fluctuations in the number of quasiparticle excitations by continuously monitoring the spontaneous quantum jumps between the states of a fluxonium qubit, in conditions where relaxation is dominated by quasiparticle loss. Resolution on the scale of a single quasiparticle is obtained by performing quantum non-demolition projective measurements within a time interval much shorter than T1, using a quantum limited amplifier (Josephson Parametric Converter). The quantum jumps statistics switches between the expected Poisson distribution and a non-Poissonian one, indicating large relative fluctuations in the quasiparticle population, on time scales varying from seconds to hours. This dynamics can be modified controllably by injecting quasiparticles or by seeding quasiparticle-trapping vortices by cooling down in magnetic field.A mesoscopic superconducting circuit, of typical size smaller than 1 mm 3 , cooled to a temperature well below the superconducting gap should be completely free of thermal quasiparticle (QP) excitations. However, in the last decade there has been growing experimental evidence that the QP density at low temperatures saturates to values orders of magnitude above the value expected at thermal equilibrium [1][2][3][4][5]. These non-equilibrium QP excitations limit the performance of a variety of superconducting devices, such as single-electron turnstiles [6], kinetic inductance [7, 8] and quantum capacitance [9] detectors, micro-coolers [10, 11], as well as Andreev bound state nano-systems [12,13]. Moreover, QP's are an important intrinsic decoherence mechanism for superconducting two level systems (qubits) [14][15][16][17][18][19]. In particular, a recent experiment performed on the fluxonium qubit showed energy relaxation times in excess of 1 ms, limited by QP's [20]. Surprisingly, the sources generating these QP excitations are not yet positively identified. The measurement of non-equilibrium QP dynamics at low temperatures could provide insight into their origin as well as an efficient tool to quantify QP suppression solutions.In this letter, we show that the quantum jumps[21] of a qubit whose lifetime is limited by QP tunneling, such as the fluxonium artificial atom, can serve as a sensitive probe of QP dynamics. A jump in the state of the qubit indicates an interaction of the qubit with a QP, and therefore fluctuations in the rate of quantum jumps are directly linked to changes in QP number. Tracking the state of the qubit in real time requires fast, single-shot projective measurement with minimal added noise, made possible by the advent of quantum-limited amplifiers [22][23][24]. In this work, we use a Josephson Parametric Converter (JPC) quantum limited amplifier [23,25] to monitor the state of our qubit with a resolution of 5 µs, two orders of magnitude faster than the qubi...

Superconducting high kinetic inductance elements constitute a valuable resource for quantum circuit design and millimeter-wave detection. Granular aluminum (grAl) in the superconducting regime is a particularly interesting material since it has already shown a kinetic inductance in the range of nH/□ and its deposition is compatible with conventional Al/AlOx/Al Josephson junction fabrication. We characterize microwave resonators fabricated from grAl with a room temperature resistivity of 4×10^{3}  μΩ cm, which is a factor of 3 below the superconductor to insulator transition, showing a kinetic inductance fraction close to unity. The measured internal quality factors are on the order of Q_{i}=10^{5} in the single photon regime, and we demonstrate that nonequilibrium quasiparticles (QPs) constitute the dominant loss mechanism. We extract QP relaxation times in the range of 1 s and we observe QP bursts every ∼20  s. The current level of coherence of grAl resonators makes them attractive for integration in quantum devices, while it also evidences the need to reduce the density of nonequilibrium QPs.