Abstract Quantum phase slip (QPS) is the fundamental manifestation of macroscopic quantum tunneling (MQT). We measure the distribution of the switching current I sw in quasi-one-dimensional superconducting NbN nanowires in the temperature range of 0.2–5.1 K. The escape temperature T esc extracted from the I sw distributions exhibits saturation at temperatures below 1.2 K, indicating a transition into the MQT state. We investigate the single-photon response of the nanowire and find that the QPS rate increases with incident light power in the MQT state. We propose the photon-enhanced quantum phase-slip model, which establishes a quantitative relationship between the quantum phase-slip rate and the incident photon energy, generalizing the phase-slip dynamics into the photon-induced non-equilibrium MQT regime.
Abstract Quantum phase slip (QPS) is the fundamental manifestation of macroscopic quantum tunneling (MQT). We measure the distribution of the switching current I sw in quasi-one-dimensional superconducting NbN nanowires in the temperature range of 0.2–5.1 K. The escape temperature T esc extracted from the I sw distributions exhibits saturation at temperatures below 1.2 K, indicating a transition into the MQT state. We investigate the single-photon response of the nanowire and find that the QPS rate increases with incident light power in the MQT state. We propose the photon-enhanced quantum phase-slip model, which establishes a quantitative relationship between the quantum phase-slip rate and the incident photon energy, generalizing the phase-slip dynamics into the photon-induced non-equilibrium MQT regime.
Quantum phase slips (QPS) or the macroscopic quantum tunneling (MQT) of a nanowire’s order parameter through an activation energy barrier have remained the subject of intense debate for many years. They are expected to occur at low enough temperature where thermally activated phase slips (TAPS) have been frozen out in analogy with Josephson junctions where the macroscopic tunneling of phase at low temperatures has been conclusively experimentally demonstrated. We address this question by following a similar experimental strategy to that employed for establishing MQT in JJs. By measuring switching current distributions, we can probe the phase slip rate and determine if it corresponds to thermal activation or quantum tunneling. Having established that the behavior we see is consistent with being in the quantum regime, we can alter properties of the nanowires and see if the response is consistent with the expectations of the quantum model. To do this we employ an in-situ modification technique using high bias voltage pulses. Using these pulses, we can change resistance, critical temperature, critical current and morphology of the nanowire. We can also change the shunting capacitance of the nanowire by altering the photolithography step used to create the nanowires electrodes. The resulting response of the nanowires agrees well with being in the quantum tunneling dominated regime. An interesting side benefit of the pulsing technique is that we can exactly set the nanowires switching current to a desired value. This may be instrumental in the development of superconducting nanowire qubits.
We perform measurements of the switching current distributions of three w ≈ 120 nm wide, 4 nm thick NbN superconducting strips which are used for single-photon detectors. These strips are much wider than the diameter of the vortex cores, so they are classified as quasi-two-dimensional (quasi-2D). We discover evidence of macroscopic quantum tunneling by observing the saturation of the standard deviation of the switching distributions at temperatures around 2 K. We analyze our results using the Kurkijärvi-Garg model and find that the escape temperature also saturates at low temperatures, confirming that at sufficiently low temperatures, macroscopic quantum tunneling is possible in quasi-2D strips and can contribute to dark counts observed in single photon detectors. At the highest temperatures the system enters a multiple phase-slip regime. In this range single phase-slips are unable to produce dark counts and the fluctuations in the switching current are reduced.
Phase slip is the intrinsic fluctuation of the order parameter and leads to the dissipation of superconductors. Here we propose a photon-assisted phase-slip model in superconducting nanowires to explain the phase-slip rate under photon irradiation. In this phenomenological model, incident photons destroy large quantities of Cooper pairs and reduce the free-energy barrier of phase slips, resulting in proliferation in the phase-slip events and leading to superconducting transition. The switching rates from the superconducting state of a niobium nitride nanowire under various photon irradiation (4.4 mW/m2--4.4 W/m2) and temperatures (2.3--3.7 K) are investigated through the distribution of switching currents in the experiment. The experimental data can be well fitted by our deduced expression of phase-slip rate after eliminating the influence of external noise. Our model develops the phase-slip theory under incident photons and is promising to reveal the intrinsic detection mechanism of the superconducting nanowire single-photon detector.
Abstract Quantum phase slip (QPS) is an important effect that occurs in one-dimensional (1D) or quasi-1D superconducting (SC) materials. Due to the extremely high requirements on the uniformity and quality of SC nanowires, it is currently very challenging to obtain repeatable and controllable QPS devices. Here, we report a systematic study of the QPS effect in NbN SC nanowires. High-quality NbN nanowires with different widths are fabricated by combining the processes of photo-lithography, electron beam lithography, and inductively coupled plasma etching. It is found that the electrical transport of the nanowires showed a systematic variation with the wire width, and a clear evolution from single to multiple QPS events with the decrease in wire width is uncovered at low temperatures. Based on these experimental results, the phase diagram reflecting the evolution of the physical states of the nanowires with wire width and temperature is obtained. In addition, we observed the features of the critical voltage, which is a characteristic of the occurrence of coherent QPS (cQPS), in samples with a width of 36 nm in the multiple QPS region. It is worth noting that this cQPS behavior was detected in a non-high-impedance environment.
Macroscopic quantum tunneling is a process by which a macroscopic object, rather than a single electron, tunnels between two macroscopically distinct quantum states. In this dissertation, we will present the study of macroscopic quantum tunneling of phase-slips in superconducting devices, including one-dimensional nanowires, quasi-two-dimensional superconducting strips and doubly connected superconducting devices composed of two nanowires connected in parallel. We observe macroscopic quantum tunneling in these devices by measuring switching current distributions. It is known that the standard deviation of switching current distributions can be measured on a single nanowire to reveal temperatures at which macroscopic quantum tunneling is responsible for phase-slips. Therefore we begin by studying higher moments of the switching current distributions, namely the skewness S (a measure of the asymmetry of a distribution) and kurtosis K (a measure of its peakedness). We find that the skewness and kurtosis of the switching current distributions obtained via the Kurkijarvi process on devices composed of single nanowires do not depend on whether the switching events are initiated by quantum or thermal phase-slips. Skewness and kurtosis deviate considerably from the values associated with a Gaussian distribution (S=0 and K=3). If, in an experiment these higher moments approach Gaussian values they indicate the existence of noise. Next we study macroscopic quantum tunneling of phase-slips in quasi-two-dimensional superconducting strips, which are commercially available devices used as single-photon detectors. These devices are composed of (250 um) long, (120 nm) wide, meandering superconducting strips. Each time a photon with sufficient energy strikes the strip a voltage pulse is produced and counted. However, the accuracy of these detectors is limited by a rate of dark counts (false events). We find that at sufficiently low temperatures, the macroscopic quantum tunneling of a vortex through an edge barrier on the wire (which results in a phase-slip as the vortex crosses the wide wire) contributes a base-level dark count rate in these detectors which must be considered during operation at low enough temperatures. After studying tunneling in single one-dimensional nanowires and wide, quasi-two-dimensional strips, we move our focus to doubly connected superconducting samples composed of two nanowires connected in parallel. We characterize and model these devices, and show their applicability as nanometer-scale superconducting memory cells. We develop precise algorithms allowing us to write and read the information onto such memory cells. We also observe signatures of macroscopic quantum tunneling in these doubly connected devices by observing a saturation in the standard deviation of switching current distributions at low temperatures. We then discuss how macroscopic quantum tunneling of the memory state can lead to a dissipationless operation of the superconducting memory cell. In each of these devices, whether single one-dimensional nanowires, wide quasi-two-dimensional superconducting strips, or two nanowires in parallel, we find, quite surprisingly, that the same equations can be used to model macroscopic quantum tunneling at the high currents we apply. Therefore we argue that the physics of macroscopic quantum tunneling in wide quasi-two-dimensional strips and doubly connected nanowire devices can be well understood in the context of tunneling in a single, one-dimensional nanowire.
In quasi-one-dimensional nanowires, superconductivity is destroyed by phase slip events. Phase slips can be caused by thermal activation over a free energy barrier (TAPS) or quantum tunneling through the barrier (QPS). Quantum phase slip is an example of macroscopic quantum tunneling. Here, we report the observation of QPS experimentally separated from interference of TAPS in aluminum nanowires. This separation between the low-temperature QPS and the high-temperature TAPS regions is made possible by a phase slip free superconducting region stabilized by the dissipative environment viz. the normal electrodes. Individual QPSs are detected by means of a single-shot voltage measurement protocol, in which they appear as stochastic switching events from the superconducting to the normal state.