Low- and High-Frequency Noise from Coherent Two-Level Systems

Shnirman, Alexander; Schön, Gerd; Martin, Ivar; Makhlin, Yuriy

doi:10.1103/physrevlett.94.127002

Cited by 168 publications

(194 citation statements)

References 26 publications

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“…These resonators are shown to exhibit flicker frequency noise, consistent with the resonator coupling to a bath of two level fluctuators, if the TLF ensemble is coherent and weakly interacting with the resonator such a bath has been theoretically shown to produce a flicker frequency spectral noise [4]. The noise level is also found to increase with increasing loss tangent, consistent with noise increasing with increasing density of TLFs [4] [22].…”

Section: (See Supplemental Materials At [Url] For [Spectral Analysis])supporting

confidence: 54%

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Slow noise processes in superconducting resonators

et al. 2013

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Slow noise processes, with characteristic timescales ∼1s, have been studied in planar superconducting resonators. A frequency locked loop is employed to track deviations of the resonator centre frequency with high precision and bandwidth. Comparative measurements are made in varying microwave drive, temperature and between bare resonators and those with an additional dielectric layer. All resonators are found to exhibit flicker frequency noise which increases with decreasing microwave drive. We also show that an increase in temperature results in a saturation of flicker noise in resonators with an additional dielectric layer, while bare resonators stop exhibiting flicker noise instead showing a random frequency walk process.Slow fluctuations in charge sensitive devices have been frequently examined over the past few decades [1]. Recently, their effects were indirectly observed in superconducting qubits [2] with supporting theoretical work [3] linking them to the presence of two level fluctuators (TLFs) [4]. We present measurements directly probing these slow noise processes in superconducting resonators using a high bandwidth feedback technique with Hz level resolution [5]. Feedback maintains a lock to the resonator centre frequency indefinitely, providing a direct measure of the nature of slow fluctuations and their behavior in varying temperature, microwave drive and TLF density. Theoretical work [3] suggests that 'slow' fluctuators can have a profound -but indirect-effect on the noise and losses in superconducting devices operating at microwave frequencies such as qubits. In simplified terms the model considers two distinct ensembles of TLFs: a coherent population that couples directly to the device, and a 'slow' population which perturbs the coherent TLFs, by changing the tunnel splitting. Hence, whereas the coherent processes between the two energy levels are expected to have short time constants, they are in turn effectively being modulated by processes that can have time constants of the order of seconds or even hours.Motivated by QIP applications, recent studies on superconducting resonators have focused heavily on sources of dissipation. Measurements have evaluated the effects of magnetic fields[6] and vortex motion [7]. The remaining dissipation channel is usually attributed to the presence of TLFs. The exact nature of TLFs remains contentious, but a variety of experiments have studied their effects [8][9][10][11][12] and recent models evaluated the contributions of TLFs in varying locations [13].In this letter we study slow noise processes in low loss niobium (Nb) on sapphire resonators [11]. Resonators are by their very nature sensitive probes: any change in the environment will produce a change in the centre frequency ν 0 of the resonator; making them ideal devices for studying noise. However, measuring this noise can be difficult due to the extrinsic low frequency noise present in equipment such as amplifiers and mixers [14]. Here we overcome this problem by using a high-bandwidth measurement metho...

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Section: (See Supplemental Materials At [Url] For [Spectral Analysis])supporting

confidence: 54%

“…Recently, their effects were indirectly observed in superconducting qubits [2] with supporting theoretical work [3] linking them to the presence of two level fluctuators (TLFs) [4]. We present measurements directly probing these slow noise processes in superconducting resonators using a high bandwidth feedback technique with Hz level resolution [5].…”

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confidence: 96%

Slow noise processes in superconducting resonators

et al. 2013

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“…For example, a key limitation in solid-state based quantum processing of information is the decoherence due to fluctuating TLSs in the dielectric layers fabricated on-chip [26][27][28] . We anticipate, resting on the motional narrowing phenomenon, that the dephasing times of the existing superconducting qubits may be dramatically improved if one is able to accelerate the dynamics of the longitudinally coupled TLSs.…”

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confidence: 99%

Motional averaging in a superconducting qubit

et al. 2013

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Superconducting circuits with Josephson junctions are promising candidates for developing future quantum technologies. Of particular interest is to use these circuits to study effects that typically occur in complex condensed-matter systems. Here we employ a superconducting quantum bit-a transmon-to perform an analogue simulation of motional averaging, a phenomenon initially observed in nuclear magnetic resonance spectroscopy. By modulating the flux bias of a transmon with controllable pseudo-random telegraph noise we create a stochastic jump of its energy level separation between two discrete values. When the jumping is faster than a dynamical threshold set by the frequency displacement of the levels, the initially separate spectral lines merge into a single, narrow, motional-averaged line. With sinusoidal modulation a complex pattern of additional sidebands is observed. We show that the modulated system remains quantum coherent, with modified transition frequencies, Rabi couplings, and dephasing rates. These results represent the first steps towards more advanced quantum simulations using artificial atoms.

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“…Understanding the origin of these spurious TLSs, their coherent quantum behavior, and their connection to ubiquitous 1/f noise is hence a challenge that will be crucial to the future of superconducting quantum devices. The behavior of a distribution of these TLSs were studied theoretically in [17,18,19]. Recently, it was proposed that TLSs can be viewed as qubits themselves [20], given their relatively long coherence times.…”

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confidence: 99%

Josephson Junction Microscope for Low-Frequency Fluctuators

Tian

Simmonds

2007

Phys. Rev. Lett.

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The high-Q harmonic oscillator mode of a Josephson junction can be used as a novel probe of spurious two-level systems (TLSs) inside the amorphous oxide tunnel barriers of the junction. In particular, we show that spectroscopic transmission measurements of the junction resonator mode can reveal how the coupling magnitude between the junction and the TLSs varies with an external magnetic field applied in the plane of the tunnel barrier. The proposed experiments offer the possibility of clearly resolving the underlying coupling mechanism for these spurious TLSs, an important decoherence source limiting the quality of superconducting quantum devices.Superconducting quantum circuits have been intensively tested in various regimes in the past few years, from superconducting qubits demonstrating long coherence times, to superconducting transmission line cavities coherently coupled to a Single Cooper Pair box [1,2,3,4,5,6]. Such circuits are extremely sensitive to very small quanta and defect states, and hence have the ability to detect individual microwave photons, charged quasiparticles, as well as spurious TLSs within or near Josephson junction tunnel barriers [7,8,9,10,11]. In recent experiments [10,11], TLSs were identified through spectroscopic measurements of a superconducting phase qubit appearing as 'gaps" or "splittings" in the energy spectrum.The TLS defects can be an unwanted source of decoherence for superconducting quantum bits. The lowfrequency noise, which has been shown to be a serious source of decoherence for superconducting qubits [10,11,12,13,14], is very probably induced by such amorphous fluctuators inside or near Josephson junctions [15,16]. Understanding the origin of these spurious TLSs, their coherent quantum behavior, and their connection to ubiquitous 1/f noise is hence a challenge that will be crucial to the future of superconducting quantum devices. The behavior of a distribution of these TLSs were studied theoretically in [17,18,19]. Recently, it was proposed that TLSs can be viewed as qubits themselves [20], given their relatively long coherence times. However, the microscopic origin and the coupling mechanism between the TLSs and the junction remains unresolved. Generally considered to be connected to the amorphous nature of the tunnel barrier [21], movement of unrestrained atoms or charges may lead to a number of possible coupling mechanisms. As originally proposed in Ref. [11], fluctuations of the TLS could lead to variations of the junction critical current. Another possibility, requires that the TLSs have fluctuating dipole moments which couple to the electric field found within the junction tunnel barrier [10].Here, we present a scheme that can resolve a variety of properties of the TLSs and distinguish between these two suggested coupling mechanisms through the use of an applied magnetic field. Consider a Josephson junction resonator operating as a high-Q, nonlinear cavity mode [22], coupling to a TLS through its canonical phase (or momentum) operator. This forms a cavity QED sys...

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Low- and High-Frequency Noise from Coherent Two-Level Systems

Cited by 168 publications

References 26 publications

Slow noise processes in superconducting resonators

Slow noise processes in superconducting resonators

Motional averaging in a superconducting qubit

Josephson Junction Microscope for Low-Frequency Fluctuators

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