Complex Inductance, Excess Noise, and Surface Magnetism in dc SQUIDs

Sendelbach, S.; Hover, David; McDermott, Robert

doi:10.1103/physrevlett.103.117001

Cited by 68 publications

(89 citation statements)

References 12 publications

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“…Our model provides a qualitative description of the origin of fluctuations in the electrical susceptibility of mesoscopic circuits, an area which has recently started to attract attention from both experiment and theory [21][22][23] . We also note that interactions between TLS have recently been observed directly in two strongly coupled defects 24 and that such coupling has been invoked as a model of noise before, e.g.…”

Section: Motivationmentioning

confidence: 99%

Interacting two-level defects as sources of fluctuating high-frequency noise in superconducting circuits

et al. 2015

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Since the very first experiments, superconducting circuits have suffered from strong coupling to environmental noise, destroying quantum coherence and degrading performance. In state-of-the-art experiments, it is found that the relaxation time of superconducting qubits fluctuates as a function of time. We present measurements of such fluctuations in a 3D-Transmon circuit and develop a qualitative model based on interactions within a bath of background two-level systems (TLS) which emerge from defects in the device material. In our model, the time-dependent noise density acting on the qubit emerges from its near-resonant coupling to high-frequency TLS which experience energy fluctuations due to their interaction with thermally fluctuating TLS at low frequencies. We support the model by providing experimental evidence of such energy fluctuations observed in a single TLS in a phase qubit circuit.

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Section: Motivationmentioning

confidence: 99%

Interacting two-level defects as sources of fluctuating high-frequency noise in superconducting circuits

et al. 2015

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“…The CZ gate fidelity is limited by three error mechanisms: Decoherence (55% of the total error), control error (24%), and state leakage (21%), see Supplementary Information. Decoherence can be suppressed with enhanced materials and optimised fabrication 24,25 . Imperfections in control arise primarily from reflections and stray inductances in wiring, and can be improved using conventional microwave techniques.…”

mentioning

confidence: 99%

Superconducting quantum circuits at the surface code threshold for fault tolerance

Barends

Kelly

Megrant

et al. 2014

Nature

1,607

1,785

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A quantum computer can solve hard problems -such as prime factoring 1,2 , database searching 3,4 , and quantum simulation 5 -at the cost of needing to protect fragile quantum states from error. Quantum error correction 6 provides this protection, by distributing a logical state among many physical qubits via quantum entanglement. Superconductivity is an appealing platform, as it allows for constructing large quantum circuits, and is compatible with microfabrication. For superconducting qubits the surface code 7 is a natural choice for error correction, as it uses only nearest-neighbour coupling and rapidly-cycled entangling gates. The gate fidelity requirements are modest: The per-step fidelity threshold is only about 99%. Here, we demonstrate a universal set of logic gates in a superconducting multi-qubit processor, achieving an average single-qubit gate fidelity of 99.92% and a two-qubit gate fidelity up to 99.4%. This places Josephson quantum computing at the fault-tolerant threshold for surface code error correction. Our quantum processor is a first step towards the surface code, using five qubits arranged in a linear array with nearest-neighbour coupling. As a further demonstration, we construct a five-qubit GreenbergerHorne-Zeilinger (GHZ) state 8,9 using the complete circuit and full set of gates. The results demonstrate that Josephson quantum computing is a high-fidelity technology, with a clear path to scaling up to large-scale, fault-tolerant quantum circuits.The high fidelity performance we demonstrate here is achieved through a combination of highly coherent qubits, a straightforward interconnection architecture, and a novel implementation of the two-qubit controlled-phase (CZ) entangling gate. The CZ gate uses a fast but adiabatic frequency tuning of the qubits 10 , which is easily adjusted yet minimises decoherence and leakage from the computational basis [Martinis, J., et al., in preparation]. We note that previous demonstrations of two-qubit gates achieving better than 99% fidelity used fixed-frequency qubits: Systems based on nuclear magnetic resonance and ion traps have shown two-qubit gates with fidelities of 99.5% 11 and 99.3% 12 . Here, the tuneable nature of the qubits and their entangling gates provides, remarkably, both high fidelity and fast control.Superconducting integrated circuits give flexibility in building quantum systems due to the macroscopic nature of the electron condensate. As shown in Fig. 1, we have designed a processor consisting of five Xmon qubits with nearestneighbour coupling, arranged in a linear array. The crossshaped qubit 14 offers a nodal approach to connectivity while maintaining a high level of coherence (see Supplementary Information for decoherence times). Here, the four legs of the cross allow for a natural segmentation of the design into coupling, control and readout. We chose a modest inter-qubit capacitive coupling strength of g/2π = 30 MHz and use alternating qubit idle frequencies of 5.5 and 4.7 GHz, enabling a CZ gate in 40 ns when two qubits are brough...

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“…Effective surface spins have recently been identified as one dominant source of low-frequency magnetic-flux noise 4,5 , detrimental to several types of superconducting qubits; however, open questions remain regarding the nature of these spins. Their noise is known to be due to local fluctuators [6][7][8][9] and the spectrum exhibits a 1/f α powerlaw dependence from hertz to tens of megahertz [10][11][12][13][14] .…”

Section: Introductionmentioning

confidence: 99%

Spectroscopy of low-frequency noise and its temperature dependence in a superconducting qubit

et al. 2012

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We report a direct measurement of the low-frequency noise spectrum in a superconducting flux qubit. Our method uses the noise sensitivity of a free-induction Ramsey interference experiment, comprising free evolution in the presence of noise for a fixed period of time followed by single-shot qubit-state measurement. Repeating this procedure enables Fourier-transform noise spectroscopy with access to frequencies up to the achievable repetition rate, a regime relevant to dephasing in ensemble-averaged time-domain measurements such as Ramsey interferometry. Rotating the qubit's quantization axis allows us to measure two types of noise: effective flux noise and effective criticalcurrent or charge noise. For both noise sources, we observe that the very same 1/f -type power laws measured at considerably higher frequencies (0.2 − 20 MHz) are consistent with the noise in the 0.01 − 100-Hz range measured here. We find no evidence of temperature dependence of the noises over 65 − 200 mK, and also no evidence of time-domain correlations between the two noises. These methods and results are pertinent to the dephasing of all superconducting qubits.

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Complex Inductance, Excess Noise, and Surface Magnetism in dc SQUIDs

Cited by 68 publications

References 12 publications

Interacting two-level defects as sources of fluctuating high-frequency noise in superconducting circuits

Interacting two-level defects as sources of fluctuating high-frequency noise in superconducting circuits

Superconducting quantum circuits at the surface code threshold for fault tolerance

Spectroscopy of low-frequency noise and its temperature dependence in a superconducting qubit

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