The flux qubit revisited to enhance coherence and reproducibility

Yan, Fei; Gustavsson, Simon; Kamal, Archana; Birenbaum, Jeffrey; Sears, Adam; Hover, David; Gudmundsen, Theodore; Rosenberg, Danna; Samach, Gabriel; Weber, Steven; Yoder, Jonilyn; Orlando, Terry P.; Clarke, John; Kerman, Andrew J.; Oliver, William

doi:10.1038/ncomms12964

Cited by 555 publications

(601 citation statements)

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“…Our results provide a quantitative picture of propagating thermal microwaves, which is especially relevant for the characterization of more advanced quantum states in the presence of unavoidable thermal background fields. With respect to superconducting qubits, we gain systematic insight into a dephasing mechanism which may become relevant for state-of-the-art devices with long coherence times [49,50].…”

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confidence: 99%

“…Our results demonstrate that the three types of propagating microwave states we investigate are reliably distinguishable below the single photon level in an experiment by their photon statistics. Therefore, both setups are promising candidates to explore decoherence mechanisms possibly limiting high-performance superconducting qubits [49,50] and the properties of more advanced quantum microwave states. The frequency-tunable transmon qubit is capacitively coupled with coupling strength g to a readout resonator, which itself is coupled with κx to a readout line.…”

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Photon Statistics of Propagating Thermal Microwaves

et al. 2017

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In experiments with superconducting quantum circuits, characterizing the photon statistics of propagating microwave fields is a fundamental task. We quantify the n 2 + n photon number variance of thermal microwave photons emitted from a black-body radiator for mean photon numbers 0.05 n 1.5. We probe the fields using either correlation measurements or a transmon qubit coupled to a microwave resonator. Our experiments provide a precise quantitative characterization of weak microwave states and information on the noise emitted by a Josephson parametric amplifier.As propagating electromagnetic fields in general [1][2][3], propagating microwaves with photon numbers on the order of unity are essential for quantum computation [4,5], communication [6], and illumination [7][8][9][10] protocols. Because of their omnipresence in experimental setups, the characterization of thermal states is especially relevant for many applications [11][12][13][14]. Specifically in the microwave regime, sophisticated experimental techniques for their generation at cryogenic temperatures, their manipulation, and detection have been developed in recent years. In this context, an important aspect is the generation of propagating thermal microwaves using thermal emitters [15][16][17]. These emitters can be spatially separated from the setup components used for manipulation and detection [18,19], which allows one to individually control the emitter and the setup temperature. Due to the low energy of microwave photons, the detection of these fields typically requires the use of nearquantum-limited amplifiers [20][21][22][23], cross-correlation detectors [17, 18, 24], or superconducting qubits [25][26][27][28].The unique nature of propagating fields is reflected in their photon statistics, which is described by a probability distribution either in terms of the number states or in terms of its moments. The former were studied by coupling the field to an atom or qubit and measuring the coherent dynamics [29][30][31] or by spectroscopic analysis [32]. The moment-based approach requires knowledge on the average photon number n and its variance Var(n) = n 2 − n 2 to distinguish many states of interest. To this end, the second-order correlation function g (2) (τ ) has been measured to analyze the photon statistics of thermal [33][34][35] or quantum [36][37][38] emitters ever since the ground-breaking experiments of Hanbury Brown and Twiss [39,40]. While these experiments use the time delay τ as control parameter, at microwave frequencies the photon number n can be controlled conveniently [15,32,[41][42][43][44]. In the specific case of a thermal field at frequency ω, the Bose-Einstein distribution yields n(T ) = [exp( ω/k B T ) − 1] −1 and Var(n) = n 2 + n, which can be controlled by the temperature T of the emitter. In practice, one wants to distinguish this relation from both the classical limit Var(n) = n 2 and the Poissonian behavior Var(n) = n characteristic for coherent states [41] or shot noise [45,46]. Hence, as shown in Fig. 1, the most relev...

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Photon Statistics of Propagating Thermal Microwaves

et al. 2017

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“…Thus, the only way for the demon to extract work from its environment is to go beyond modular control, dynamically changing the energetic coupling between its memory and environment. Controllable bistable thermodynamic systems, such as Bose-Einstein condensates [41], nanoelectromechanical systems [42], flux qubits [43,44], and feedback traps [45], can store information bistably and so are candidates for experimentally implementing both the demon and its environment in a Szilard engine.…”

Section: Prior Thermodynamics Of Correlationmentioning

confidence: 99%

Thermodynamics of Modularity: Structural Costs Beyond the Landauer Bound

2018

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Information processing typically occurs via the composition of modular units, such as the universal logic gates found in discrete computation circuits. The benefit of modular information processing, in contrast to globally integrated information processing, is that complex computations are more easily and flexibly implemented via a series of simpler, localized information processing operations that only control and change local degrees of freedom. We show that, despite these benefits, there are unavoidable thermodynamic costs to modularity-costs that arise directly from the operation of localized processing and that go beyond Landauer's bound on the work required to erase information. Localized operations are unable to leverage global correlations, which are a thermodynamic fuel. We quantify the minimum irretrievable dissipation of modular computations in terms of the difference between the change in global nonequilibrium free energy, which captures these global correlations, and the local (marginal) change in nonequilibrium free energy, which bounds modular work production. This modularity dissipation is proportional to the amount of additional work required to perform a computational task modularly, measuring a structural energy cost. It determines the thermodynamic efficiency of different modular implementations of the same computation, and so it has immediate consequences for the architecture of physically embedded transducers, known as information ratchets. Constructively, we show how to circumvent modularity dissipation by designing internal ratchet states that capture the information reservoir's global correlations and patterns. Thus, there are routes to thermodynamic efficiency that circumvent globally integrated protocols and instead reduce modularity dissipation to optimize the architecture of computations composed of a series of localized operations.

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“…It is worth mentioning that dynamical decoupling techniques are available to suppress the effect of the dephasing, which can improve the sensitivity for AC magnetic fields [6,24]. With a well-controlled dynamical decoupling technique, the coherence time of the solid state qubit can be in principle limited by the energy relaxation process, which is observed in several systems such as superconducting qubits [25,26]. However, the energy relaxation induces not only bit-flip noise but also phase-flip noise.…”

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Magnetic-field sensing with quantum error detection under the effect of energy relaxation

Matsuzaki

Benjamin

2017

Phys. Rev. A

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A solid state spin is an attractive system with which to realize an ultra-sensitive magnetic field sensor. A spin superposition state will acquire a phase induced by the target field, and we can estimate the field strength from this phase. Recent studies have aimed at improving sensitivity through the use of quantum error correction (QEC) to detect and correct any bit-flip errors that may occur during the sensing period. Here, we investigate the performance of a two-qubit sensor employing QEC and under the effect of energy relaxation. Surprisingly, we find that the standard QEC technique to detect and recover from an error does not improve the sensitivity compared with the single-qubit sensors. This is a consequence of the fact that the energy relaxation induces both a phase-flip and a bit-flip noise where the former noise cannot be distinguished from the relative phase induced from the target fields. However, we have found that we can improve the sensitivity if we adopt postselection to discard the state when error is detected. Even when quantum error detection is moderately noisy, and allowing for the cost of the postselection technique, we find that this two-qubit system shows an advantage in sensing over a single qubit in the same conditions. Measurement of small magnetic fields is an important objective in the field of metrology because of many practical applications in material science, biology, and medical science. It is known that SQUID [1], Hall sensors [2], and force sensors [3] show excellent performance for such field detection.A two-level system coupled with magnetic fields is an alternative way to detect small magnetic field. The magnetic field typically shifts the resonant frequency of the qubit, and one can readout the shift from Ramsey type interference experiment, to which we refer as single-qubit sensors. Atomic vapor magnetometry is one of such ways to use a qubit for sensing [4]. Nitrogen vacancy centers in diamond is another candidate to realize such a sensor [5][6][7], and this typically has a better spatial resolution compared with other conventional devices.One of the obstacles to sense small magnetic fields with the qubit is decoherence on the system [8]. The frequency shift from the magnetic fields induces a relative phase between coherent superpositions of the qubit, and this provides us with measurable signals [9]. This means that any deteriorating effect of the phase coherence decreases the signal to noise ratio of the sensing. Since unwanted coupling with environment is inevitable, the decoherence effect is one of the main challenges to realize an ultra-sensitive field sensor with qubits.Recently, magnetic field sensing with quantum error correction (QEC) has been proposed to improve the sensitivity of qubit-based metrology [10][11][12][13]. QEC is a technique to detect and recover errors by using an encoded state where ancillary qubits are used to employ redundancy in the code space [14]. QEC has been proposed in the context of scalable quantum computation, and proof of principle ...

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The flux qubit revisited to enhance coherence and reproducibility

Cited by 555 publications

References 52 publications

Photon Statistics of Propagating Thermal Microwaves

Photon Statistics of Propagating Thermal Microwaves

Thermodynamics of Modularity: Structural Costs Beyond the Landauer Bound

Magnetic-field sensing with quantum error detection under the effect of energy relaxation

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