Macroscopic realism is the name for a class of modifications to quantum theory that allow macroscopic objects to be described in a measurement-independent manner, while largely preserving a fully quantum mechanical description of the microscopic world. Objective collapse theories are examples which aim to solve the quantum measurement problem through modified dynamical laws. Whether such theories describe nature, however, is not known. Here we describe and implement an experimental protocol capable of constraining theories of this class, that is more noise tolerant and conceptually transparent than the original Leggett–Garg test. We implement the protocol in a superconducting flux qubit, and rule out (by ∼84 s.d.) those theories which would deny coherent superpositions of 170 nA currents over a ∼10 ns timescale. Further, we address the ‘clumsiness loophole' by determining classical disturbance with control experiments. Our results constitute strong evidence for the superposition of states of nontrivial macroscopic distinctness.

Vacuum Rabi splitting is demonstrated in a GaAs double quantum dot system coupled with a coplanar waveguide resonator. The coupling strength g, the decoherence rate of the quantum dot γ, and the decay rate of the resonator κ are derived, assuring distinct vacuum Rabi oscillation in a strong coupling regime [(g,γ,κ)≈(30,25,8.0) MHz]. The magnitude of decoherence is consistently interpreted in terms of the coupling of electrons to piezoelectric acoustic phonons in GaAs.

The hybridization of distinct quantum systems is now seen as an effective way to engineer the properties of an entire system leading to applications in quantum metamaterials, quantum simulation, and quantum metrology. One well known example is superconducting circuits coupled to ensembles of microscopic natural atoms. In such cases, the properties of the individual atom are intrinsic, and so are unchangeable. However, current technology allows us to fabricate large ensembles of macroscopic artificial atoms such as superconducting flux qubits, where we can really tailor and control the properties of individual qubits. Here, we demonstrate coherent coupling between a microwave resonator and several thousand superconducting flux qubits, where we observe a large dispersive frequency shift in the spectrum of 250 MHz induced by collective behavior. These results represent the largest number of coupled superconducting qubits realized so far. Our approach shows that it is now possible to engineer the properties of the ensemble, opening up the way for the controlled exploration of the quantum many-body system.Quantum science and technology have reached a very interesting stage in their development where we are now beginning to engineer the properties that we require of our quantum systems [1,2]. Hybridization is a core technique in achieving this. An additional (or ancilla) system can be used to greatly change not only the properties of the overall system, but also its environment [3][4][5].Specifically, a hybrid system composed of many qubits and a common field such as cavity quantum electrodynamics [6,7] may provide an excellent way of realizing such quantum engineering, leading to an interesting investigation of many-body phenomena including quantum simulations [8,9], superradiant phase transitions [10][11][12][13][14][15], spin squeezing [16][17][18], and quantum metamaterials [19][20][21][22][23][24][25]. In this regard, one of the ways to realize such a system is to employ superconducting circuits coupled to electron spin ensembles where basic quantum control such as memory operations have been demonstrated [26][27][28][29][30]. If we are to investigate quantum many-body phenomena, we will need control over the ensemble. In most typical superconducting circuit-ensemble hybrid experiments, the ensemble has been formed from a collection of either atoms or molecules with examples including nitrogen vacancy centers [26][27][28]31], ferromagnetic magnons [32], and bismuth donor spins in silicon [33]. In these cases, the properties of the atomic ensemble system are basically defined as the ensemble is formed, and are difficult to change. However, our ensembles could be composed of artificial atoms such as superconducting qubits.Superconducting qubits are macroscopic two-level systems with a significant degree of design freedom [34,35]. Josephson junctions provide the superconducting circuit with non-linearity, and we can tailor the qubit properties by changing the design of the circuit. Moreover, in contrast to natural a...

Electron paramagnetic resonance (EPR) spectroscopy is an important technology in physics, chemistry, materials science, and biology [1]. Sensitive detection with a small sample volume is a key objective in these areas, because it is crucial, for example, for the readout of a highly packed spin based quantum memory or the detection of unlabeled metalloproteins in a single cell. In conventional EPR spectrometers, the energy transfer from the spins to the cavity at a Purcell enhanced rate [2] plays an essential role [1,3,4] and requires the spins to be resonant with the cavity, however the size of the cavity (limited by the wavelength) makes it difficult to improve the spatial resolution. Here, we demonstrate a novel EPR spectrometer using a single artificial atom as a sensitive detector of spin magnetization. The artificial atom, a superconducting flux qubit, provides advantages both in terms of its quantum properties and its much stronger coupling with magnetic fields. We have achieved a sensitivity of ∼400 spins/ √ Hz with a magnetic sensing volume around 10 −14 λ 3 (50 femto-liters). This corresponds to an improvement of two-order of magnitude in the magnetic sensing volume compared with the best cavity based spectrometers while maintaining a similar sensitivity as those spectrometers [5,6]. Our artificial atom is suitable for scaling down and thus paves the way for measuring single spins on the nanometer scale.EPR spectroscopy is an essential tool for characterizing the properties of electron spins in materials. Due to the wide variety of EPR applications, significant efforts have been devoted to improving both its sensitivity and spatial resolution. A conventional EPR spectrometer relies on energy exchange (transverse) coupling, where the spins and detector should be resonant. In particular, in a leaky cavity limit, the spins mainly emits photons to the measurement chain at the Purcell enhanced relaxation rate [3], and the detector absorbs the photon energy as a signal. Recently, sensitive EPR spectrometers based on a superconducting resonator have been realized [4][5][6][7] with a measurement chain that uses a quantum limited amplifier. This approach limits the size of the device according to the wavelength, and so such spectrometers may not scale well at a smaller size. On the other hand, it is also possible to observe the EPR phenomenon without a cavity and magnetization detection [8] is one such example. Magnetically induced force detection [9] has recently been demonstrated that achieves high sensitivity and spatial resolution. In these cases, energy transfer between spins and the detector is suppressed due to the large detuning, thus the signal is detected without significant disturbance to the spin system. However, such non-resonant methods still require improved in their sensitivity.In this paper, we demonstrate sensitive local EPR spectroscopy using an artificial atom (a superconducting flux qubit [10]) as a magnetic field sensor [11,12]. The superconducting flux qubit has two distinct states correspo...

We demonstrate electron spin polarization detection and electron paramagnetic resonance (EPR) spectroscopy using a direct current superconducting quantum interference device (dc-SQUID) magnetometer. Our target electron spin ensemble is directly glued on the dc-SQUID magnetometer that detects electron spin polarization induced by a external magnetic field or EPR in micrometer-sized area. The minimum distinguishable number of polarized spins and sensing volume of the electron spin polarization detection and the EPR spectroscopy are estimated to be ∼10 6 and ∼10 −10 cm 3 (∼0.1 pl), respectively. Electron paramagnetic resonance EPR spectroscopy is a widely-used method to obtain material properties such as the Landé factor of electron spins in various materials 1 . Conventional EPR spectrometers use a microwave cavity as a detector of permeability change induced by electron spin polarization 1 . Recent technological progress in superconducting circuits including Josephson junctions enables us to use these sua) Electronic perconducting devices as a sensitive detector of permeability at low temperatures. Using superconducting coplanar waveguide resonators, EPR spectroscopy of various materials, such as nitrogen vacancy (NV) centers 2 and nitrogen substitution (P1) centers 3 in diamond, chromium doped aluminum oxide 3 , and erbium impurities in yttrium orthosilicate (Y 2 SiO 5 , YSO) 4 has been demonstrated. By hybridizing a superconducting resonator and a superconducting transmon qubit, highly sensitive EPR spectroscopy was also demonstrated 5 . In these devices, coplanar waveguide resonators play the role of detectors of spin polarization 2-4 as the mi-1 arXiv:1511.04832v1 [cond-mat.mes-hall]

We report the experimental realization of a 3D capacitively-shunt superconducting flux qubit with long coherence times. At the optimal flux bias point, the qubit demonstrates energy relaxation times in the 60-90 µs range, and Hahn-echo coherence time of about 80 µs which can be further improved by dynamical decoupling. Qubit energy relaxation can be attributed to quasiparticle tunneling, while qubit dephasing is caused by flux noise away from the optimal point. Our results show that 3D c-shunt flux qubits demonstrate improved performance over other types of flux qubits which is advantageous for applications such as quantum magnetometry and spin sensing.Improving the performance of superconducting flux qubits is crucial for the development of emerging quantum technologies, such as quantum annealing 1 , quantum magnetometry 2 , and spin sensing 3 . A conventional flux qubit consists of a superconducting loop interrupted by three Josephson junctions, and transition frequencies between qubit states can be controlled by applying external magnetic flux through the qubit loop 4,5 . The decoherence of qubit states is caused by unwanted interactions with the environment, and different techniques have been employed to mitigate their effects. One of the major sources of the decoherence of a conventional flux qubit is magnetic flux noise, and it was shown theoretically that susceptibility of a flux qubit to the flux noise (as well as charge noise) can be significantly reduced by shunting the smaller Josephshon junction by an additional capacitance 6 . Initial experimental studies of c-shunt flux qubits coupled to a coplanar resonator (2D c-shunt flux qubits) reported a modest improvement of coherence times 7,8 , but later experiments demonstrated energy relaxation times T 1 of up to 55 µs 9 and spin-echo decoherence times T 2E of about 80 µs 10 . Besides magnetic flux noise, another possible qubit decoherence mechanism is the unintended interaction between a qubit and spurious microwave modes. As originally demonstrated in experiments with transmon qubits 11,12 , it is possible to create a well-controlled electromagnetic environment for a qubit by coupling it to a 3D microwave cavity. Moreover, it was found that surface dielectric losses can be reduced by using 3D qubit designs 13 . In the case of flux qubits, it was shown that, by embedding a conventional flux qubit in a 3D cavity, intrinsic energy relaxation time can reach 20 µs and pure dephasing times can be up to 10 µs 14 . In this work, we combined the above approaches in order to reduce qubit decoherence due to magnetic flux noise, charge noise and coupling to higher microwave modes.Here we present the realization of a c-shunt flux qubit coupled to a 3D microwave cavity. At the optimal working point of magnetic flux bias, we observed energyrelaxation times T 1 in the range of 60-90 µs, and spina) Electronic mail: leonid.abdurakhimov.nz@hco.ntt.co.jp 200¡m 20 mm (a) 2 m (e) (d) (b) (c) FIG. 1. (a) A photograph of the c-shunt flux qubit mounted inside the 3D cavity. (b)...

In this Letter, we propose a counterintuitive use of a hybrid system where the coherence time of a quantum system can be significantly improved by coupling it with a system of a shorter coherence time. Coupling a two-level system with a single nitrogen-vacancy (NV^{-}) center, a dark state of the NV^{-} center naturally forms after the hybridization. We show that this dark state becomes robust against noise due to the coupling even when the coherence time of the two-level system is much shorter than that of the NV^{-} center. Our proposal opens a new way to use a quantum hybrid system for the realization of robust quantum information processing.

We report on electron spin resonance spectroscopy measurements using a superconducting flux qubit with a sensing volume of 6 fl. The qubit is read out using a frequency-tunable Josephson bifurcation amplifier, which leads to an inferred measurement sensitivity of about 20 spins in a 1 s measurement. This sensitivity represents an order of magnitude improvement when compared to flux-qubit schemes using a direct current-superconducting quantum interference device switching readout. Furthermore, noise spectroscopy reveals that the sensitivity is limited by flicker (1/f) flux noise.