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...