We demonstrate magnetometry of cultured neurons on a polymeric film using a superconducting flux qubit that works as a sensitive magnetometer in a microscale area. The neurons are culturedin Fe3+ rich medium to increase magnetization signal generated by the electron spins originating from the ions. The magnetometry is performed by insulating the qubit device from the laden neurons with the polymeric film while keeping the distance between them around several micrometers. By changing temperature (12.5 – 200 mK) and a magnetic field (2.5 – 12.5 mT), we observe a clear magnetization signal from the neurons that is well above the control magnetometry of the polymeric film itself. From electron spin resonance (ESR) spectrum measured at 10 K, the magnetization signal is identified to originate from electron spins of iron ions in neurons. This technique to detect a bio-spin system can be extended to achieve ESR spectroscopy at the single-cell level, which will give the spectroscopic fingerprint of cells.
Parasitic two-level-system (TLS) defects are one of the major factors limiting the coherence times of superconducting qubits. Although there has been significant progress in characterizingbasic parameters of TLS defects, exact mechanisms of interactions between a qubit and various types of TLS defects remained largely unexplored due to the lack of experimental techniques able to probe the form of qubit-defect couplings. Here we present an experimental method of TLS defect spectroscopy using a strong qubit drive that allowed us to distinguish between various types of qubit-defect interactions. By applying this method to a capacitively shunted flux qubit, we detected a rare type of TLS defects with a nonlinear qubit-defect coupling due to critical-current fluctuations, as well as conventional TLS defects with a linear coupling to the qubit caused by charge fluctuations. The presented approach could become the routine method for high-frequency defect inspection and quality control in superconducting qubit fabrication, providing essential feedback for fabrication process optimization. The reported method is a powerful tool to uniquely identify the type of noise fluctuations caused by TLS defects, enabling the development of realistic noise models relevant to fault-tolerant quantum control.
We control transition frequency of a superconducting flux qubit coupled to a frequency-tunable resonator comprising a direct current superconducting quantum interference device (dc-SQUID)by microwave driving. The dc-SQUID mediates the coupling between microwave photons in the resonator and a flux qubit. The polarity of the frequency shift depends on the sign of the flux bias for the qubit and can be both positive and negative. The absolute value of the frequency shift becomes larger by increasing the photon number in the resonator. These behaviors are reproduced by a model considering the magnetic interaction between the flux qubit and dc-SQUID. The tuning range of the transition frequency of the flux qubit reaches ≈ 1.9 GHz, which is much larger than the ac Stark/Lamb shift observed in the dispersive regime using typical circuit quantum electrodynamics devices.
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 Josephsonbifurcation 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 with flux-qubit schemes using a dc-SQUID switching readout. Furthermore, noise spectroscopy reveals that the sensitivity is limited by flicker (1/f) flux noise.
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 energyrelaxation 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.
We theoretically analyze the performance of the nuclear magnetic resonance (NMR) spectroscopy with a superconducting flux qubit (FQ). Such NMR with the FQ is attractive because of thepossibility to detect the relatively small number of nuclear spins in a local region (∼μm) with low temperatures (∼ mK) and low magnetic fields (∼ mT), in which other types of quantum sensing schemes cannot easily access. A sample containing nuclear spins is directly attached on the FQ, and the FQ is used as a magnetometer to detect magnetic fields from the nuclear spins. Especially, we consider two types of approaches to NMR with the FQ. One of them is to use spatially inhomogeneous excitations of the nuclear spins, which are induced by a spatially asymmetric driving with radio frequency~(RF) pulses. Such an inhomogeneity causes a change in the DC magnetic flux penetrating a loop of the FQ, which can be detected by a standard Ramsey measurement on the FQ. The other approach is to use a dynamical decoupling on the FQ to measure AC magnetic fields induced by Larmor precession of the nuclear spins. In this case, neither a spin excitation nor a spin polarization is required since the signal comes from fluctuating magnetic fields of the nuclear spins. We calculate the minimum detectable density (number) of the nuclear spins for the FQ with experimentally feasible parameters. We show that the minimum detectable density (number) of the nuclear spins with these approaches is around 1021 /cm3 (108) with an accumulation time of a second.
Electron paramagnetic resonance (EPR) spectroscopy is an important technology in physics, chemistry, materials science, and biology. Sensitive detection with a small sample volume isa 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 plays an essential role 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 10−14λ3 (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 . Our artificial atom is suitable for scaling down and thus paves the way for measuring single spins on the nanometer scale.
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, quantumsimulation, 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.
Macroscopic realism is the name for a class of modifications to quantum theory that allow macroscopic objects to be described in a measurement-independent fashion, while largely preservinga fully quantum mechanical description of the microscopic world. Objective collapse theories are examples which attempt to provide a physical mechanism for wavefunction collapse, and thereby aim to solve the quantum measurement problem. Here we describe and implement an experimental protocol capable of constraining theories of this class, and show how it is related to the original Leggett-Garg test, yet more noise tolerant and conceptually transparent. By conducting a set of simple ‚prepare, shuffle, measure‘ tests in a superconducting flux qubit, we rule out (by over 77 standard deviations) those theories which would deny coherent superpositions of 170 nA currents over a 9 ns timescale. Further, we address the ‚clumsiness loophole‘ by determining classical disturbance in a set of control experiments.
One of the promising systems to realize quantum computation is a hybrid system where a superconducting flux qubit plays a role of a quantum processor and the NV center ensemble is usedas a quantum memory. We have theoretically and experimentally studied the effect of magnetic fields on this hybrid system, and found that the lifetime of the vacuum Rabi oscillation is improved by applying a few mT magnetic field to the NV center ensemble. Here, we construct a theoretical model to reproduce the vacuum Rabi oscillations with/without magnetic fields applied to the NV centers, and we determine the reason why magnetic fields can affect the coherent properties of the NV center ensemble. From our theoretical analysis, we quantitatively show that the magnetic fields actually suppress the inhomogeneous broadening from the strain in the NV centers.