We report repeated single-shot measurements of the two-electron spin state in a GaAs double quantum dot. The readout scheme allows measurement with fidelity above 90% with a ∼ 7 µs cycle time. Hyperfine-induced precession between singlet and triplet states of the two-electron system are directly observed, as nuclear Overhauser fields are quasi-static on the time scale of the measurement cycle. Repeated measurements on millisecond to second time scales reveal evolution of the nuclear environment.Qubits constructed from spin states of confined electrons are of interest for quantum information processing [1], for investigating decoherence and controlled entanglement, and as probes of mesoscopic nuclear spin environments. For logical qubits formed from pairs of electron spins in quantum dots [2], several requirements for quantum computing [3] have been realized [4,5,6,7]. To date, however, measurements of these systems have constituted ensemble averages over time, while protocols for quantum control, including quantum error correction, typically require high-fidelity single-shot readout. Coherent evolution conditional on individual measurement outcomes can give rise to interesting non-classical states [8,9]. Rapidly repeated single-shot measurements can also give access to the dynamics of the environment, allowing, for instance, feedback-controlled manipulation of the nuclear state. Single-shot measurements of solidstate quantum systems have been reported for superconducting qubits [10], the charge state of a single quantum dot [11], the spin of a single electron in a quantum dot in large magnetic fields [12,13], and the two-electron spin state in a single quantum dot [14].In this Letter, we demonstrate rapidly repeated highfidelity single-shot measurements of a two-electron spin (singlet-triplet) qubit in a double quantum dot. Singlet and triplet spin states are mapped to charge states [4], which are measured by a radio-frequency quantum point contact (rf-QPC) that is energized only during readout. The measurement integration time required for > 90% readout fidelity is a few microseconds. On that time scale, nuclear Overhauser fields are quasi-static, leading to observed periodic precession of the qubit. By measuring over longer times, the evolution of the Overhauser fields from milliseconds to several seconds can be seen as well. We apply a model of single-shot readout statistics that accounts for T 1 relaxation, and find good agreement with experiment. Finally, we examine the evolution of the two-electron spin state at the resonance between the singlet (S) and the m = +1 triplet (T + ) via repeated single-shot measurement, and show that the transverse component of the Overhauser field difference is not quasistatic on the time scale of data acquisition, as expected theoretically.The double quantum dot is formed by Ti/Au depletion gates on a GaAs/Al 0.3 Ga 0.7 As heterostructure with a two-dimensional electron gas (density 2 × 10 15 m −2 , mobility 20 m 2 /Vs) 100 nm below the surface. In order to split the three t...

Semiconductor spins are one of the few qubit realizations that remain a serious candidate for the implementation of large-scale quantum circuits. Excellent scalability is often argued for spin qubits defined by lithography and controlled via electrical signals, based on the success of conventional semiconductor integrated circuits. However, the wiring and interconnect requirements for quantum circuits are completely different from those for classical circuits, as individual direct current, pulsed and in some cases microwave control signals need to be routed from external sources to every qubit. This is further complicated by the requirement that these spin qubits currently operate at temperatures below 100 mK. Here, we review several strategies that are considered to address this crucial challenge in scaling quantum circuits based on electron spin qubits. Key assets of spin qubits include the potential to operate at 1 to 4 K, the high density of quantum dots or donors combined with possibilities to space them apart as needed, the extremely long-spin coherence times, and the rich options for integration with classical electronics based on the same technology.npj Quantum Information (2017) 3:34 ; doi:10.1038/s41534-017-0038-y INTRODUCTIONThe quantum devices in which quantum bits are stored and processed will form the lowest layer of a complex multi-layer system. 1-3 The system also includes classical electronics to measure and control the qubits, and a conventional computer to control and program these electronics. Increasingly, some of the important challenges involved in these intermediate layers and how they interact have become clear, and there is a strong need for forming a picture of how these challenges can be addressed.Focusing on the interface between the two lowest layers of a quantum computer, each of the quantum bits must receive a long sequence of externally generated control signals that translate to the steps in the computation. Furthermore, given the fragile nature of quantum states, large numbers of quantum bits must be read out periodically to check whether errors occurred along the way, and to correct them. 4 Such error correction is possible provided the probability of error per operation is below the accuracy threshold, which is around 1% for the so-called surface code, a scheme which can be operated on two-dimensional (2D) qubit arrays with nearest-neighbor couplings. 5,6 The read-out data must be processed rapidly and fed back to the qubits in the form of control signals. Since each qubit must separately interface with the outside world, the classical control system must scale along with the number of qubits, and so must the interface between qubits and classical control.The estimated number of physical qubits required for solving relevant problems in quantum chemistry or code breaking is in the 10 6 -10 8 range, using currently known quantum algorithms and quantum error correction methods. 7,8 For comparison, state-

Coherent spin states in semiconductor quantum dots offer promise as electrically controllable quantum bits (qubits) with scalable fabrication. For few-electron quantum dots made from gallium arsenide (GaAs), fluctuating nuclear spins in the host lattice are the dominant source of spin decoherence. We report a method of preparing the nuclear spin environment that suppresses the relevant component of nuclear spin fluctuations below its equilibrium value by a factor of approximately 70, extending the inhomogeneous dephasing time for the two-electron spin state beyond 1 microsecond. The nuclear state can be readily prepared by electrical gate manipulation and persists for more than 10 seconds.

Inflammatory monocyte-derived effector cells play an important role in the pathogenesis of numerous inflammatory diseases. However, no treatment option exists that is capable of modulating these cells specifically. We show that infused negatively charged, immune-modifying microparticles (IMPs), derived from polystyrene, microdiamonds, or biodegradable poly(lactic-co-glycolic) acid, were taken up by inflammatory monocytes, in an opsonin-independent fashion, via the macrophage receptor with collagenous structure (MARCO). Subsequently, these monocytes no longer trafficked to sites of inflammation; rather, IMP infusion caused their sequestration in the spleen through apoptotic cell clearance mechanisms and, ultimately, caspase-3–mediated apoptosis. Administration of IMPs in mouse models of myocardial infarction, experimental autoimmune encephalomyelitis, dextran sodium sulfate–induced colitis, thioglycollate-induced peritonitis, and lethal flavivirus encephalitis markedly reduced monocyte accumulation at inflammatory foci, reduced disease symptoms, and promoted tissue repair. Together, these data highlight the intricate interplay between scavenger receptors, the spleen, and inflammatory monocyte function and support the translation of IMPs for therapeutic use in diseases caused or potentiated by inflammatory monocytes.

We report high-bandwidth charge sensing measurements using a GaAs quantum point contact embedded in a radio frequency impedance matching circuit (rf-QPC). With the rf-QPC biased near pinch-off where it is most sensitive to charge, we demonstrate a conductance sensitivity of 5 × 10−6 e 2 /h Hz −1/2 with a bandwidth of 8 MHz. Single-shot readout of a proximal few-electron double quantum dot is investigated in a mode where the rf-QPC back-action is rapidly switched.Mesoscopic charge sensors such as the single-electron transistor (SET) [1] and the quantum point contact (QPC) [2] are at the heart of many readout technologies for quantum information processing. As electrometers, their intrinsic sensitivity [3,4] provides efficient measurement with detector noise close to the minimum allowed by quantum mechanics [5]. When combined with highbandwidth operation, these devices are attractive for application in metrology [6,7], single photon detection [8], and as non-invasive charge probes at the nanoscale [9,10,11].Combining sensitivity with fast operation is challenging because of the large RC time of the detector resistance (> 50 kΩ) and shunt capacitance of wire between the cold stage of a cryostat and room-temperature electronics (hundreds of pF). Nevertheless several demonstrations of single-electron detection have been reported with bandwidths in the tens of kHz [12,13,14]. An approach [15] that circumvents the difficulty of wiring capacitance uses an impedance matching network on resonance to transform the high resistance of the detector towards the Z 0 = 50 Ω characteristic impedance of a transmission line. Changes in device resistance modulate the reflected or transmitted [16] power of a radio frequency (rf) carrier, tuned to the resonance frequency. Application of this reflectometry technique to aluminum based SETs has demonstrated charge sensing [9], near quantum-limited sensitivity [4,17], and bandwidths above 100 MHz [15].In this Letter, we extend rf reflectometry to a semiconductor quantum point contact. We describe the reflectometer circuit in detail and explore its performance as a fast, cryogenic charge sensor, demonstrating singleelectron sensing with 8 MHz bandwidth. In the regime of operation, intrinsic shot noise of the QPC makes up approximately 80% of the total system noise. Previous work [18,19] has demonstrated modulation of rf power by a point contact operated as a voltage-controlled resistor. Here, we demonstrate application of the QPC as a fast charge sensor by performing charge-state readout of an integrated double quantum dot [20] with fixed total charge. In addition, we present single-shot measurements of the double dot in a mode where the rf-QPC carrier is rapidly switched, modulating the measurement back-action on a time-scale of 50 ns.The device, shown in Fig. 1(a), consists of a GaAs/Al 0.3 Ga 0.7 As heterostructure with two dimensional electron gas (density 2 × 10 15 m −2 , mobility 20 m 2 /Vs) 100 nm below the surface. Ti/Au top gates define a few-electron double dot with proximal QPC....

Solid-state quantum computer architectures with qubits encoded using single atoms are now feasible given recent advances in the atomic doping of semiconductors. Here we present a charge qubit consisting of two dopant atoms in a semiconductor crystal, one of which is singly ionized. Surface electrodes control the qubit and a radio-frequency single-electron transistor provides fast readout. The calculated single gate times, of order 50 ps or less, are much shorter than the expected decoherence time. We propose universal one-and two-qubit gate operations for this system and discuss prospects for fabrication and scale up. DOI: 10.1103/PhysRevB.69.113301 PACS number͑s͒: 03.67.Lx, 73.21.Ϫb, 85.40.Ry In the search for an inherently scalable quantum computer ͑QC͒ technology solid-state systems are of great interest. One of the most advanced proposals is based on superconducting qubits, 1 where coherent control of qubits has been demonstrated and decoherence times measured.2 The Kane scheme, 3 in which qubits are defined by nuclear spin states of buried phosphorus dopants in a silicon crystal, has also attracted considerable attention due to its promise of very long ͑ms or longer͒ decoherence times below 1 K. Recent advances in single-dopant fabrication, 4 -6 together with the demonstration of fast single-electron transistor ͑SET͒ charge detection, 7,8 bring the Kane Si:P architecture closer to reality. These important results notwithstanding, the demonstration of single-spin readout remains a major challenge. Here we consider a Si:P dopant-based qubit in which the logical information is encoded on the charge degrees of freedom. This system, which is complementary to the Kane concept, is not dependent on single-spin readout and, given the present availability of fabrication 4 -6 and readout 7,8 technologies, can now be built. A two-qubit gate based on the charge qubit scheme we describe will enable an experimental determination to be made of the key sources of decoherence and error in a nanoscale silicon QC architecture. Such devices therefore provide an important and necessary pathway towards the longer term goal of real-spin Si:P devices.Semiconductor quantum-dot charge-based qubits were first considered in 1995 by Barenco et al.,9 where quantum information was encoded in excitation levels, and later by Fedichkin et al. 10 for position-based charge qubits in GaAs. Very recently, coherent oscillations have been observed 11 in a GaAs double quantum dot providing realization of a chargebased qubit with coherence times above 1 ns, accessible by existing fast pulse technology. In this paper we assess the potential of Si:P donor-based charge qubits by calculating the energetics and gate operation times for realistic device configurations and gate potentials and find that both one-and two-qubit operations times are well within the relevant decoherence times for the system.The buried donor charge qubit is shown in Fig. 1 for the case of P dopants in Si, although a number of other dopantsubstrate systems could also be consi...

One proposal for a solid-state-based quantum bit (qubit) is to control coupled electron spins on adjacent semiconductor quantum dots. Most experiments have focused on quantum dots made from III-V semiconductors; however, the coherence of electron spins in these materials is limited by hyperfine interactions with nuclear spins. Ge/Si core/shell nanowires seem ideally suited to overcome this limitation, because the most abundant nuclei in Ge and Si have spin zero and the nanowires can be chemically synthesized defect-free with tunable properties. Here, we present a double quantum dot based on Ge/Si nanowires in which we can completely control the coupling between the dots and to the leads. We also demonstrate that charge on the double dot can be detected by coupling it capacitively to an adjacent nanowire quantum dot. The double quantum dot and integrated charge sensor serve as an essential building block to form a solid-state qubit free of nuclear spin.

We report the dispersive charge-state readout of a double quantum dot in the few-electron regime using the in situ gate electrodes as sensitive detectors. We benchmark this gate-sensing technique against the well established quantum point contact (QPC) charge detector and find comparable performance with a bandwidth of ∼ 10 MHz and an equivalent charge sensitivity of ∼ 6.3 × 10 −3 e/ √ Hz. Dispersive gate-sensing alleviates the burden of separate charge detectors for quantum dot systems and promises to enable readout of qubits in scaled-up arrays.Non-invasive charge detection has emerged as an important tool for uncovering new physics in nanoscale devices at the single-electron level and allows readout of spin qubits in a variety of material systems [1][2][3][4][5][6][7][8][9]. For quantum dots defined electrostatically by the selective depletion of a two dimensional electron gas (2DEG), the conductance of a proximal quantum point contact (QPC) [4][5][6][7]9] or single electron transistor (SET) [3,8] can be used to detect the charge configuration in a regime where direct transport is not possible. This method can, in principle, reach quantum mechanical limits for sensitivity [10] and has enabled the detection of single electron spin-states [4, 7, 11] with a 98% readout fidelity in a single-shot [12].An alternate approach to charge-state detection, long used in the context of single electron spectroscopy [13], is based on the dispersive signal from shifts in the quantum capacitance [14,15] when electrons undergo tunnelling. Similar dispersive interactions are now the basis for readout in a variety of quantum systems including atoms in an optical resonator [16], superconducting qubits [17][18][19] and nanomechanical devices [20].In this Letter we report dispersive readout of quantum dot devices using the standard, in situ gate electrodes coupled to lumped-element resonators as highbandwidth, sensitive charge-transition sensors.We demonstrate the sensitivity of this gate-sensor in the fewelectron regime, where these devices are commonly operated as charge or spin qubits [21] and benchmark its performance against the well established QPC charge sensor. We find that because the quantum capacitance is sufficiently large in these devices, gate-sensors have similar sensitivity to QPC sensors. In addition, we show that gate-sensors can operate at elevated temperatures in comparison to QPCs.Previous investigations, in the context of circuit quantum electrodynamics (c-QED), have engineered a dispersive interaction between many-electron dots and superconducting coplanar waveguide resonators [22][23][24][25]. Recently, the charge and spin configuration of double quantum dots has also been detected by dispersive changes in a radio frequency resonator coupled directly to the source or drain contacts of the device [25][26][27][28]. The present work advances these previous studies by demonstrating that the gates, already in place to define the quantum dot system, can also act as fast and sensitive readout detectors in the single...