We report the observation of an even-denominator fractional quantum Hall (FQH) state at ν = 1/4 in a high quality, wide GaAs quantum well. The sample has a quantum well width of 50 nm and an electron density of ne = 2.55 × 10 11 cm −2 . We have performed transport measurements at T ∼ 35 mK in magnetic fields up to 45 T. When the sample is perpendicular to the applied magnetic field, the diagonal resistance displays a kink at ν = 1/4. Upon tilting the sample to an angle of θ = 20.3 o a clear FQH state at emerges at ν = 1/4 with a plateau in the Hall resistance and a strong minimum in the diagonal resistance.PACS numbers: 73.21. Fg, 73.43.Qt, 73.63.Hs Interest in the even-denominator fractional quantum Hall (FQH) state at ν = 5/2 in the first excited Landau level continues to remain high over twenty years after its discovery [1]. Generally believed to be due to the p−wave pairing of composite fermions [2,3,4,5], the quasi-particle excitations of this state are thought to obey nonabelian statistics and thus may be relevant to faulttolerant, topological quantum computing schemes [6,7].To date, observations of even-denominator FQH states have been rare beyond the ν = 5/2 state in single-layer systems [8]. In particular, experimental evidence for a FQH state at ν = 1/2, the lowest Landau level counterpart of the ν = 5/2 state, does not exist, although previous theoretical work has suggested that it may form in thick two-dimensional electrons systems (2DES) [9].In bilayer systems, however, the situation is different. The presence of two nearby interacting electron layers introduces an additional degree of freedom which can allow the formation of a FQH state at ν = 1/2. Observations of such a state at ν = 1/2 have been made in both double quantum wells [10] and wide single quantum wells (WSQWs) [11,12]. In both of these cases the ν = 1/2 state has been shown to have a large overlap with the so-called {331} wave function [13]. Originally proposed by Halperin[14] to describe two-component FQH states, the {331} wave function can also be characterized as a p−wave pairing state, although with albelian statistics [15,16,17]. A crude way to interpret the {331} wave function, or in general any {nnm} wave function, is to consider two electron layers each with a filling factor of ν * = 1/n. The electrons in each layer are bound to correlation holes in the other, represented by a filling factor of 1/m. Together the filling factor of the entire system is ν = 2/(n + m) [18].Beyond the {331} state the model should generalize to other even-denominator states. For example, both the {771} and {553} wave functions would be possible candidates to describe a FQH state at ν = 1/4. In contrast to ν = 1/2, relatively little theoretical work has been done concerning a FQH state ν = 1/4 and an experimental observation of this state has yet to be reported. On the one hand, an observation of the ν = 1/4 state would be demanding experimentally, including a high mobility 2DES and ultra high magnetic fields for high density samples. On the other ...

Spin-orbit coupling is relatively weak for electrons in bulk silicon, but enhanced interactions are reported in nanostructures such as the quantum dots used for spin qubits. These interactions have been attributed to various dissimilar interface effects, including disorder or broken crystal symmetries. In this Letter, we use a double-quantum-dot qubit to probe these interactions by comparing the spins of separated singlet-triplet electron pairs. We observe both intravalley and intervalley mechanisms, each dominant for [110] and [100] magnetic field orientations, respectively, that are consistent with a broken crystal symmetry model. We also observe a third spin-flip mechanism caused by tunneling between the quantum dots. This improved understanding is important for qubit uniformity, spin control and decoherence, and two-qubit gates.

We use a cryogenic high-electron-mobility transistor circuit to amplify the current from a single electron transistor, allowing for demonstration of single shot readout of an electron spin on a single P donor in Si with 100 kHz bandwidth and a signal to noise ratio of ∼9. In order to reduce the impact of cable capacitance, the amplifier is located adjacent to the Si sample, at the mixing chamber stage of a dilution refrigerator. For a current gain of ∼2.7×103, the power dissipation of the amplifier is 13 μW, the bandwidth is ∼1.3 MHz, and for frequencies above 300 kHz the current noise referred to input is ≤70 fA/Hz. With this amplification scheme, we are able to observe coherent oscillations of a P donor electron spin in isotopically enriched 28Si with 96% visibility.

Abstract:We demonstrate a capability of deterministic doping at the single atom level using a combination of direct write focused ion beam and solid-state ion detectors. The focused ion beam system can position a single ion to within 35 nm of a targeted location and the detection system is sensitive to single low energy heavy ions. This platform can be used to deterministically fabricate single atom devices in materials where the nanostructure and ion detectors can be integrated, including donor-based qubits in Si and color centers in diamond.Deterministic placement of single atoms is a key capability for fabrication of nanometer scale and single atom solid-state devices in a range of material systems including Si, diamond, and III-V compounds. Examples of Si-based devices include: donors coupled to quantum dots [1] for charge [2], electron [3,4], and nuclear spin [5,6] qubits (quantum bits) and acceptors coupled to silicon cavities to create phononic qubits [7]. Single color (defect) centers in diamond have a range of applications including metrology [8], quantum computing using nitrogen-vacancy (NV) centers [9] and coupling silicon-vacancy (SiV) centers to photonic cavities for cavity QED experiments [9,10]. In III-V materials, deterministic seeding of nucleation sites for controlling the quantum dot growth locations [11] has many potential applications including the development of single photon sources [12]. In many applications, placement of single ions within small volumes is critical. Ion implantation has been widely applied in the semiconductor industry for introducing dopants with a nominal depth and dose by varying the implant energy and the exposure time, respectively. The key challenges to extending this technique down to single atom control are the precise control over the atom's position and the implantation of one and only one atom. Techniques include in-situ ion detection using PIN diode detectors [13][14][15] and FinFETs [16,17] and detection of secondary electrons [18].We present a "top down" ion implantation approach to deterministic single atom device fabrication in Si and in other material systems suitable for ion detection including diamond [19] and GaAs [20]. This requires the ability to place the implanted ions with high positioning precision and deterministic control over the number of ions implanted. We use the nanoImplanter (nI) at Sandia National Labs (SNL), which is a direct-write focused ion beam platform to control the positioning of the implanted ion and in-situ solid-state detectors for single ion detection. We demonstrate single ion targeting to less than 35 nm allowing for deterministic single ion implantation. The combination of focused ion beams, direct write lithography, fast beam blanking and chopping, ion mass selectivity, in-situ detection and electrical probing are key features that enable rapid prototyping, customized implantation and high throughput fabrication of deterministic single atom devices. As a test of our "top-down" ion implantation and detection capability we...

Even as today's most prominent spin-based qubit technologies are maturing in terms of capability and sophistication, there is growing interest in exploring alternate material platforms that may provide advantages, such as enhanced qubit control, longer coherence times, and improved extensibility. Recent advances in heterostructure material growth have opened new possibilities for employing hole spins in semiconductors for qubit applications. Undoped, strained Ge/SiGe quantum wells are promising candidate hosts for hole spin-based qubits due to their low disorder, large intrinsic spin-orbit coupling strength, and absence of valley states. Here, we use a simple one-layer gated device structure to demonstrate both a single quantum dot as well as coupling 2 between two adjacent quantum dots. The hole effective mass in these undoped structures, m* ~ 0.08 m0, is significantly lower than for electrons in Si/SiGe, pointing to the possibility of enhanced tunnel couplings in quantum dots and favorable qubit-qubit interactions in an industry-compatible semiconductor platform. CONCLUSIONSWe have demonstrated lithographically defined single and double hole quantum dots in high quality strained Ge/SiGe quantum wells. These results strongly suggest that this material system may serve as a viable host for spin-based qubits (compatible with CMOS processing) and enable quantitative comparisons between quantum dots in electron and hole systems. Multi-metal-layer devices, as are commonly used in Si/SiGe, should be directly applicable to Ge/SiGe to increase the sharpness of tunnel barriers and provide more orthogonal control of coupling between adjacent quantum dots. Development of successful qubit architectures will ultimately call for indepth studies of qubit decoherence mechanisms in this system, in particular the impact of charge noise due to the enhanced spin-orbit coupling. AUTHOR INFORMATION