Session F29: Semiconductor Qubits - Spin Qubit Read-out II

Live

Fault-tolerant quantum computation requires qubit measurements to be both high fidelity and fast to minimise errors on measured and idling qubits and reduce the integrated measurement noise over the course of an experiment. Towards this goal, we demonstrate single-shot readout of semiconductor single-spin qubits with 97% fidelity in 1.5 μs. In particular, we show that we can engineer donor-based single-electron transistors (SETs) in silicon with atomic precision to measure single spins much faster than the spin decoherence times in isotopically purified silicon (270 μs). By designing the SET to have a large capacitive coupling between the SET and target charge, we can optimally operate in the “strong-response” regime to ensure maximal signal contrast. We demonstrate single-charge detection with a signal-to-noise ratio (SNR) of 12.7 at 10 MHz bandwidth, corresponding to a SET charge sensitivity (integration time for SNR=2) of 2.5 ns. We present a theory of the shot-noise sensitivity limit for the strong-response regime which predicts that the present sensitivity is about one order of magnitude above the shot-noise limit. By reducing cold amplification noise to reach the shot-noise limit, it should be theoretically possible to achieve high-fidelity, single-shot readout of an electron spin in silicon with a total readout time of approximately 36 ns.   

Live

We investigate gate-induced quantum dots in silicon nanowire fabricated using a foundry-compatible fully depleted silicon-on-insulator (FD-SOI) process. A series of split gates overlapping the silicon nanowire naturally produces a 2 × n bilinear array of quantum dots. We present the capacitive coupling of quantum dots within such a 2 × 2 array and then show how such couplings can be extended across two parallel silicon nanowires coupled together by shared, electrically isolated, “floating” electrodes. With one quantum dot operating as a single-electron-box sensor, the floating gate serves to enhance the charge sensitivity range, enabling it to detect charge state transitions in a separate silicon nanowire. By comparing measurements from multiple devices, we illustrate the impact of the floating gate by quantifying both the charge sensitivity decay as a function of dot-sensor separation and configuration within the dual-nanowire structure.   

Live

Adiabatic conversion schemes are commonly used to
measure quantum states. These schemes may include, e.g., spin-to-charge
conversion for spins in quantum dots or parity-to-charge conversion for qubits
based on Majorana zero modes. In all of these schemes, a common element
is that dephasing is minimized at an "operating point" (e.g., for the 'spin' or
'parity' quantum number), but measurement-induced dephasing is maximized
at the "measurement point". The balance between 'adiabaticity' and
dephasing can be optimized to improve performance of these readout
schemes. We give an explicit construction that allows for optimal state
conversion in qubit readouts. Applying this scheme to the specific case of
spin qubits in quantum dots, we show that a high-fidelity (better than 99.9%)
single-shot all-electrical readout is possible.   

Live

One practical challenge of realizing spin-based quantum processors is the tuning of many different control voltages to a desirable region within its high-dimensional gate-voltage space. For a capacitively-coupled network of quantum dots, Coulomb blockade can stabilize many different charge configurations depending on applied gate voltages. In gate-voltage space, each ground state is associated with a region that is (within the constant-interaction model) a convex polytope. A common method of locating these polytopes is by dense rastering of gate-voltage space. From these charge stability maps information of Coulomb-blockade boundaries is extracted, thereby diminishing the information content of most measured pixels and making this approach expensive for larger arrays.
To learn Coulomb-blockade boundaries from only a sparse set of measurements, we develop a convex-polytope-finding algorithm based on active learning and large-margin classifiers suitable for noisy measurements. By applying this algorithm to a quadruple dot implemented in silicon we demonstrate the automatic discovery of charge-state transitions from a small number of noisy measurements obtained via high-frequency reflectometry off one gate electrode.   

Live

Parametric amplification through pumping a non-linear or variable reactive element of a resonator can approach quantum-limited noise performance. Josephson junction parametric amplifiers (JPAs) based on a non-linear inductance have been instrumental in enabling rapid, high fidelity readout of superconducting qubits and, recently, semiconductor quantum dots (QDs).

We analyse through a semi-classical model and demonstrate experimentally parametric amplification using the tuneable tunnelling capacitance of a QD-reservoir electron transition in a CMOS nanowire split-gate transistor embedded in a 1.8 GHz superconducting spiral inductor microwave cavity. Pumping through one gate the QD-reservoir detuning at twice the cavity resonant frequency, while probing the hybrid QD-cavity in reflectometry, we achieve phase-sensitive (de)amplification of the reflection coefficient by (-)+3dB (cf. zero pump amplitude). The performance here was limited by the Sisyphus dissipation in the QD; however, we identify a clear path towards achieving gains comparable to JPAs using only the same devices already present for dispersive gate-based readout.   

Live

We present our latest experimental results on sub-GHz Josephson parametric amplifiers (JPAs) fabricated with our Nb/Al-AlO_x/Nb junction process [1]. The early generation of our flux-driven reflection-type JPA has found applications in the radio-frequency readout of bolometers [2], as well as quantum dots [3]. We have recently addressed bandwidth limitations by engineering a partially reconfigurable impedance transformer. With a bandwidth of 10 MHz at 20 dB gain at 600 MHz, the improved JPA is in line with the requirements of fast readout schemes for, e.g., semiconductor quantum dots. In addition, we have experimented with a variant where unwanted non-linearities are suppressed by alternating the dipolar orientation of superconducting nonlinear asymmetric inductive elements (SNAILs) in an array.

[1] Leif Grönberg et al., Supercond. Sci. Technol. 30, 125016 (2017)
[2] Roope Kokkoniemi et al., Commun. Phys. 2, 124 (2019)
[3] P. Apostolidis et al., arXiv:2007.03588   

Live

Nuclear spins show long coherence times and are well isolated from the environment, which are properties making them promising for quantum information applications. However, these same qualities make the readout of nuclear spin qubits challenging. Here, we present a method for nuclear spin readout by probing the transmission of a microwave resonator. We consider the flopping mode spin qubit [1,2] formed by a single electron in a silicon double quantum dot subjected to a homogeneous magnetic field and a transverse magnetic field gradient. This qubit interacts with a microwave resonator via the electric dipole coupling allowing for strong spin photon coupling [3,4]. In our scenario, the electron spin interacts with a ³¹P defect nuclear spin via the hyperfine interaction. Our theoretical investigation demonstrates a ³¹P nuclear spin state dependent change of the cavity transmission that could be resolved in experiments and used to readout the state of the nuclear spin.

[1] Benito et al., Phys. Rev. B 100, 125430 (2019)
[2] Croot et al., Phys. Rev. Research 2, 012006 (2020)
[3] Mi et al., Nature 555, 599 (2018)
[4] Benito et al., Phys. Rev. B 96, 235434 (2017)   

Live

Interacting nuclear spins in diamond are a promising new platform for simulating many-body physics phenomena, due to their naturally realised spin-spin interactions combined with high-fidelity control and selective readout. Recently, we demonstrated the 3D imaging of a cluster of 27 coupled nuclear spins using a nitrogen vacancy (NV) centre in diamond [1], as well as a fully connected 10-qubit register formed of 9 nuclear spins combined with the NV centre electron spin [2].

Building on these recent results, I will show how such a system might be used to simulate many-body physics phenomena. I will also present new methods that allow us to extend control over more nuclear spins towards this goal. By combining our precise knowledge of the nuclear spin environment with dynamic nuclear polarisation, dynamical decoupling and selective readout protocols, we can prepare, measure and isolate the dynamics of individual nuclear spins and subsets of spins within a large interacting cluster. These techniques open the door to the realisation of quantum simulations of many-body physics phenomena using nuclear spins in diamond.

[1] M. H. Abobeih et al. Nature, 576, 411-415 (2019)
[2] C. E. Bradley et al. Phys. Rev. X 9, 031045 (2019)   

Live

The long-sought scalable quantum information processor is a critical challenge since it requires long coherence time as well as full control and readout of every single qubit. Here we propose methods to use closely spaced diamond nitrogen-vacancy (NV) centers for realizing quantum information processing and quantum computing. The NV centers are coupled with their adjacent color centers by spin-spin interactions. By applying a strain gradient, the position of each NV center is encoded and hence more than 100 NV centers can be read out individually due to the narrow linewidths and the dispersive distributions of the optical transition frequencies. At the same time, the individual control of the spin states can be realized by a position-dependent magnetic field. The strain and magnetic field gradient provide the flexibility to independently manipulate and selectively couple the electron spins. We also present a universal set of quantum gates with high fidelity combined utilizing optimal control methods for this solid-state system.   

Live

Understanding and protecting the coherence of individual quantum systems is a central challenge in quantum science and technology. A variety of methods to protect coherence have been demonstrated, including clock states, dynamical decoupling, quantum error correction, isotopic purification, and decoherence-protected subspaces. Here we introduce a new type of long-lived quantum system: a pair consisting of two identical coupled nuclear spins. These spin pairs naturally combine clock states with decoherence-protected subspaces making them intrinsically robust to external perturbations. We study three carbon-13 nuclear spin pairs and realize high-fidelity control and single-shot readout using a single NV center in their vicinity. We demonstrate a long inhomogeneous dephasing time, T2* = 113(18) seconds. Finally, we demonstrate complete control over these qubits by preparing an entangled state of two spin pairs through projective quantum parity measurements. The long-lived spin pairs demonstrated here are naturally abundant in diamond and other solid-state systems, and provide new opportunities for quantum bits, quantum networks and precision measurements.   

Live

In recent years, the cQED toolkit has been successfully applied to push the boundary of measurement sensitivity in electron spin resonance (ESR) spectroscopy [1,2,3]. In particular, the adoption of Josephson Parametric Amplifiers has allowed the noise in ESR spectrometers to approach the quantum limit. Here we report the use of a degenerate parametric amplifier (DPA) to perform in-situ amplification of spin echo signals in pulsed ESR measurements of ²⁰⁹Bi donors in Silicon. Unlike previous work, the spins here have a direct inductive coupling to the DPA, which is constructed from a quarter-wavelength resonator in a thin NbTiN film. The DPA serves as both the ESR cavity and first-stage amplifier for spin echo signals. We show that the device is capable of operating in a magnetic field of 250 mT and greatly enhances the SNR of pulsed ESR measurements.

[1] A. Bienfait, et. al. Reaching the quantum limit of sensitivity in electron spin resonance. Nature Nano. 11(3):253 (2016)
[2] C. Eichler, et. al. Electron spin resonance at the level of 10⁴ spins using low impedance superconducting resonators. Phys. Rev. Lett. 118(3): 037701 (2017)
[3] V. Ranjan, et. al. Electron spin resonance spectroscopy with femtoliter detection volume. Appl. Phys. Lett. 116:184002 (2020)   

Live

For spin-based quantum processors, controlled transitions in quantum-dot arrays between one ground state to other competing ground states are of significant operational significance, as these allow movements of quantum information within otherwise empty arrays (single-electron shuttling), or wave function overlap of one spin with another (coherent Heisenberg spin exchange). Even for small arrays, dense raster scans in control-voltage space become impractical, due to the large number of measurements needed to sample the high-dimensional gate-voltage cube, and the comparatively little information (Coulomb diamond boundaries) one gains.
Instead, we develop a hardware-triggered detection method using high-frequency reflectometry, to acquire sparse measurements directly corresponding to transitions between competing ground states within the array. Instead of digitizing reflectometry voltages, the acquisition computer only records timestamps (triggered changes in reflectometry signal) and reconstructs their location in gate-voltage space based on the applied voltage ramps. Applying this sparse acquisition technique to a quadruple quantum dot implemented in silicon, we find good agreement between our method and traditional raster scans.   

On Demand

Recent experimental and theoretical work on single-electron spin qubits in silicon has opened the possibility of realizing long-distance entangling gates mediated by microwave photons. Currently proposed iSWAP gates, however, require qubits to be tuned to resonance with one another, and have limited fidelity due to charge noise. We present here a novel, photon-mediated cross-resonance gate which requires no resonant tuning, as well as a nested entangling gate sequence capable of suppressing gate errors due to quasistatic charge noise.