Active Super Spin Number

[Theodora Andy]

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Feedback control of qubits is a highly demanded technique for advanced quantum information protocols such as fault-tolerant quantum error correction. Here we demonstrate active reset of a silicon spin qubit using feedback control. The active reset is based on quantum non-demolition (QND) readout of the qubit and feedback according to the readout results, which is enabled by hardware data processing and sequencing. We incorporate a cumulative readout technique to the active reset protocol, enhancing initialization fidelity above a limitation imposed by the single-shot QND readout fidelity. An analysis of the reset protocol implies a pathway to achieve the initialization fidelity sufficient for fault-tolerant quantum computation. These results provide a practical approach to high-fidelity qubit operations in realistic devices.

In this work, we report feedback-based active reset of an electron spin qubit in a natural silicon quantum dot. The spin-qubit state is read out by QND measurements8,9,23,24,25, whose outcome is used to generate a feedback pulse to reset the qubit. A combination of a digital signal processing (DSP) hardware26 and a hardware sequencer (Keysight M3300A module with the option for hardware virtual instrument programming) enables us to reset the qubit much faster than spin relaxation. First, we have tested a simple reset protocol based on a single-shot QND measurement. Next, we have developed an active reset protocol based on a cumulative readout technique23,24,25. The initialization fidelity of 98.3% has been obtained, much higher than the readout fidelity of the individual QND measurement (91.7%) which imposes a limitation for the simple reset protocol. We have also analyzed the reset protocol and propose a pathway to achieve an initialization fidelity of >99.5%. Feedback protocols based on cumulative readout are applicable even if a high-fidelity single-shot readout is not available owing to physical and hardware constraints, providing a practical approach to qubit operations in realistic devices.

FI obtained by the active reset protocol in Fig. 2a is limited by the low fR value of the QND measurement. We attempt to incorporate a cumulative readout technique to improve the readout fidelity23,24,25 in the active reset protocol. Increase of the readout fidelity by cumulative readout is tested using a quantum circuit shown in Fig. 3c. After the reset protocol and the spin rotation, the data qubit is read out 21 times (instead of once as in Fig. 3a) in a QND manner. Here we use only the last QND measurement outcome for active reset, but we use the rest of the ancilla readout outcomes, μ20, to analyze the cumulative readout process.

In conclusion, we have demonstrated a deterministic initialization scheme of a spin qubit based on the QND measurement. Combination of fast data processing and sequencing enables us to implement feedback according to the QND-readout estimators before the data-qubit state relaxes. This scheme works properly regardless of isolation of a qubit from the electron reservoirs. We also demonstrate that cumulative readout techniques can be incorporated to improve the initialization fidelity by the reset protocol. This scheme opens a pathway to develop silicon spin quantum information architectures suitable for scaling up demanded for quantum information processors.

T.K. carried out the experiments and analyzed the data with input from T.N., K.T., A.N., and J.Y., T.N. programmed the DSP. T.K. programmed the sequencer with input from T.N. K.T., A.N., and J.Y. set up the measurement hardware. K.T. fabricated the device. S.T. supervised the project. All authors discussed the results. T.K. wrote the manuscript with contributions from all authors.

For more intense workouts, such as spinning, interval training, or running, the number of Active Zone Minutes you earn might exceed the length of your workout. For example, if you run for a total of 20 minutes and spend 10 minutes in the fat burn zone and 10 minutes in the cardio or peak zones, you earn 30 Active Zone Minutes.

1 MET is the rate of energy you expend during rest or sitting quietly, while you typically expend 3 METs or more during a moderately-intense activity such as walking. You earn active minutes for activities at or above about 3 METs.

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Doping-induced evolution of an antiferromagnetic insulating state into a ferromagnetic metallic state has been observed for many transition metal compounds, including cobaltates and several other strongly correlated systems. The physics described here in terms of frustrated ferromagnetic states and superspin glass behavior may therefore be relevant to those systems, too.

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Now in its fourth-generation, SH-AWD has evolved to become more compact and even more capable. Compared to the system's previous generation, the quickness of its reactions has jumped 30%, and to handle the output of Acura's all-new 355-horsepower Type S Turbo V6, its torque capacity has been increased 40%. SH-AWD is standard on the brand's high-performance Type S models and is available on the Acura TLX, RDX and MDX.

History of the SH-AWD System

Two earlier technologies used in Honda and Acura vehicles laid the groundwork for SH-AWD. Active Torque Transfer System (ATTS) was introduced on the 1997 Honda Prelude Type SH, followed by the Variable Torque Management four-wheel drive system (VTM-4) on the 2001 Acura MDX.

ATTS used a pair of electronically controlled clutches to actively send up to 80% of engine torque to the outside wheel when cornering, actively overdriving that wheel up to 15%. ATTS not only offered traction management enhancement similar to a limited-slip differential, but the active torque vectoring of overdriving the outside wheel helped significantly reduce understeer, giving the Prelude Type SH handling characteristics more commonly associated with all-wheel drive cars.

For the 2001 MDX, Acura's first crossover, engineers sought an all-wheel drive system that would automatically distribute torque to all four wheels as needed, but in a compact and lightweight package. The resulting Variable Torque Management all-wheel drive system (VTM-4) used a single-speed torque transfer unit bolted directly to the transaxle and an electromagnetic clutch mounted on each side of the rear differential.

During normal operation the clutches were disengaged, allowing the rear axles to freewheel, delivering all engine power to the front wheels. However, if the system's sophisticated ECU anticipated wheel slippage, the clutches would lock, routing power to the rear wheels to maximize available traction, such as when launching hard from a stop or in slippery conditions. Drivers could also press a VTM-Lock button on the dash to temporarily engage the rear wheels to help aid traction management in situations such as a snow bank or slippery ditch.

Mechanics of the SH-AWD System

To enhance both handling and stability, SH-AWD can send up to 70% of engine torque to the rear wheels and actively distribute up to 100% of that torque to a single left or right wheel. This is achieved by routing the power generated by the vehicle's engine to a transaxle, while power is sent to the rear wheels through a torque transfer unit mounted up front rather than a traditional center differential.

Mounted alongside the front transaxle, the torque-transfer unit receives torque from a helical gear that is attached to the front differential's ring gear. From there, a short horizontal shaft and hypoid gear set within the torque-transfer unit's case send power to the rear drive unit though a lightweight driveshaft.

Controlling the torque split between the rear wheels falls to two hydraulically operated clutch packs, one for each rear wheel. The clutch packs are activated with an electric motor that powers a single hydraulic pump for each pack. The SH-AWD Electronic Control Unit (ECU) controls a pair of linear solenoids by selectively sending pressure to the packs, which controls the amount of power sent to each rear wheel. Together, the clutch packs control the front-to-rear torque split, and when controlled independently, they can send all of the rear-wheel torque to a single rear wheel.

Performance Benefits of Torque Vectoring SH-AWD

In addition, SH-AWD overdrives the vehicle's rear axle by 2.7%, giving the rear wheels the capability to spin faster than the fronts to create a yaw moment at the rear of the car. This is key to the system's torque-vectoring function.

The left and right turning motion of a vehicle is known as "yaw." In conventional cars, this motion is created almost entirely by steering input, applying rotational torque to the vehicle body. However, in some dynamic situations, the front wheels can become overwhelmed with the multiple tasks of steering the car, supporting a large part of the vehicle's weight, and in the case of front-wheel drive vehicles, delivering power. This can cause the front tires to lose traction well before the rear tires, a condition known as "understeer."

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