A measurement system is developed for studying a lateral double quantum dot (DQD) device formed in the two-dimensional electron gas (2DEG) of a GaAs/AlGaAs heterostructure. Copper powder and RC filters are constructed to isolate the device from sources of instrumentation and thermal noise, allowing the temperature of the 2DEG to reach 70 mK. Coaxial cables and bias tees allow Gaussian shaped pulses to be delivered to several depletion gates of the device for the purpose of quantum dot qubit manipulation. A conventional charge detector based on a quantum point contact (QPC) and a room temperature current preamplifier is implemented. This readout has a bandwidth of 25 kHz. To improve readout bandwidth, a radio-frequency QPC (RF-QPC) circuit is also developed. A superconducting niobium inductor, manufactured using photolithography techniques, is the basis of a 520 MHz matching network. The RF-QPC system noise temperature is 5.2 K, limited by the cryogenic semiconductor amplifier. The RF-QPC has a bandwidth of 15 MHz. A summary of the first quantum dot physics measurements is provided. Data related to fundamental phenomena such as conductance quantization of a QPC, Coulomb blockade for a single dot, and spin blockade for a DQD are presented. A qubit based on two-electron spin states in a DQD (S-T₊ qubit) is created and coherent oscillations of the qubit known as Landau-Zener-Stückelberg oscillations are observed. Analysis of the spin-to-charge conversion mechanism associated with the qubit readout reveals the role played by metastable charge states, a result published in Physical Review B 92, 125434 (2015).
We demonstrate a low loss, chip-level frequency multiplexing scheme for readout of scaled-up spin qubit devices. By integrating separate bias tees and resonator circuits on-chip for each readout channel, we realise dispersive gate-sensing in combination with charge detection based on two radio frequency quantum point contacts. We apply this approach to perform multiplexed readout of a double quantum dot in the few-electron regime and further demonstrate operation of a 10-channel multiplexing device. Limitations for scaling spin qubit readout to large numbers of multiplexed channels are discussed.
The extension of quantum trajectory theory to incorporate realistic imperfections in the measurement of solid-state qubits is important for quantum computation, particularly for the purposes of state preparation and error correction as well as for readout of computations. Previously this has been achieved for low-frequency (dc) weak measurements. In this paper we extend realistic quantum trajectory theory to include radio frequency (rf) weak measurements where a low-transparency quantum point contact (QPC), coupled to a charge qubit, is used to damp a classical oscillator circuit. The resulting realistic quantum trajectory equation must be solved numerically. We present an analytical result for the limit of large dissipation within the oscillator (relative to the QPC), where the oscillator slaves to the qubit. The
rf+dc mode of operation is considered. Here the QPC is biased (dc) as well as subjected to a small-amplitude sinusoidal carrier signal (rf). The
rf+dc QPC is shown to be a low-efficiency charge-qubit detector, which may nevertheless be higher than the dc-QPC (which is subject to
1∕f noise).