Many architectures for quantum information processing rely on qubits dedicated for the readout of a larger quantum register. These ancilla readout qubits present a physical overhead not contributing to the computational resource. A common implementation in spin qubit architectures is the readout schemes based on Pauli exclusion of charges confined in a double quantum dot, with one dot serving as the ancilla qubit. Here, using a three-qubit spin register and a Pauli exclusion-based readout, we present z-basis measurements of the entire register constructed with tomography, eliminating the physical overhead. We validate our approach with simulations which provide insight into potential sources of errors in the reconstruction. We also demonstrate our reconstruction by performing quantum state tomography on a GHZ state of a spin-qubit based device.
Many architectures for quantum information processing rely on qubits dedicated for the readout of a larger quantum register. These ancilla readout qubits present a physical overhead not contributing to the computational resource. A common implementation in spin qubit architectures is the readout schemes based on Pauli exclusion of charges confined in a double quantum dot, with one dot serving as the ancilla qubit. Here, using a three-qubit spin register and a Pauli exclusion-based readout, we present z-basis measurements of the entire register constructed with tomography, eliminating the physical overhead. We validate our approach with simulations which provide insight into potential sources of errors in the reconstruction. We also demonstrate our reconstruction by performing quantum state tomography on a GHZ state of a spin-qubit based device.
Joint measurements of two-Pauli observables are powerful tools for both the control and protection of quantum information. By following a simple recipe for measurement choices, single- and two- qubit rotations using two-Pauli parity and single qubit measurements are guaranteed to be unitary while requiring only a single ancilla qubit. This language for measurement based quantum computing is shown to be directly applicable to encoded double quantum dot singlet-triplet spin qubits, by measuring spin-parity between dots from neighboring qubits. Along with exchange interaction, a complete, leakage free, measurement based gate set can be shown, up to a known Pauli correction. Both theoretically exact spin-parity measurements and experimentally demonstrated asymmetric spin-parity measurements are shown to be viable for achieving the proposed measurement based scheme, provided some extra leakage mitigating measurement steps. This method of spin qubit control offers a leakage suppressed, low resource overhead implementation of a measurement-based control that is viable on current spin qubit processor devices.
Single-electron spin qubits defined in quantum dots are used as building blocks of a semiconductor-based quantum computer. Readout in a scaled-up version of such a quantum computer is expected to rely on the Pauli Spin Blockade (PSB) mechanism. A desired functionality of PSB readout is that it reveals two bits of information on the two spin qubits that are involved in the process, such that the four computational basis states can be discriminated. In this work, we propose and quantitatively analyze an experimental procedure, based on gate reflectometry, which enables this four-state discrimination in a single measurement. We provide an intuitive recipe to maximize the contrast between the quantum capacitances of the four basis states. Focusing on silicon double quantum dots equipped with a micromagnet, we quantify how amplifier noise and phonon-mediated relaxation influence readout fidelity. Our results highlight a realistic opportunity to mitigate the overhead of readout ancilla qubits in a spin-based quantum computer.
We propose and demonstrate complete spin state readout of a two-electron system in a double quantum dot probed by an electrometer. The protocol is based on repetitive single-shot measurements using Pauli spin blockade and our ability to tune on fast timescales the detuning and the interdot tunnel coupling between the GHz and sub-Hz regime. A sequence of three distinct manipulations and measurements allows establishing if the spins are in S, T_{0}, T_{+}, or T_{−} state. This work points at a procedure to reduce the overhead for spin readout, an important challenge for scaling up spin-qubit platforms.
A quantum compiler is required to run a quantum circuit on a quantum computer, similar to how classical compilers are required to run programs on classical computers. Spin qubits are a promising candidate for scalable quantum computers. General-purpose quantum compilers can be used for spin qubit devices, however they do not consider additional constraints imposed due to Double-Quantum-Dot (DQD) readout. For DQD readout, when a qubit in an algorithm is measured, the compiler must remap and re-route it to be adjacent to a special-purpose readout qubit, which is preserved in a known state. Moreover, this should be done in a way that minimizes overhead and maximizes the fidelity, in particular, due to noise in the readout process. This work formulates readout constraints to extend SMT (Satisfiability Modulo Theory)-based layout synthesis techniques proposed in [1] to spin qubit architectures. We define a metric, readout depth, to quantify the overheads incurred. Preliminary results, obtained from benchmarking GHZ and variational quantum eigensolver circuits on architectures with grid topologies, highlight the algorithmic tradeoffs that arise. We expect ongoing work on scaling up the methods to a larger set of algorithms will help inform the decisions of spin qubit hardware designers.
Spin qubits in semiconductor quantum circuits have been identified as a compelling platform for large scale quantum computing. Today an ongoing effort of the community is aiming at the scaling of such systems by filling larger and larger arrays. In this context the development of tuning and readout protocols allowing to reduce the inherent complexity of such systems seems to be a necessity.In the first part of my thesis, I demonstrate array partitioning of a linear triple quantum dot defined in a AlGaAs/GaAs heterostructure via a highly tunable interdot tunnel coupling. This allows us to operate independently smaller sections of the array which reduces the number of available charge states for the system and therefore reduces its complexity during the tuning.The second part of my thesis is focused on reducing the hardware overhead necessary to extract information during the readout of the spin qubits. In semiconductor spin qubits, high fidelity read-out is obtained through Pauli spin blockade in a double quantum dot with two electrons probed by a local electrometer. One electron is the qubit, the second is the ancilla and only one bit of information is extracted. To obtain the full two-bit information of the two-electron spin states, more quantum hardware is necessary (either dots, electrometers, or reservoirs).Based on an original protocol, I develop in this thesis a complete single-shot read-out procedure for two-electron spin states in a double quantum dot discriminating between S, T0, T+ and T-. It uses successively three distinct spin-to-charge conversion protocols interleaved with charge configuration measurements. The strategy is based on four novel experimental achievements:First, the control from the GHz to sub-Hz regime of the interdot tunnel coupling at the nanosecond timescale.Second, the separation of the spin-to-charge conversion and the charge read-out in the voltage gate space.Third, the suppression of spin relaxation process during the read-out procedure.Finally, the repetition of three distinct spin-to-charge conversion protocols.The read-out procedure is tested on various initialized populations and implemented in more complex spin manipulation sequences to assess its validity.
We investigate a range of methods to perform tomography in a solid-state qubit device, for which a priori initialization and measurement of the qubit is restricted to a single basis of the Bloch sphere. We explore and compare several methods to acquire precise descriptions of additional states and measurements, quantifying both stochastic and systematic errors, ultimately leading to a tomographically complete set that can be subsequently used in process tomography. We focus in detail on the example of a spin qubit formed by the singlet-triplet subspace of two electron spins in a GaAs double quantum dot, although our approach is quite general.