In every parameter-estimation experiment, the final measurement or the postprocessing incurs a cost. Postselection can improve the rate of Fisher information (the average information learned about an unknown parameter from a trial) to cost. We show that this improvement stems from the negativity of a particular quasiprobability distribution, a quantum extension of a probability distribution. In a classical theory, in which all observables commute, our quasiprobability distribution is real and nonnegative. In a quantummechanically noncommuting theory, nonclassicality manifests in negative or nonreal quasiprobabilities. Negative quasiprobabilities enable postselected experiments to outperform optimal postselection-free experiments: postselected quantum experiments can yield anomalously large information-cost rates. This advantage, we prove, is unrealizable in any classically commuting theory. Finally, we construct a preparation-and-postselection procedure that yields an arbitrarily large Fisher information. Our results establish the nonclassicality of a metrological advantage, leveraging our quasiprobability distribution as a mathematical tool.

Surface acoustic waves (SAWs) can create moving quantum dots in piezoelectric materials. Here we show how electron-spin qubits located on dynamic quantum dots can be entangled. Previous theoretical and numerical models of quantum-dot entanglement generation have been insufficient to study quantum dynamics in realistic experimental devices. We utilize state-of-the-art graphics processing units to simulate the wave function dynamics of two electrons carried by a SAW through a 2D semiconductor heterostructure. We build a methodology to implement a power-of-SWAP gate via the Coulomb interaction. A benefit of the SAW architecture is that it provides a coherent way of transporting the qubits through an electrostatic potential. This architecture allows us to avoid problems associated with fast control pulses and guarantees operation consistency, providing an advantage over static qubits. For inter-dot barrier heights where the double occupation energy is sufficiently greater than the double-dot hopping energy, we find that parameters based on experiments in GaAs/AlGaAs heterostructures can produce a high-fidelity root-of-SWAP operation. Our results provide a methodology for a crucial component of dynamic-qubit quantum computing. arXiv:2001.05502v1 [quant-ph]

Quantum simulators have recently enabled experimental observations of quantum many-body systems' internal thermalisation. Often, the global energy and particle number are conserved, and the system is prepared with a well-defined particle number-in a microcanonical subspace. However, quantum evolution can also conserve quantities, or charges, that fail to commute with each other. Noncommuting charges have recently emerged as a subfield at the intersection of quantum thermodynamics and quantum information. Until now, this subfield has remained theoretical. We initiate the experimental testing of its predictions, with a trapped-ion simulator. We prepare 6-15 spins in an approximate microcanonical subspace, a generalisation of the microcanonical subspace for accommodating noncommuting charges, which cannot necessarily have well-defined nontrivial values simultaneously. We simulate a Heisenberg evolution using laser-induced entangling interactions and collective spin rotations. The noncommuting charges are the three spin components. We find that small subsystems equilibrate to near a recently predicted non-Abelian thermal state. This work bridges quantum many-body simulators to the quantum thermodynamics of noncommuting charges, whose predictions can now be tested.

No abstract

We present a numerically-optimized multipulse framework for the quantum control of a singleelectron charge qubit. Our framework defines a set of pulse sequences, necessary for the manipulation of the ideal qubit basis, that avoids errors associated with excitations outside the computational subspace. A novel control scheme manipulates the qubit adiabatically, while also retaining high speed and ability to perform a general single-qubit rotation. This basis generates spatially localized logical qubit states, making readout straightforward. We consider experimentally realistic semiconductor qubits with finite pulse rise and fall times and determine the fastest pulse sequence yielding the highest fidelity. We show that our protocol leads to improved control of a qubit. We present simulations of a double quantum dot in a semiconductor device to visualize and verify our protocol. These results can be generalized to other physical systems since they depend only on pulse rise and fall times and the energy gap between the two lowest eigenstates.

We demonstrate the possibility of engineering a single donor transistor directly from a phosphorous doped quantum dot by making use of the intrinsic glassy behaviour of the structure as well as the complex electron dynamics during cooldown. Characterisation of the device at low temperatures and in magnetic field shows single donors can be electrostatically isolated near one of the tunnel barrier with either a single or a doubly occupancy. Such a model is well supported by capacitance-based simulations. Ability of using the D0 of such isolated donor as a charge detector is demonstrated by observing the charge stability diagram of a nearby and capacitively coupled semi-connected double quantum dot.

No abstract