13 research outputs found
Witnesses of non-classicality for simulated hybrid quantum systems
The task of testing whether quantum theory applies to all physical systems
and all scales requires considering situations where a quantum probe interacts
with another system that need not obey quantum theory in full. Important
examples include the cases where a quantum mass probes the gravitational field,
for which a unique quantum theory of gravity does not yet exist, or a quantum
field, such as light, interacts with a macroscopic system, such as a biological
molecule, which may or may not obey unitary quantum theory. In this context a
class of experiments has recently been proposed, where the non-classicality of
a physical system that need not obey quantum theory (the gravitational field)
can be tested indirectly by detecting whether or not the system is capable of
entangling two quantum probes. Here we illustrate some of the subtleties of the
argument, to do with the role of locality of interactions and of
non-classicality, and perform proof-of-principle experiments illustrating the
logic of the proposals, using a Nuclear Magnetic Resonance quantum
computational platform with four qubits.Comment: Revised and extende
Efficient Hamiltonian programming in qubit arrays with nearest-neighbour couplings
We consider the problem of selectively controlling couplings in a practical
quantum processor with always-on interactions that are diagonal in the
computational basis, using sequences of local NOT gates. This methodology is
well-known in NMR implementations, but previous approaches do not scale
efficiently for the general fully-connected Hamiltonian, where the complexity
of finding time-optimal solutions makes them only practical up to a few tens of
qubits. Given the rapid growth in the number of qubits in cutting-edge quantum
processors, it is of interest to investigate the applicability of this control
scheme to much larger scale systems with realistic restrictions on
connectivity. Here we present an efficient scheme to find near time-optimal
solutions that can be applied to engineered qubit arrays with local
connectivity for any number of qubits, indicating the potential for practical
quantum computing in such systems.Comment: 5 pages, 5 figures. Shortened and clarified from previous versio
Witnesses of non-classicality for hybrid quantum systems
The task of testing whether quantum theory applies to all physical systems
and all scales requires considering situations where a quantum probe interacts
with another system that need not be fully quantum. Important examples include
the cases where a quantum mass probes the gravitational field; or a quantum
field, such as light, interacts with a macroscopic system, such as a biological
molecule. In this context a class of experiments has recently been proposed,
where the non-classicality of a physical system can be tested indirectly by
detecting whether or not it is capable of entangling two quantum probes. Here
we illustrate some of its subtleties and perform proof-of-principle experiments
using a Nuclear Magnetic Resonance (NMR) quantum computational platform with
four qubits.Comment: 4 pages, 4 figure
Rescaling interactions for quantum control
A powerful control method in experimental quantum computing is the use of
spin echoes, employed to select a desired term in the system's internal
Hamiltonian, while refocusing others. Here we address a more general problem,
describing a method to not only turn on and off particular interactions but
also to rescale their strengths so that we can generate any desired effective
internal Hamiltonian. We propose an algorithm based on linear programming for
achieving time-optimal rescaling solutions in fully coupled systems of tens of
qubits, which can be modified to obtain near time-optimal solutions for
rescaling systems with hundreds of qubits.Comment: Minor corrections and clarification
Transforming pure and mixed states using an NMR quantum homogeniser
The universal quantum homogeniser can transform a qubit from any state to any
other state with arbitrary accuracy, using only unitary transformations to
perform this task. Here we present an implementation of a finite quantum
homogeniser using nuclear magnetic resonance (NMR), with a four-qubit system.
We compare the homogenisation of a mixed state to a pure state, and the reverse
process. After accounting for the effects of decoherence in the system, we find
the experimental results to be consistent with the theoretical symmetry in how
the qubit states evolve in the two cases. We analyse the implications of this
symmetry by interpreting the homogeniser as a physical implementation of pure
state preparation and information scrambling
Coherent control for quantum information processing
An exquisite control over the dynamics of a quantum system is essential for realizing any quantum technology. This thesis presents some practical and efficient strategies for coherent quantum control: a paradigm for controlling quantum dynamics using electromagnetic fields. Coherent control is a common framework for describing the physical implementation of quantum logic gates in several quantum information processing platforms including superconducting qubits and trapped ions. For the purpose of demonstration, however, liquid-state Nuclear Magnetic Resonance (NMR) is used in this thesis as it is an ideal test-bed for developing, testing and benchmarking coherent control tools and techniques.
Coherent control was revitalised in 2005 by the development of Gradient Ascent Pulse Engineering (GRAPE), a powerful optimal control technique that is widely used for designing shaped pulses for a variety of tasks. GRAPE, like any other closed-loop optimal control algorithm, requires multiple evaluations of the computationally expensive matrix exponential and the full time-evolution propagator. To this end, I present strategies to sidestep the explicit evaluation of matrix exponentials, thereby offering substantial speed-ups over conventional implementations of the GRAPE algorithm. The results are demonstrated experimentally in NMR, but also in simulations of ultracold molecules and superconducting qubits.
Beyond GRAPE, this thesis also considers methods for designing spin-echo sequences for implementing any arbitrary network of controlled-Z gates in a system of qubits with zz-couplings. First, a method based on linear-programming provides near time-optimal sequences in fully-coupled systems having up to a few hundred qubits. Further, an analytic method based on graph colouring finds near time-optimal spin echo sequences in engineered systems with any number of qubits having only nearest and next-nearest neighbour-couplings