At the core of Quantum Information Science (QIS) is the idea that information processing is limited by the laws of physics that the ADQC is a new model of quantum computation that combines the advantages of gate-based and measurement-based quantum computation. ADQC is very well suited to experimental situations as it naturally uses static, long lived qubits as register qubits which are addressed sequentially by a flying, easy to manipulate qubit, called the "ancilla''. In each step the ancilla interacts with a fixed interaction, E, with a single register qubit or at most two, see Figure. carrier of information must obey. Most of today's information is encoded in a classical way, as the on/off of voltage in an electrical circuit for instance, or the point depth engraved on a CD. However, it is conceivable and increasingly realistic to encode information on genuinely quantum objects. For example the polarisation of a single photon, horizontal or vertical, is a way to encode a bit. The point of this is that quantum physics adds a new dimension to processing logic. The ability to bring a quantum object in two states at the same time, in superposition, was made famous by Schrödinger with his cat. Exactly this superposition can be used to perform a new kind of "coherent" information processing that results in the exponential power increase of a quantum computer over the classical one. Moreover, quantum objects can share correlations much stronger than their classical counterpart. This "spooky action" (Einstein) is known as quantum entanglement. Part of our research is to clarify the role of entanglement in quantum computing. Specifically, we found that the computing power of correlations can be characterised using a novel framework. The framework describes how an external control computer can, by interacting with quantum correlated resource states, perform calculations beyond its own power. We also introduced a new programmable version for building a quantum computer. Ideally suited for experiments, Ancilla-Driven Quantum Computation (ADQC) requires only a single moving quantum system, the ancilla, and a single interaction. An increasing part of our research is devoted to thermodynamics in the quantum regime, and the links between information theory and thermodynamics. We confirmed that while quantum entanglement is known to enable many counterintuitive effects, entanglement is not able to violate the second law of thermodynamics. However, entanglement is not just a low temperature effect. In the right environment it can persist at high temperatures and even exists in biological systems, for instance, between the electronic clouds of DNA base pairs. Currently we are working on characterising the behaviour of quantum systems in non-equilibrium. We are also involved in an experiment with a tiny nanosphere.
In a recent paper we demonstrate the validity of Landauer’s erasure principle in the strong coupling quantum regime by treating the system-reservoir interaction in a thermodynamic way. We show that the initial coupling to the reservoir modifies both the energy and the entropy of the system, and provide explicit expressions for the latter for a damped quantum harmonic oscillator. These contributions are related to the Hamiltonian of mean force and dominate in the strong damping limit. They need to be fully taken into account in any low temperature thermodynamic analysis of quantum systems. A popular version of the paper is here.
After coupling the ancilla is
measured in a suitable basis and this results in a back-action that,
step by step, 'steers' the register's state. The interactions suitable for universal, stepwise deterministic ADQC are locally
equivalent to the Ising model or the Heisenberg XX model with maximal
coupling strength. Apart from unitary evolution, any generalized
measurement can be implemented with the help of a second ancilla. The architectural advantage of the model is that only the ancilla parameters, i.e. initial state and measurement basis, have to be manipulated, while the register itself is always only addressed with a single type of interaction. This is suited to many physical systems where the necessary register-ancilla interaction is available, such as neutral atoms in optical lattices, micro ion trap arrays, nuclear-electron spin systems and cavity QED-superconducting qubits. Besides, computations in the ADQC model can be translated into patterns for standard MBQC and vice versa. This implies the existence of a wider class of graph states for quantum computation, which includes so-called "twisted graph states" generated from non-commuting coupling operations.
Experiments showing the violation of Bell inequalities have formed our belief
that the world at its smallest is genuinely non-local. While many non-locality
experiments use the first quantised picture, the physics of fields of
indistinguishable particles, such as bosonic gases, is captured most
conveniently by second quantisation. This implies the possibility of non-local
correlations, such as entanglement, between modes of the field. In this paper
we propose an experimental scheme that tests the theoretically predicted
entanglement between modes in space occupied by massive bosons. Moreover, the
implementation of the proposed scheme is capable of proving that the particle
number superselection rule is not a fundamental necessity of quantum theory but
a consequence of not possessing a distinguished reference frame.
Measurement-based
quantum computation is an approach to computation radically different
to conventional circuit models. Instead of processing information
through a network of logical gates as in conventional circuit models,
computation is implemented by a sequence of adaptive single-qubit
measurements on a highly entangled multi-partite resource state. In the speculative paper
Harmonic lattices are an important class of quantum many-body systems reaching beyond discrete systems and providing the tools to describe continuous systems such as trapped ions. Telling practically whether a state of a harmonic lattice is entangled was considered difficult and only sufficient criteria, such as a generalisation of the discrete PPT-criterion and the violation of entanglement witnesses, were known. In
In In
In In
We participated in the development of a fully tomographic quantum key distribution protocol that is
unconditionally secure even when the channel noise rises above the
trust-threshold of the standard BB84 protocol. A description of the Singapore
protocol is here: |