Events

Giulia Rubino (University of Bristol)

17 Sep 2025

Quantum superposition of thermodynamic evolutions with opposing time’s arrows

Microscopic physical laws are time-symmetric, hence, a priori there exists no preferential temporal direction. However, the second law of thermodynamics allows one to associate the “forward” temporal direction to a positive variation of the total entropy produced in a thermodynamic process, and a negative variation with its “time-reversal” counterpart. This definition of a temporal axis is normally considered to apply in both classical and quantum contexts. Yet, quantum physics admits also superpositions between forward and time-reversal processes, whereby the thermodynamic arrow of time becomes quantum-mechanically undefined. In this talk, I will demonstrate that a definite thermodynamic time’s arrow can be restored by a quantum measurement of entropy production, which effectively projects such superpositions onto the forward (time-reversal) time-direction when large positive (negative) values are measured. Furthermore, for small values (of the order of plus or minus one), the amplitudes of forward and time-reversal processes can interfere, giving rise to entropy-production distributions featuring a more or less reversible process than either of the two components individually, or any classical mixture thereof.

Michael Stern (Bar-Ilan University)

23 May 2024

Coupling single electrons spins to a superconducting flux qubit

The realization of a quantum computer represents a tremendous scientific and technological challenge due to the extreme fragility of quantum information. The physical support of information, namely the quantum bit or qubit, must at the same time be strongly coupled to other qubits by gates to compute information, and well decoupled from its environment to keep its quantum behavior.  

An interesting physical system for realizing such qubits are magnetic impurities in semiconductors, such as bismuth spins in silicon. Indeed, spins in semiconductors can reach extremely long coherence times - of the order of seconds. Yet it is extremely difficult to establish and control efficient gates between distant spins. Here we experimentally demonstrate a protocol where single spins can coherently transfer their quantum information to a superconducting device, which acts as a mediator or quantum bus. This superconducting device allows to connect distant spins on-demand without compromising their coherent behavior.

Hannah Stern (University of Manchester)

16 May 2024

A new 2D platform for quantum technology: A quantum coherent spin in hexagonal boron nitride that operates at ambient conditions

Solid-state spin-photon interfaces that combine single-photon generation and long-lived spin coherence with scalable device integration, ideally at ambient conditions, hold great promise for the implementation of quantum networks and sensors. Despite rapid progress reported across several candidate systems, those possessing quantum coherent single spins at room temperature remain extremely rare. In this talk I will show quantum coherent control under ambient conditions of a single-photon emitting defect spin in a layered van der Waals material, hexagonal boron nitride. We identify that the carbon-related defect has a spin-triplet electronic ground-state manifold  and demonstrate that the spin coherence is governed predominantly by coupling to only a few proximal nuclei and is prolonged by decoupling protocols. These results serve to introduce a new platform to realise a room-temperature spin qubit coupled to a multi-qubit quantum register or quantum sensor with nanoscale sample proximity.

Jonas Glatthard (University of Nottingham)

9 May 2024

Page-curve-like entanglement dynamics in open quantum systems

The entanglement entropy of a black hole and that of its Hawking radiation are expected to follow the so-called Page curve: After an increase in line with Hawking’s calculation, it is expected to decrease back to zero once the black hole has fully evaporated, as demanded by unitarity. Recently, a simple system-plus-bath model has been proposed which shows a similar behavior. Here, we make a general argument as to why such a Page-curve-like entanglement dynamics should be expected to hold generally for system-plus-bath models at small coupling and low temperatures, when the system is initialized in a pure state far from equilibrium. The interaction with the bath will then generate entanglement entropy, but it eventually has to decrease to the value prescribed by the corresponding mean-force Gibbs state. Under those conditions, it is close to the system ground state. We illustrate this on two paradigmatic open-quantum-system models, the exactly solvable harmonic quantum Brownian motion and the spin-boson model, which we study numerically. In the first example we find that the intermediate entropy of an initially localized impurity is higher for more localized initial states. In the second example, for an impurity initialized in the excited state, the Page time—when the entropy reaches its maximum—occurs when the excitation has half decayed.

Gloria Patero (Materials Science Institute of Madrid (CSIC))

29 Feb 2024

Long-range quantum state transfer in semiconductor quantum dots arrays

The fabrication and control of semiconductor quantum dot arrays open the possibility to use these systems as quantum links, for transferring quantum information between distant sites, an indispensable part of large-scale quantum information processing. Great effort is currently being devoted to the investigation of hole spin qubits in quantum dots owing to their long coherence time resulting from the weak hyperfine coupling to nuclear spins and rapid operation time due to the inherently strong spin–orbit coupling (SOC). In this talk I will discuss different protocols, both adiabatic and those based on shortcuts to adiabaticity, to transfer spin holes directly between edges of a quantum dot chain with high fidelity. I will show how the spin polarization of the transferred holes can be controlled by tuning the ratio between the SOC and the spin conserving tunneling rate. Also, I will discuss how to transfer entangled hole spins between edge dots and the feasibility of quantum dot arrays as high-fidelity quantum buses to distribute information between distant sites and perform one qubit gates in parallel.

Recently, quantum dot arrays have been proposed as quantum simulators of complex lattices, as those which present non trivial topology. An alternative way to transfer directly information between distant sites with high fidelity, is to use protected topological edge states in systems with non-trivial topology. I will discuss particle transfer mediated by edge states in different quantum dot array configurations which present edge states protected against certain type of disorder. It opens a new avenue for quantum state transfer protocols in low dimensional topological lattices.

Micheline Soley (University of Wisconsin-Madison)

22 Feb 2024

The power of data science and optics in quantum mechanics 

Today, exact quantum dynamics simulations face the “curse of dimensionality,” in which computational cost grows exponentially as the dimensionality of molecular systems increases. This limits exact grid-based quantum dynamics simulations to the smallest molecular systems. Low-rank tensor-network approaches provide a way to surmount this curse for many chemical systems, as they can exponentially reduce computational cost for weakly coupled molecular systems. Our work capitalizes on the native advantages of tensor networks and the high degree of entanglement possible on quantum computers to develop new approaches to simulate molecular systems. In addition, we demonstrate that reflectionless scattering mode theory from optics can be reformulated in quantum mechanics to reveal the existence of long-sought-after quantum mechanical parity-time reversal (PT) symmetry behavior in standard ultracold-atom scattering experiments, a discovery which opens the door to the development of powerful quantum technologies.

Janet Anders (University of Exeter)

24 Nov 2021

Quantum Brownian motion for magnets

In this talk I will discuss how a system+bath Hamiltonian, similar to the Caldeira-Leggett and spin-boson models, can be used to derive a general spin dynamics equation. I will show how in the Ohmic (Markovian) limit the new equation reduces to the Landau Lifshitz Gilbert equation, a phenomenological equation widely used in magnetism. 

I will demonstrate how resonant Lorentzian couplings can be used as a general tool for the systematic comparison of spin dynamics of Markovian and non-Markovian regimes, and present numerical results of a classical spin's dynamics under classical and quantum noise. We find much quicker decay to steady state for couplings that invoke memory effects, and quantum flattening of the magnetization curve at low temperatures. 

In the second part of the talk we will explore long time steady states. The dynamical convergence to the Gibbs state is a standard assumption across much of classical and quantum thermodynamics. However, for nanoscale and quantum systems the interaction with their environment becomes non-negligible.  Is the system steady state then still the Gibbs state? And if not, how exactly does it depend on the interaction details? I will briefly outline several aspects of this timely topic.

Eddy Chen (UC San Diego)

20 Oct 2021

Strong determinism

A strongly deterministic theory of physics is one that permits exactly one possible history of the universe. In the words of Penrose (1990), "it is not just a matter of the future being determined by the past; the entire history of the universe is fixed, according to some precise mathematical scheme, for all time.” Such an extraordinary feature, at first glance, may appear difficult to achieve in physics. In this talk, I first clarify the meaning of strong determinism and explain how it differs from standard determinism and superdeterminism. Next, I show that Everettian quantum mechanics, with the help of the Past Hypothesis, provides an easy route to strong determinism. On a theory that I call the Everettian Wentaculus, the quantum state of the multiverse is a fundamental mixed state with exactly one possible history. As a consequence of physical laws, the history of the multiverse could not have been otherwise. Since the theory postulates no fundamental measures of probability or typicality, any such notions will have to be “emergent.” 

Nicholas Maxwell (Science and Technology Studies, UCL)

15 Sept 2021

In praise of natural philosophy – a synthesis of physics and metaphysics

I will discuss the overwhelming case for conceiving, and doing, physics within the framework of aim-oriented empiricism – and its implications for discovery, for quantum theory, for the role of metaphysics and philosophy of physics within physics.

Carlo Rovelli (University of Marseille)

16 Dec 2020

The relational interpretation of quantum mechanics

An intuitive and compelling interpretation of quantum mechanics talk from one of the world’s best-known figures in theoretical physics, Carlo Rovelli.

Jim Baggott

9 Dec 2020

Quantum reality: The interpretations of quantum mechanics and the purpose of science

A non-technical introductory overview of the meaning of quantum mechanics from the perspective of the philosophy of science lead by freelance science writer Jim Baggott.

Dorje Brody (University of Surrey)

18 Nov 2020

Quantum geometry and space-time

The manifold of pure quantum states — the quantum “phase” space — can be regarded as a complex projective space endowed with the unitary-invariant Fubini-Study metric. According to the principles of geometric quantum mechanics, the physical characteristics of a given quantum system can be represented by geometrical features that are preferentially  identified in this complex manifold. What might come as a surprise is that the phase space of (classical) relativistic mechanics — the future tube — carries structures that are almost identical to those of quantum phase space. This observation leads to a new way of formulating relativistic mechanics, and relativistic quantum theory of space-time events. In the first half of this talk I will review some of the ideas on geometric formulation of basic quantum theory. In the second half I will illustrate how essentially identical structures emerge in relativistic mechanics, and based on this show how measurement theory for relativistic quantum mechanics — a theory that has hitherto missing in the literature — can be formulated. (Talk based on joint work with L.P. Hughston.) 

Mohammed Sanduk (University of Surrey)

4 Nov 2020

Is there a physical meaning for the complex harmonic oscillator? Circles theory and quantum mechanics

A seminar on a project that goes back 30 years trying to find a deeper interpretation of the quantum mechanical wave function. 

Chris Dewdney (University of Portsmouth)

21 Oct 2020

The de Broglie-Bohm approach to quantum theory: Particles and fields

This seminar covered the Bohmian interpretation of quantum mechanics.