Calendar of Events

Venue: CQT Level 3 Seminar Room, S15-03-15

Title: The Schmidt rank for the commuting operatorframework

Abstract:  A fundamental question in quantum theory is: When can a quantum system be considered effectively finite-dimensional? A natural ansatz for approaching this question is to consider the Schmidt rank,which gives a fundamental measure for the entanglement dimension of a purebipartite state. Its usual definition uses the Schmidt decomposition of vectorson bipartite Hilbert spaces. Unfortunately, this definition has limitationssince these local Hilbert spaces do not exist (or are at least not canonicallygiven) if the observable algebras of the local systems are allowed to begeneral C*-algebras.

In our work, we generalize the Schmidt rank to the commuting operator frameworkwhere the joint system is not necessarily described by the minimal tensorproduct but by a general bipartite algebra. We give algebraic and operationaldefinitions for the Schmidt rank and show their equivalence.
We analyze bipartite states and compute the Schmidt rank in several examples:the vacuum in quantum field theory, Araki–Woods-Powers states, as well asground states and translation invariant states on spin chains which are viewedas bipartite systems for the left and right half chains. We conclude with alist of open problems for the commuting operator framework.

Reference: 
- L. v. Luijk, R. Schwonnek, A. Stottmeister, and R.F. Werner, Commun. Math.Phys. (2024) 405:152 https://doi.org/10.1007/s00220-024-05011-9
(arxiv: 2307.11619)

Venue:CQT Level 3 Seminar Room, S15-03-15

Title: Dual atom interferometry with strontium atoms

Abstract: Light pulse atomic interferometry (AI) has important uses both in fundamental studies and technological applications. Recent works have begun to use alkaline earth atoms like strontium to perform AI. The 1S0 ground and the 3P0 metastable states of strontium are insensitive to magnetic field, making them useful for precision measurements and sensing. I will present our lab’s progress in the development of strontium AI for applications in inertial sensing and test of weak equivalence principle (WEP). In one project, we use 813 nm Bragg pulses to perform a dual AI. At this magic wavelength, both 1S0 and 3P0 states have the same polarizability, and therefore, they experience simultaneously the same interferometric sequence. This scheme opens up the possibility of performing a quantum test of WEP when the two states are in coherent superposition. On a more practical side, the dual AI scheme can also play an important role for trapped AI inertial sensing, by removing the unwanted effects coming from the atomic traps. For real-life applications, one limitation of cold strontium AI comes in its typically complicated and bulky setup. Another project in our lab aims to overcome this limitation, by implementing a compact cold atom source based on thermal laser ablation. This laser ablation source can be readily adapted to produce cold atoms of other atomic species.

Venue: Level 5 Seminar Room, S15-05-14

Title: TBA

Abstract: TBA

Venue: CQT Level 3 Seminar Room, S15-03-15

Title:Dipolar interactions between ultracold RbCs molecules in magic traps and optical tweezers

Abstract:Ultracold polar molecules are an exciting platform for quantum science and technology. The combination of rich internal structure of vibration and rotation, controllable long-range dipolar interactions and strong coupling to applied electric and microwave fields has inspired many applications. These include quantum simulation of strongly interacting many-body systems, the study of quantum magnetism, quantum metrology and molecular clocks, quantum computation, precision tests of fundamental physics and the exploration of ultracold chemistry. Many of these applications require full quantum control of both the internal and motional degrees of freedom of the molecule at the single particle level.In Durham, we study ultracold ground-state RbCs molecules formed by associating Rb and Cs atoms using a combination of magnetoassociation and stimulated Raman adiabatic passage [1]. This talk will report our work on the development of full quantum control of the molecules. Specifically, I will explain how we have mastered the ac Stark shift due to the trapping light [2] to demonstrate robust storage qubits in the molecule [3] and will describe the development of magic traps [4] that support second-scale rotational coherences giving access to controllable dipolar interactions [5]. I will also report on new experiments that produce single molecules in optical tweezers starting from a single Rb and a single Cs atom [6]. Using this platform, we prepare the molecules in the motional ground state of the trap and can perform addressing and detection of single molecules [7]. Using mid-sequence detection of formation errors, we demonstrate rearrangement to produce small defect-free arrays [8]. By transferring the molecules into magic-wavelength tweezers, we can prepare long-lived rotational coherences that support spin-exchange interactions between molecules, enabling the preparation of maximally entangled entangled Bell states with high fidelity.
Finally, as an outlook, we demonstrate a new hybrid platform that combines single ultracold molecules with single Rydberg atoms [9], opening up the prospect of non-destructive readout of the molecular state and fast entangling gates.

[1]    P.K.Molony et al., “Creation of Ultracold RbCs Molecules in the Rovibrational Ground State”,
Phys. Rev. Lett. 113, 255301 (2014).
[2]    P.D.Gregory et al., “ac Stark effect in ultracold polar RbCs molecules”,
Phys. Rev. A 96, 021402(R) (2017).
[3]    P.D.Gregory et al., “Robust storage qubits in ultracold polar molecules”,
Nature Physics 17, 1149-1153 (2021).
[4]    Q.Guan et al., “Magic conditions for multiple rotational states of bialkali molecules in optical lattices”, Phys. Rev. A 103, 043311 (2021).
[5]    P.D.Gregory et al., “Second-scale rotational coherence and dipolar interactions in a gas of ultracold polar Molecules”, Nature Physics 20, 415–421 (2024).
[6]    R.V.Brooks et al., “Preparation of one Rb and one Cs atom in a single optical tweezer”,
New J. Physics 23, 065002 (2021).
[7]    D.K.Ruttley, A.Guttridge et al., “Formation of ultracold molecules by merging optical tweezers”,Phys. Rev. Lett. 130, 223401 (2023).
[8]    D.K.Ruttley et al., “Enhanced Quantum Control of Individual Molecules Using Optical Tweezer Arrays”, PRX Quantum 5, 020333 (2024).
[9]    A.Guttridge, D.K.Ruttley et al., “Observation of Rydberg blockade due to the charge-dipole interaction between an atom and a polar molecule”, Phys. Rev. Lett. 131, 013401 (2023).

Venue:CQT Level 3 Seminar Room, S15-03-15

Title:An optical ground station in Singapore for satellite-to-ground quantum communication (CQT-OGS)

Abstract:Fiberoptic network acts as the backbone of global internet. These networks span thousands of kilometers connecting continents. However, due to high transmission losses the same network cannot be used to perform quantum communication, or quantum key distribution (QKD) at a global scale. Satellite-based quantum communication has emerged as a promising solution to overcome the range limitations of ground-based systems. Here, network nodes in space can connect multiple nodes coherently across different ground points on the globe. There have been successful demonstrations of this technology, and several new satellite QKD missions are to be launched in near future. In this connection, we have developed an optical ground station (CQT-OGS) on National University of Singapore campus to demonstrate entanglement sharing and quantum key distribution with low Earth orbit Satellites. In this talk, we will discuss the engineering challenges and design considerations for developing a quantum OGS in an urban environment and how these challenges are met by our CQT-OGS construction.
To achieve a global quantum network, cross-compatibility among optical ground stations and quantum satellites is crucial. In this light, we will discuss how cross-compatibility among several quantum satellite missions and ground stations can be systematically studied. Finally, we will elucidate some  applications of the OGS beyond quantum communication.