Abstract : The goal of this project was to demonstrate the feasibility of realizing a large quantum computer by linking a series of elementary quantum computers (EQCs), using ensembles of atoms as qubits. We built all the tools for making an EQC.However, experimental work revealed that use of light shift imbalance only is not enough to achieve the required blockade. We have now shown why this is the case, and have identified how the blockade can be restored by using Rydberg interaction assisted light-shift imbalance. However, due to lack of necessary lasers, we were unable to demonstrate the EQC using this scheme. Instead, we focused on exploring other aspects of collective state entanglement for precision metrology. Specifically, we have shown how to realize spin-squeezed collective state atomic interferometer (COSAIN) and collective state atomic clock (COSAC), capable of improving the performance by a factor of root-N, thus reaching the Heisenberg limit. Significant experimental work has been carried out to demonstrate these devices, and this work will be carried to completion in the near future.
Abstract : The goal of this project was to demonstrate the feasibility of realizing a large quantum computer by linking a series of elementary quantum computers (EQCs), using ensembles of atoms as qubits. We built all the tools for making an EQC.However, experimental work revealed that use of light shift imbalance only is not enough to achieve the required blockade. We have now shown why this is the case, and have identified how the blockade can be restored by using Rydberg interaction assisted light-shift imbalance. However, due to lack of necessary lasers, we were unable to demonstrate the EQC using this scheme. Instead, we focused on exploring other aspects of collective state entanglement for precision metrology. Specifically, we have shown how to realize spin-squeezed collective state atomic interferometer (COSAIN) and collective state atomic clock (COSAC), capable of improving the performance by a factor of root-N, thus reaching the Heisenberg limit. Significant experimental work has been carried out to demonstrate these devices, and this work will be carried to completion in the near future.
Quantum control of many body atomic spins is often pursued in the context of an atom-light quantum interface, where a quantized light field acts as a "quantum bus" that can be used to entangle distant atoms. One key challenge is to improve the coherence of the atom-light interface and the amount of atom-light entanglement it can generate, given the constraints of working with multilevel atoms and optical fields in a 3D geometry. We have explored new ways to achieve this, through rigorous optimization of the spatial geometry, and through control of the internal atomic state. Our basic setup consists of a quantized probe beam passing through an atom cloud held in a dipole trap, first generating spin-probe entanglement through the Faraday interaction, and then using backaction from a measurement of the probe polarization to squeeze the collective atomic spin. The relevant figure of merit is the metrologically useful spin squeezing determined by the enhancement in the resolution of rotations of the collective spin, relative to the commonly used spin coherent state. With an optimized free-space geometry, and by using a 2-color probe scheme to suppress tensor light shifts, we achieve 3(2) dB of metrologically useful spin squeezing. We can further increase atom-light coupling by implementing internal state control to prepare spin states with larger initial projection noise relative to the spin coherent state. Under the right conditions this increase in projection noise can lead to stronger measurement backaction and increased atom-atom entanglement. With further internal state control the increased atom-atom entanglement can then be mapped to a basis where it corresponds to improved squeezing of, e.g., the physical spin-angular momentum or the collective atomic clock pseudospin. In practice, controlling the collective spin of N ~ 10⁶ atoms in this fashion is an extraordinarily difficult challenge because errors in the control of individual atoms tend to be highly correlated. By employing precise internal state control, we have prepared and detected projection noise limited "cat" states (which have initial projection noise that is larger by a factor of 2f = 8 for Cs relative to the spin coherent state) and estimate that we can generate up to 6.0(5) dB of metrologically useful spin squeezing, demonstrating the advantage of using the internal atomic structure as a resource for ensemble control.
Devices that harness the surprising properties of quantum systems at scale hold the possibility of revolutionary advances in information technology. Algorithms for quantum computers can take advantage of quantum parallelism to solve problems that are intractable with classical hardware, including prime factorization and simulating chemical processes. A quantum communication network that carries qubits instead of bits can leverage the irreversibility of projective measurement to guarantee secure communication. Because quantum states are fragile, building such devices requires exquisite control over many isolated quantum systems. One of the most critical capabilities is the interaction of stationary and flying qubits. In this thesis we consider how to engineer atom-light interactions as an interface for networked quantum systems. Quantum atom-light couplers require the efficient and reversible interaction of atomic and optical qubits, but the natural interaction of atoms and photons is weak. It is this very lack of interaction between light and its environment that makes light such an excellent carrier of quantum information. There are three promising strategies for stronger coherent atom-light interactions. First, the light field may be confined to a cavity so that it interacts with the atom over a longer time. Second, the light field may be focussed to a small spot with high aperture optics and matched to the natural atomic radiation pattern. Third, we may pass the light field through an ensemble of many atoms and store the qubit in a distributed state. In practice, atom-light couplers employ some combination of all three strategies. We will focus our research on the second two approaches: first by coupling one and two trapped ions with high aperture optics, and then operating a quantum optical memory with an ensemble of approximately 10 billion neutral atoms. Atoms that have been ionized and trapped in a common electromagnetic potential are an advanced few-qubit computation platform. Qubit states are associated with an electronic excitation at each atom, controlled coherently with lasers, and made to interact in two (or many) qubit gates by mutual Coulomb repulsion. One path to quantum advantage with ion-trap quantum processors is to scale small qubit registers by distributing entanglement with photonic interconnects. We consider the feasible efficiency of free-space atom-light couplers for atomic dipole transitions, and derive the atomic image by common free-space couplers. We couple a trapped-ion register with high-aperture lenses and operate a single trapped ion as a photon source. We model the dynamic character of the source and show spin-orbit coupling in the single-photon spatial mode. Although the total collection efficiency is only approximately 0.01, the source has exceptional photon-number purity A = (1.9 +/- 0:2) * 10^-3 such that the higher-order quantum nature of the field persists even after most of the field has been discarded. We derive an efficient quantum non-Gaussian witness and surpass it, the first such demonstration for a trapped-atom single-photon source. We entangle two trapped atoms by single-photon detection, and observe an interference pattern in the spatial mode of the bipartite state. Constructive interference between the entangled components enhances the emission probability by up to 29%, an example of collective enhancement by a two-atom ensemble. The efficiency of free-space atom-light couplers is limited by our capability to engineer high-precision, high-aperture optics. We fabricate ultra-precise hemispheric mirrors with numerical aperture 0.996 by single point diamond turning. The mirrors are amongst the smoothest hemispheric surfaces ever manufactured with root-mean-square (RMS) errors consistently below 25 nm. The smoothest of our mirrors has a RMS error of 14 nm and peak-to-valley error of 88 nm. A mirror with this surface is capable of suppressing or enhancing the spontaneous emission of an atom into free space by 96% in a proposed quantum electrodynamics experiment. We show how these mirrors, with a simple modification, can shape the spatial mode of a trapped atom by similar vacuum-mode engineering. We derive a near-hemispheric mirror coupling scheme that should be 72% efficient with mirrors as precise as ours. We design an ion trap for use with such high-aperture optics. With careful conditioning and control, ensembles of many atoms can be made to store and release photonic qubits on demand. Such optical quantum memories leverage the collective interaction of a light field with many billions of atoms to achieve storage and recall efficiencies approaching unity. We show how stationary light fields for photonic phase gates can be generated in optically deep ensembles, an effect which has since been observed. We implement the gradient echo memory (GEM) scheme in an ultra-high optical depth cold-atom ensemble. The performance of a practical quantum memory is contingent on the chosen qubit encoding. We extend the GEM protocol to allow the simultaneous storage of frequency separated signals and demonstrate that this `dual-rail' memory is suitable for high-fidelity frequency qubits. Dual-rail signals are recalled with 35% efficiency, 82% interference fringe visibility and 6 degrees of phase stability. We describe how the fidelity of the scheme is limited by frequency-dependent polarization rotation and how this may be addressed in an improved configuration. Finally, we demonstrate single-rail storage by GEM with 87% efficiency and 1 ms memory lifetime. Our memory surpasses the no-cloning limit for up to 600 us of storage, out-performing an optical fibre delay line by a factor of six. This is the first quantum memory to beat this important benchmark.
The ultimate goal of this research is to develop the essential physical ingredients required for the shared quantum gate processing unit (S-QGPU) architecture for distributed quantum computing. In this architecture, local quantum processors are connected through flying photonic qubits and a shared central gate processor, providing a route toward scalable quantum computation beyond the limitations of isolated local devices. Realizing such an architecture requires two key components: a reliable quantum link between distant nodes and a hybrid controlled gate capable of implementing nonlocal quantum operations.This thesis investigates these two ingredients in cold 87Rb atomic ensembles. The first part focuses on narrowband biphoton generation through spontaneous four-wave mixing in laser-cooled atoms. These biphotons serve as heralded single-photon sources and quantum links between distant nodes, while remaining naturally compatible with atomic quantum memories. A central result of this work is the discovery and analysis of symmetry-protected biphoton coherence time, in which the biphoton coherence becomes immune to loss associated with electromagnetically induced transparency. This effect provides a new mechanism for generating ultranarrow-band quantum light with improved robustness, and enhances both synchronization and light–matter interfacing in the S-QGPU architecture. The role of atomic-state preparation in biphoton generation is also investigated through opticalpumping-based configurations. The second part of this thesis explores atom–photon hybrid controlled gates based on the combination of quantum memory and Rydberg blockade. In these schemes, photonic qubits are mapped onto collective atomic excitations, conditional operations are implemented through strong Rydberg interactions, and the quantum states are subsequently converted back into photonic qubits. These hybrid gate schemes provide the physical mechanism for remotely controlled operations between distant quantum nodes. Together, the narrowband biphoton source and the atom–photon hybrid gate constitute the two essential building blocks of the S-QGPU architecture. This work establishes a physical foundation for distributed quantum computing with cold atomic systems and advances the development of scalable quantum networks based on efficient photon–atom interfaces.
Ultimate quantum fluctuations which limit the fundamental precision in metrology can be only reduced by entanglement of particles. In particular, squeezing of the fluctuations called the spin projection noise by means of generation of entanglement in an ensemble of atoms can improve the precision of spectroscopy and of atomic clocks. A powerful tool for generation of multiparticle entanglement is a quantum nondemolition measurement (QND). We will describe the fundamentals of QND measurement of atomic collective state with light and present a recent experiment [1] where this method has been used to reduce the projection noise on the clock transition in an ensemble of 105 cold and trapped Cesium atoms. Another method for generation of spin squeezing via electron-nucleus entanglement applicable only for atoms with angular momentum higher than ½ will be also discussed [2].
Thesis chapter. The fragility of quantum information is a fundamental constraint faced by anyone trying to build a quantum computer. A truly useful and powerful quantum computer has to be a robust and scalable machine. In the case of many qubits which may interact with the environment and their neighbors, protection against decoherence becomes quite a challenging task. The scalability and decoherence issues are the main difficulties addressed by the distributed model of quantum computation. A distributed quantum computer consists of a large quantum network of distant nodes — stationary qubits which communicate via flying qubits. Quantum information can be transferred, stored, processed and retrieved in decoherence-free fashion by nodes of a quantum network realized by an atomic medium — an atomic quantum memory. Atomic quantum memories have been developed and demonstrated experimentally in recent years. With the help of linear optics and laser pulses, one is able to manipulate quantum information stored inside an atomic quantum memory by means of electromagnetically induced transparency and associated propagation phenomena. Any quantum computation or communication necessarily involves entanglement. Therefore, one must be able to entangle distant nodes of a distributed network. In this article, we focus on the probabilistic entanglement generation procedures such as well-known DLCZ protocol. We also demonstrate theoretically a scheme based on atomic ensembles and the dipole blockade mechanism for generation of inherently distributed quantum states so-called cluster states. In the protocol, atomic ensembles serve as single qubit systems. Hence, we review single-qubit operations on qubit defined as collective states of atomic ensemble. Our entangling protocol requires nearly identical single-photon sources, one ultra-cold ensemble per physical qubit, and regular photodetectors. The general entangling procedure is presented, as well as a procedure that generates in a single stepQ-qubit GHZ states with success probability psuccess ~ ηQ/2, where η is the combined detection and source efficiency. This is significantly more efficient than any known robust probabilistic entangling operation. The GHZ states form the basic building block for universal cluster states, a resource for the one-way quantum computer.
We present a complete protocol for cavity-free quantum networking based on collective enhancement in Rydberg atom ensembles. The scheme combines Rydberg blockade, collectively enhanced light--matter coupling, and phase-matched directional emission to remove the need for optical cavities while retaining efficiencies comparable to cavity-assisted interfaces. The protocol proceeds in three steps: (i)~local control--ensemble entanglement generated by Rydberg blockade with gate fidelity Fgate≈99.93%; (ii)~atom--photon conversion through Raman emission from an oblate spheroidal ensemble, yielding directional emission efficiency ηdir≈73% and single-node efficiency ηnode≈40%; and (iii)~remote atom--atom entanglement via Hong--Ou--Mandel interference, producing Bell states with fidelity F>97.5%. Incorporating quantum memories allows up to M≈100 retry attempts within a coherence time T2>100μs, enabling entanglement generation rates of approximately 4kHz over a 20~km separation. Collectively enhanced Rydberg ensembles thus provide a practical, cavity-free interface for scalable distributed quantum computing and secure quantum communication.