omg blueprint for trapped ion quantum computing with metastable states
D. T. C. Allcock, Wesley C. Campbell, John Chiaverini, Isaac L. Chuang, Eric R. Hudson, I. D. Moore, Anthony Ransford, Conrad Roman, Jeremy Sage, David J. Wineland
Quantum computers, much like their classical counterparts, will likely benefit from flexible qubit encodings that can be matched to different tasks. For trapped ion quantum processors, a common way to access multiple encodings is to use multiple, co-trapped atomic species. Here, we outline an alternative approach that allows flexible encoding capabilities in single-species systems through the use of long-lived metastable states as an effective, programmable second species. We describe the set of additional trapped ion primitives needed to enable this protocol and show that they are compatible with large-scale systems that are already in operation.
Quantum computers, much like their classical counterparts, will likely benefit from flexible qubit encodings that can be matched to different tasks. For trapped ion quantum processors, a common way to access multiple encodings is to use multiple, co-trapped atomic species. Here, we outline an alternative approach that allows flexible encoding capabilities in single-species systems through the use of long-lived metastable states as an effective, programmable second species. We describe the set of additional trapped ion primitives needed to enable this protocol and show that they are compatible with large-scale systems that are already in operation.
Coherent storage of quantum information is crucial to many quantum technologies. Long coherence times have been demonstrated in trapped-ion qubits, typically using the hyperfine levels within the ground state of a single ion. However, recent research suggests qubits encoded in metastable states could provide architectural benefits for quantum information processing, such as the possibility of effective dual-species operation in a single-species system and erasure-error conversion for fault-tolerant quantum computing. Here we demonstrate long-lived encoding of a quantum state in the metastable states of a trapped ion. By sympathetically cooling with another ion of the same species and constantly monitoring for erasure errors, we demonstrate a coherence time of 136(42) seconds with a qubit encoded in the metastable 5D5/2 state of a single 137Ba+ ion. In agreement with a model based on empirical results from dynamical-decoupling-based noise spectroscopy, we find that dephasing of the metastable levels is the dominant source of error once erasure errors are removed.
Today's most advanced ion trap quantum computers have significant overhead due to the need for dual-species operation. Looking ahead, logical qubit register sizes will be limited by the encoding rate needed to correct generic Pauli errors. We address both of these issues by establishing high-fidelity control of metastable qubits, a key component of \textit{omg} or dual-type architectures, which enables converting a significant fraction of gate errors to erasures. We first implement an erasure conversion scheme which enables detection of ∼94% of spontaneous Raman scattering errors during logic gates and nearly all errors from qubit decay. Second, we perform a two-ion geometric phase gate using far-detuned (-44\,THz) stimulated Raman transitions to produce an entangled state with a raw Bell state fidelity of 97.73\% and a SPAM-corrected Bell state fidelity of 98.61\%. When subtracting erasure errors, this fidelity becomes 99.16\%. These results, along with projections based on our detailed error budget, demonstrate metastable trapped-ion qubits as a platform for low-overhead, fault-tolerant quantum computing.
Abstract The first generation of quantum computers are on the horizon, fabricated from quantum hardware platforms that may soon be able to tackle certain tasks that cannot be performed or modelled with conventional computers. These quantum devices will not likely be universal or fully programmable, but special-purpose processors whose hardware will be tightly co-designed with particular target applications. Trapped atomic ions are a leading platform for first-generation quantum computers, but they are also fundamentally scalable to more powerful general purpose devices in future generations. This is because trapped ion qubits are atomic clock standards that can be made identical to a part in 1015, and their quantum circuit connectivity can be reconfigured through the use of external fields, without modifying the arrangement or architecture of the qubits themselves. In this forward-looking overview, we show how a modular quantum computer with thousands or more qubits can be engineered from ion crystals, and how the linkage between ion trap qubits might be tailored to a variety of applications and quantum-computing protocols.
Large-scale quantum information processors must be able to transport and maintain quantum information and repeatedly perform logical operations. Here, we show a combination of all of the fundamental elements required to perform scalable quantum computing through the use of qubits stored in the internal states of trapped atomic ions. We quantified the repeatability of a multiple-qubit operation and observed no loss of performance despite qubit transport over macroscopic distances. Key to these results is the use of different pairs of 9Be+ hyperfine states for robust qubit storage, readout, and gates, and simultaneous trapping of 24Mg+ "re-cooling" ions along with the qubit ions.
Quantum computers can solve certain problems more efficiently compared to conventional classical methods. In the endeavor to build a quantum computer, several competing platforms have emerged that can implement certain quantum algorithms using a few qubits. However, the demonstrations so far have been done usually by tailoring the hardware to meet the requirements of a particular algorithm implemented for a limited number of instances. Although such proof of principal implementations are important to verify the working of algorithms on a physical system, they further need to have the potential to serve as a general purpose quantum computer allowing the flexibility required for running multiple algorithms and be scaled up to host more qubits. Here we demonstrate a small programmable quantum computer based on five trapped atomic ions each of which serves as a qubit. By optically resolving each ion we can individually address them in order to perform a complete set of single-qubit and fully connected two-qubit quantum gates and alsoperform efficient individual qubit measurements. We implement a computation architecture that accepts an algorithm from a user interface in the form of a standard logic gate sequence and decomposes it into fundamental quantum operations that are native to the hardware using a set of compilation instructions that are defined within the software. These operations are then effected through a pattern of laser pulses that perform coherent rotations on targeted qubits in the chain. The architecture implemented in the experiment therefore gives us unprecedented flexibility in the programming of any quantum algorithm while staying blind to the underlying hardware. As a demonstration we implement the Deutsch-Jozsa and Bernstein-Vazirani algorithms on the five-qubit processor and achieve average success rates of 95 and 90 percent, respectively. We also implement a five-qubit coherent quantum Fourier transform and examine its performance in the period finding and phase estimation protocol. We find fidelities of 84 and 62 percent, respectively. While maintaining the same computation architecture the system can be scaled to more ions using resources that scale favorably (O(N^2)) with the number of qubits N.
Trapped-ion systems are leading candidates for quantum computing, offering long coherence times and high-fidelity gate operations. Scaling them, however, requires advanced capabilities such as mid-circuit measurement, qubit reset, and cooling, which together enable quantum error correction. One approach is the use of mixed qubit types, for example employing different ion species for cooling and computation. Recently, the omg blueprint [1] has gained attention as a method for coherently converting between qubit types within a single ion species, removing the overhead of mixed-species architectures while retaining their flexibility. In this thesis, we realise mixed qubit types in 137Ba+ using optical and metastable qubits. We demonstrate coherent control of both, as well as their coherent interconversion, achieving error rates at the 10−4 level suitable for NISQ applications. To enable selective control in multi-ion chains, we integrate a novel photonic chip for individual addressing of Raman beams [2]. We achieve cross-talk levels at or below 10−3 across all zones, and as low as 10−5 in some cases, establishing the scalability of our approach. Building on this, we present a new protocol for heralded state preparation and measurement (SPAM) [3]. This approach combines high-fidelity measurement with coherent conversion between qubit types to mitigate errors associated with qubit loss, decay, and imperfect SPAM. We demonstrate record-low SPAM error rates across optical, metastable, and ground-state qubits, with a minimum of 5(4)×10−6. This protocol can be integrated with erasure conversion techniques to enable in-sequence qubit loss correction, providing a pathway to scalable error correction even with finite qubit lifetimes. Finally, we introduce two-qubit operations through an implementation of the optical-transition dipole force (OTDF) gate [4]. This is the first implementation of the OTDF gate in 137Ba+, or in any ion with hyperfine structure, and it is directly compatible with our existing 532 nm laser system. Crucially, this gate operates in the σZ basis, avoiding the need for phase coherence between the driving field and the qubit, making it an excellent candidate for use in mixed-qubit-type systems. These results demonstrate the application of mixed-qubit-type architectures within a single ion species and establish a foundation for scaling trapped-ion quantum computers.