Today, we are announcing a major technical milestone on photonic interconnects, our core technology that we believe will enable quantum networking between and within quantum computers. We believe that this is the first demonstration of ion-photon entanglement outside of an academic environment.
Networking: Scaling Qubits through Photonic Interconnects
Last year, we provided additional details on our near-term technical roadmap, which is focused on improving measurable performance, increasing scale, and enabling system manufacturing. Today, we are excited to share progress against one of the tenets of the roadmap – increasing scale. Our scaling strategy is anchored on two areas of research and development: adding more qubits to newly designed traps and networking traps together via photonic interconnects.
IonQ’s technical roadmap, Q4 2023.
Photonic interconnects enable the entanglement of remote qubits across multiple, physical locations. Ions have unique advantages in the use of photons for networking – as the interactions between atoms and photons are a well-studied and understood area of quantum information science. As a result, IonQ’s vision has always been to scale our technology through photonic interconnects. In addition to ions’ high fidelity, high connectivity, and long coherence, ions’ compatibility with photonic networking is a core reason IonQ chose this modality of quantum computing.
While the science and procedure behind photonic interconnects has been understood for years in a research setting, an important endeavor for IonQ has been transitioning this technology from a lab setting to commercial grade.
Advancing Photonic Interconnects at IonQ
At IonQ, we are bringing this technology to the technical readiness level needed to be deployed in commercially available quantum computers. Our path to photonic interconnects consists of four main milestones, each of which expands on the previous milestone and culminates in a large-scale, networked, multi quantum processing unit (QPU).
Milestone 1: Ion-Photon Entanglement
The first – and one of the most challenging milestones in entangling quantum information across a network of QPUs – is generating and manipulating single photons entangled with a qubit to form a network node. Such a node must be capable of three key capabilities. First, the node must have the ability to generate “interconnect photons” entangled with the interconnect qubit. Second, the node must be capable of sending these interconnect photons through fiber optics to a detection hub. Lastly, the detection hub must be able to manipulate and measure the state of the interconnect photon to confirm ion-photon entanglement.
Milestone 2: Photon-Mediated Ion-Ion Entanglement
Milestone 2 expands upon Milestone 1 by entangling two ion-based qubits from separate nodes using their entangled photons. To achieve this, we are developing systems to collect interconnect photons from two different nodes, and to route these photons to a single detection hub, where they interfere and are measured, leaving an entangled state between the qubits at each node.
Milestone 3: Swapping Ion-Ion Entanglement to the QPU
After establishing this remote entanglement between interconnect qubits, Milestone 3 is to demonstrate that we can transfer this entanglement from interconnect qubits to computation qubits for more complex algorithms. This entanglement can be transferred via two-qubit swap gates to establish two entangled QPUs. With this entanglement, we can expand the number of qubits available for quantum computations.
Milestone 4: Multi-QPU Programmatic Entanglement
The final milestone is our ultimate goal of scaling photonic interconnects beyond two nodes. With many QPUs networked together in a programmatic fashion, we can execute extremely wide circuits by harnessing all of the qubits in the network, in parallel. To achieve this, we are in the process of developing single-photon switching techniques and devices, allowing us to collect interconnect photons from many interconnect qubits across many traps, to selectively entangle qubits across the network based on the parameters of the circuit.
Achieving Our Photonic Interconnect Milestone 1
Recently, we successfully demonstrated Milestone 1, ion-photon entanglement, a major step forward in our path to scaling our qubit counts. IonQ’s approach relies on well-established quantum principles that govern the interaction of photons and electrons.
First, an interconnect qubit is loaded into the ion trap. Next, the interconnect qubit is excited, with a highly specialized laser system, into an excited state. The excited state of the interconnect qubit then decays into a superposition of two possible qubit states.
As the electron decays, it releases a photon whose properties are entangled with the two quantum states of the ion qubit.
This emission and subsequent entanglement are natural quantum behaviors and don't require any coaxing from the interconnect technology. Next, we need to collect this valuable, entangled photon. We do this with highly specialized optical systems, which redirect the photon into a fiber optic cable. This cable is then connected to our detection hub, where we can control and manipulate the photon, as needed.
The state of the photon is measured in this photon state detection hub. We used this hub to perform specific measurements of the photon qubit states. After measurements of the ion-qubit, we confirmed that the photon was entangled with the original interconnect ion qubit state and sent through our network, successfully demonstrating Milestone 1.
Physics to Commercial Engineering Transfer
As we mentioned, there is a wealth of existing knowledge in the academic field on the strength of trapped ions for this photonic interconnect technology. IonQ is drawing upon this academic research and the expertise of specialists employed at IonQ to efficiently spin out commercial technology transfer from the academic world into an IonQ trapped ion system. Incorporating a photonic interconnect resource to realize a fully capable quantum computer is a cutting-edge frontier in the field. We are also integrating this technology with protocol development and advanced optical design towards high performance and reliable products.
We’ll keep pushing boundaries on this technology for many years to come.