Understanding computations with neutral atoms comes down to knowing how to realize, control, manipulate and measure qubits through this particular technology. Here, we dive into those details.
Atoms consist of a nucleus and their electron cloud. At QuEra, we use Rubidium atoms. When the positive and negative charges of an atom balance each other, the atom is said to be neutral; in fact, that is how most atoms are found in nature. By shining a laser at the atom, one can increase the energy of the atom, making it excited. Each atom has a wide range of potential excited states, each at different energy levels. This offers a great platform for storing and processing quantum information. Any choice of two such levels can be named ‘0’ and ‘1’ to form a qubit.
Using lasers as tweezers, we trap individual atoms in their place. The lasers suppresses the atomic movement, effectively cooling them down to nearly absolute zero temperature. At these temperatures, the individual discrete energy levels of the atoms can be resolved and manipulated. Some of them leading to huge coherence times exceeding one second!
When atoms are excited to high-energy states, called Rydberg states, their electron clouds are puffed up to relatively enormous scale. Such atoms grow by more than a thousand times their usual size. In this Rydberg state, different atoms can interact over long distances, enabling quantum information to be transferred between them. This enables entanglement, one of the key ingredients for quantum information manipulation.
The precise mechanism by which Rydberg atoms interact is called the ‘van der Waals’ interaction, originating from the strong dipole moments of the puffed atoms. This interaction decays with the 6th power of the interatomic distance, meaning that only when atoms are close to eachother they interact very strongly. In fact, that interaction can be so strong that it can lead to the so-called "Rydberg blockade" effect, under which no two neighboring atoms can be excited at a given time. Through this mechanism, conditional quantum logic and two-qubit gates can be implemented.
Remarkably, the size of Rydberg atoms can include several proximate qubits, enabling them to all mutually interact. While most quantum computers can only perform 1-qubit and 2-qubit gates natively, the Rydberg blockade mechanism enables the creation of native multi-qubit gates.
Such gates, like the Toffoli gate (see image on the left), play an important role in many quantum algorithms. Encoding these multi-qubit gates natively can strongly reduce and algorithm’s circuit depth, which also means a strong mitigation of errors. A good example is the constant-depth implementation of Shor’s algorithm.
All of this happens on a miniature scale: one can pack thousands of laser-trapped atoms in a square millimeter. And with integrated photonic chips, precise control of multiple qubits can be achieved by just a few lasers.
As lasers can move freely through space, neutral atoms can also be arranged in nearly arbitrary configurations, which can be used to adjust and adapt their connectivity to the specific needs of a given problem. It also leads to significantly shorter development cycles, as new applications can make use of new configurations without reassembly of the hardware.
With a proper choice of atomic levels for qubit encoding, atoms can even be coherently moved around during a calculation! This feature is a key advantage of neutral-atom technology. It enables an efficient memory bus service, allowing all-to-all connectivity of the qubits at large scale. This is particularly beneficial in error-correction, as it offers new opportunities for error-correcting code choice and gate choice, including the first and only realization of Kitaev's paradigmatic toric code with periodic boundary conditions. Furthermore, it enables zoned architectures for memory and processing, which further improves the ratio of control lines to number of qubits.
Together, these features make neutral-atom quantum computers the architecture of choice for utility and scalability.
There are various quantum operation modes that can realize a computation. Neutral atoms support different modes: digital and analog, which each have their own advantages.
The digital gate-based mode decomposes a complex operation into a few elementary steps (gates) that operate on one or two qubits at a time and changes qubits between their two computational states. Sequences of gates change one computational state into another. When measured, the final state becomes a bitstring, which captures the outcome of the computation.
The gate-based operation mode provides an elegant recipe to enable universal functionality and programmability, with only a small set of elementary operations.
At the physical level, digital quantum operations are continuous state transitions expressed by a Hamiltonian: the function that describes the forces that make a physical system change over time. If one can control a Hamiltonian well-enough, the gate-based mode can be fully bypassed. That might be suitable for only a subset of problems, but when it is possible, this can lead us directly to answers without decomposing algorithms into elementary steps.
In the digital gate-based mode, errors can be introduced at each step, and accumulate over the course of a computation. The analog mode does not suffer from accumulation of faulty gates and is therefore much less susceptible to errors and therefore very suitable for the current early stage of quantum computing maturity.
Combined with simplified control requirements, the analog quantum mode enables the efficient manipulation of many more qubits with significantly fewer control signals.