Building quantum computers from
neutral-atom arrays

How does it work?

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.

Neutral atom's unique characteristics
Nature's perfect qubits
Efficient error correction
High-quality multi-qubit gates
Small footprint
Field programmable atom arrays
Highly scalable architecture
Modular architecture
Hybrid operation modes
Post-classical compute power

Qubits made of Neutral atoms

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.

Laser control

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!

Nature's perfect qubits
Atoms are nature's perfect qubits, all identical to one another. They are simultaneously capable of storing and processing quantum information, without significant cryogenic requirements. This gives an edge over manufactured qubits that can suffer from manufacturing imperfections.

Computations through puffing atoms

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.

Efficient error correction
Rydberg atoms interact on-demand. When atoms are not excited, they are protected from errors, regardles of the total number of qubits in the system. This significantly increases the computational power of our machines.

Multi-qubit entanglement

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.

High-quality multi-qubit gates
Long-range Rydberg interactions can be used to reduce the gate-decomposition overhead of multi-qubit gates. This minimizes errors and increases processing speed.

Small footprint

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.

Small footprint
The atoms and control system for a neutral-atom quantum computer can easily fit in a room. This small footprint makes a neutral atom computers easily deployable in a lab or a data center.

Flexible configurations

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.

Field programmable atom arrays
The flexible arrangements of qubits enable reconfigurable layout. This can be leveraged in algorithm design to limit gate overhead. It leads to significantly shorter development cycles, as new applications requiring different configurations can be implemented without reassembly of the hardware.

Qubit shuttling

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.

Highly scalable architecture
The atoms and control system for a neutral-atom quantum computer can easily fit in a room. This small footprint makes a neutral atom computers easily deployable in a lab or a data center.

Digital and analog quantum operations

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 mode

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.

A modular architecture
The core neutral atom processor can be equipped with modules for different functionalities, to enable quantum gates, error correction, memory and processing zones. This enables rapid hardware development cycles with a flexible roadmaps to accommodate new applications.

The analog quantum mode

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.

Hybrid operation modes
The atoms and control system for a neutral-atom quantum computer can easily fit in a room. This small footprint makes a neutral atom computers easily deployable in a lab or a data center.

Scalability vs. programmability

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.

Post-classical compute power
Robustness to errors, combined with efficient control mechanisms enabled the assembly of our 256 qubits machine. This machine is powerful enough to enter a nonsimulatable regime for a range of practical problems, surpassing the capabilities of classical supercomputers.