Quantum Computer Hardware: How Quantum Computers Are Implemented – New Technology

In the previous article titled “The Basics: How Quantum
Computers Work and Where the Technology is Heading,” we
provided an overview of foundational quantum computing concepts,
including qubits (quantum bits), superposition, and entanglement.
Building on these principles, in this article we will provide an
overview of how qubits are implemented in real, physical systems to
make quantum computing possible. There are many leading approaches
to quantum hardware design, each with different techniques for
maintaining qubit stability and minimizing decoherence. Among them,
promising approaches include (1) neutral atom quantum computers,
(2) trapped ion quantum computers, (3) superconducting quantum
computers, and (4) spin qubit quantum computers. We explore the
underlying physical systems of each approach, discussing their
advantages and trade-offs, while recognizing that the development
of each creates different implications.

Neutral Atom Quantum Computers

Neutral atom quantum computers operate using lasers or
electromagnetic fields to trap neutral atoms in a localized region.
In such systems, neutral atoms serve as qubits by storing quantum
information in the energy levels of the atoms’ electrons or
hyperfine states of the atoms. The internal states of the neutral
atoms serve as the logical |0⟩ and |1⟩ states of the
qubits.

Precisely controlled laser or electromagnetic pulses are used to
manipulate the neutral atoms to initialize the qubits, carry out
quantum operations, and perform measurements of their quantum
states. Single-qubit operations can be achieved by applying tuned
laser or electromagnetic pulses to couple two selected internal
energy levels of a single atom. The duration, frequency, and
intensity of the pulses can be adjusted to change the qubit’s
state from |0⟩ or |1⟩, or any superposition in
between, to perform different gates with the single atom.

Neutral atom quantum computers can implement two-qubit gates by
leveraging the “Rydberg blockade” effect. To do so,
lasers or electromagnetic pulses are used to excite a qubit to a
high-energy state known as the “Rydberg state.” When
shifting to this high-energy state, the excited atom shifts the
energy levels of neighboring atoms, preventing those neighboring
atoms from being excited to similar energy states. The Rydberg
blockade effect can be used to establish superposition between two
neighboring atoms, producing multi-qubit gates.

Advantages of neutral atom quantum computers include high
scalability and uniform qubit characteristics through large arrays
of common atoms. Laser and electromagnetic pulses also provide
precise control over qubit states. However, trapping and
stabilizing neutral atoms is challenging because minor
misalignments or power changes in the laser beams can cause atoms
to drift or escape. Additionally, all neutral atoms must be
maintained in a controlled vacuum environment to prevent
decoherence due to collisions with background gases.

Trapped Ion Quantum Computers

In contrast to neutral atom quantum computers, trapped ion
quantum computers confine (e.g., immobilize) charged
atomic particles (ions) in free space using electromagnetic fields.
Motion of each trapped ion is mitigated by laser-cooling the ions
to near ground state. Like neutral atom quantum computers, each
atom in a trapped ion system operates as a qubit that encodes
quantum information in the electronic or hyperfine states of each
ion (e.g., using laser or microwave pulses).

Multi-qubit operations are realized in trapped ion quantum
computers by coupling the ions’ internal qubit states to shared
vibrational modes between proximate ions. As the ions are trapped
within the electromagnetic field, any vibrational motion of one ion
(e.g., induced via laser pulses) causes multiple ions to exhibit
quantized motion through Coulomb interactions (i.e.,
electromagnetic repulsion between the charged ions). With precisely
controlled laser or electromagnetic pulses, the induced vibrations
can cause selected ions to become entangled, allowing for various
multi-qubit gates and quantum operations.

Trapped ion quantum computers have several advantages, including
low error rates with long coherence times, high-fidelity quantum
gates, extensive possible arrangements of entangled particles
(i.e., with each ion in a trap capable of being entangled with any
other ion in the trap), along with well-understood and standardized
gate schemes that facilitate straightforward implementation of
quantum algorithms.

However, these systems typically involve slower processing times
compared with systems that use solid-state qubits (e.g.,
superconducting circuits or silicon-based qubits). Additionally,
trapped ion systems typically require complex laser and
electromagnetic infrastructure to achieve useful results. Like
neutral atom systems, trapped ion systems require controlled vacuum
conditions to prevent decoherence due to collisions with background
gases or other particles.

Superconducting Quantum Computers

Superconducting quantum computers leverage the properties of
superconducting materials to create and control qubits.
Superconducting materials, or “superconductors” are
materials that can conduct electricity with no resistance or energy
loss. Superconductors operate at extremely low temperatures,
typically within a few kelvins of absolute zero (e.g., 1 to 20 K or
-272 to -253°C). Superconducting quantum computers use a
superconductor structure called “Josephson junction” to
implement superconducting qubits (e.g., superconducting electronic
circuits).

Josephson junctions are thin, insulating barriers positioned
between superconducting materials, which can be used to create a
variety of different qubits, including transmon qubits and flux
qubits. Transmon qubits are created by connecting a Josephson
junction in parallel with a relatively large capacitor. This
creates a non-linear LC oscillator, which enables the creation of
discrete energy levels to implement quantum computing. Quantum
information can be encoded in the energy states of transmon qubits
using microwave pulses, which induce transitions between the
discrete energy levels of the qubit. Transmon qubits can be
entangled using capacitive coupling or inductive coupling of
adjacent qubits, or through coupling to a common resonator, such as
a microwave cavity or transmission line.

On the other hand, flux qubits are formed using a
“superconducting loop,” or current path, that includes
one or more Josephson junctions. Flux qubits encode quantum
information in the magnetic flux of each qubit, mediated by the
direction of current through the superconducting loop. The current
directions (e.g., clockwise or counterclockwise) serve as the
logical |0⟩ and |1⟩ states of the qubits. Flux qubits
are controlled using a combination of external magnetic flux tuning
and electromagnetic pulses. More specifically, the pulses and
magnetic flux can be used to externally adjust the state of the
qubit, inducing transitions between |0⟩ and |1⟩
states and implementing single-qubit gates. Multi-qubit gates can
be achieved through entangling flux qubits via inductive coupling
of adjacent qubits, shared Josephson junctions, or coupling to
common resonators.

Implementing quantum computers using superconductors has a
number of benefits, including very fast gate operations and the
ability to manufacture such systems using lithographic techniques
that are similar to those used to manufacture conventional
semiconductor circuits. Superconducting quantum computers are also
easy to scale due to their ease in manufacturing, and benefit from
a well-established research community. However, superconducting
suffers from limited coherence times and can only operate at
ultracold temperatures. Such systems are also susceptible to noise,
particularly from crosstalk produced from electromagnetic pulses
used to control individual qubits.

Spin Qubit Quantum Computers

Spin qubit quantum computers implement spin qubits, which encode
quantum information within the spin of charge carriers (e.g.,
electrons) in semiconductor materials. Spin is a quantum property
of subatomic particles that can be in a superposition of up or
down, which can respectively represent the logical |0⟩ and
|1⟩ states of the qubits. Quantum information of each
electron (e.g., spin-up and spin-down states) can be controlled by
rotating the spin of an electron in “quantum dots.”
Quantum dots are regions within the semiconductor material that
confine electrons in all three spatial dimensions. In some systems,
donor atoms implanted in semiconductor material, such as atoms of
phosphorous implanted within a silicon substrate, can be used to
isolate the spin of electrons.

Spin qubits can be controlled using a variety of optical,
electromagnetic, or thermal techniques. In some systems,
single-qubit gates can be implemented by controlling the state
through the application of an oscillating magnetic field tuned to
the frequency of the target qubit. Oscillating electric fields may
also be used to transition the spin of one or more qubits to a
desired state.

Multi-qubit gates can be implemented by providing a tunnel
barrier between two quantum dots or donor atoms. The interaction
between spins causes the two adjacent electrons to become
entangled, which can be used to create gates. The multi-qubit gates
can be controlled by applying a voltage across the gate, which
affects how the spins of the adjacent electrons interact.

Spin qubit quantum computers have a number of advantages,
including long coherence times, small footprint, and compatibility
with existing semiconductor manufacturing processes. However, such
approaches still require extremely low temperatures to operate
(e.g., near absolute zero). Additionally, spin qubit quantum
computers suffer from poor scalability due to complex circuitry
required to precisely route high-frequency voltage signals to each
qubit within the system.

Nitrogen-Vacant Qubits

A promising new paradigm in quantum computing leverages
nitrogen-vacancy (NV) centers in diamond to create robust,
room-temperature qubits. NV centers are atomic-scale defects in the
diamond lattice, consisting of a nitrogen atom adjacent to a
vacancy, which exhibit spin-dependent photoluminescence and
exceptionally long spin coherence times. These properties allow the
NV centers to function as potentially highly stable qubits that can
be initialized, manipulated, and read out optically or through
microwave fields. Unlike many other quantum systems that require
extreme cooling, NV center qubits maintain quantum coherence even
at room temperature, opening the door to more practical and
scalable quantum devices. Additionally, their sensitivity to
magnetic, electric, and thermal environments enables advanced
quantum sensing applications alongside quantum computation. This
diamond-based approach combines biocompatibility, durability, and
potential for integration with existing semiconductor technology,
making it a strong candidate for the next generation of quantum
technologies.

The Implications

These quantum computing approaches or implementations have
different technological advantages, timelines for maturity and
commercial readiness, supply chains, potential strategic
partnerships, use cases, product impacts, costs, etc., as
suggested. Understanding the various implications will be important
for assessing impact, strategic and otherwise. As the race for
quantum supremacy and for consumer-grade quantum computing
intensifies, it will shape investments, strategic decisions,
business timelines, and risk analysis. While it is too early to
ascertain which approach(es) may achieve dominance, now is the time
for strategists, analysts, and decision-makers to become familiar
with the candidates.

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