Today's quantum computers are complicated to build, difficult to scale up,
and require temperatures colder than interstellar space to operate. These
challenges have led researchers to explore the possibility of building
quantum computers that work using photons—particles of light. Photons can
easily carry information from one place to another, and photonic quantum
computers can operate at room temperature, so this approach is promising.
However, although people have successfully created individual quantum "logic
gates" for photons, it's challenging to construct large numbers of gates and
connect them in a reliable fashion to perform complex calculations.
Now, Stanford University researchers have proposed a simpler design for
photonic quantum computers using readily available components, according to
a paper published Nov. 29 in Optica. Their proposed design uses a laser to
manipulate a single atom that in turn, can modify the state of the photons
via a phenomenon called "quantum teleportation." The atom can be reset and
reused for many quantum gates, eliminating the need to build multiple
distinct physical gates, vastly reducing the complexity of building a
quantum computer.
"Normally, if you wanted to build this type of quantum computer, you'd have
to take potentially thousands of quantum emitters, make them all perfectly
indistinguishable, and then integrate them into a giant photonic circuit,"
said Ben Bartlett, a Ph.D. candidate in applied physics and lead author of
the paper. "Whereas with this design, we only need a handful of relatively
simple components, and the size of the machine doesn't increase with the
size of the quantum program you want to run."
This remarkably simple design requires only a few pieces of equipment: A
fiber optic cable, a beam splitter, a pair of optical switches and an
optical cavity.
Fortunately, these components already exist and are even commercially
available. They're also continually being refined since they're currently
used in applications other than quantum computing. For example,
telecommunications companies have been working to improve fiber optic cables
and optical switches for years.
"What we are proposing here is building upon the effort and the investment
that people have put in for improving these components," said Shanhui Fan,
the Joseph and Hon Mai Goodman Professor of the School of Engineering and
senior author on the paper. "They are not new components specifically for
quantum computation."
A novel design
The scientists' design consists of two main sections: A storage ring and a
scattering unit. The storage ring, which functions similarly to memory in a
regular computer, is a fiber optic loop holding multiple photons that travel
around the ring. Analogous to bits that store information in a classical
computer, in this system, each photon represents a quantum bit, or "qubit."
The photon's direction of travel around the storage ring determines the
value of the qubit, which like a bit, can be 0 or 1. Additionally, because
photons can simultaneously exist in two states at once, an individual photon
can flow in both directions at once, which represents a value that is a
combination of 0 and 1 at the same time.
The researchers can manipulate a photon by directing it from the storage
ring into the scattering unit, where it travels to a cavity containing a
single atom. The photon then interacts with the atom, causing the two to
become "entangled," a quantum phenomenon whereby two particles can influence
one another even across great distances. Then, the photon returns to the
storage ring, and a laser alters the state of the atom. Because the atom and
the photon are entangled, manipulating the atom also influences the state of
its paired photon.
"By measuring the state of the atom, you can teleport operations onto the
photons," Bartlett said. "So we only need the one controllable atomic qubit
and we can use it as a proxy to indirectly manipulate all of the other
photonic qubits."
Because any quantum logic gate can be compiled into a sequence of operations
performed on the atom, you can, in principle, run any quantum program of any
size using only one controllable atomic qubit. To run a program, the code is
translated into a sequence of operations that direct the photons into the
scattering unit and manipulate the atomic qubit. Because you can control the
way the atom and photons interact, the same device can run many different
quantum programs.
"For many photonic quantum computers, the gates are physical structures that
photons pass through, so if you want to change the program that's running,
it often involves physically reconfiguring the hardware," Bartlett said.
"Whereas in this case, you don't need to change the hardware—you just need
to give the machine a different set of instructions."
Reference:
Ben Bartlett et al, Deterministic photonic quantum computation in a
synthetic time dimension, Optica (2021).
DOI: 10.1364/OPTICA.424258
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