Image: Christine Daniloff |
Quantum
computers are largely theoretical devices that would exploit the weird
properties of matter at extremely small scales to perform calculations, in some
cases much more rapidly than conventional computers can. To date, the most
promising approach to building quantum computers has been to use ions trapped
in electric fields. Using photons instead would have many advantages, but it’s
notoriously difficult to get photons to interact: Two photons that collide in a
vacuum simply pass through each other.
In
Science, researchers at the Massachusetts Institute of Technology (MIT)
and Harvard University describe an experiment that allows a single photon to
control the quantum state of another photon. The result could have wide-ranging
consequences for quantum computing and quantum communication, the quantum
analog to conventional telecommunications.
A
quantum particle has the odd property that it can be in “superposition,”
meaning it’s in two different states at the same time: Fire a single photon at
a barrier with two slits in it, for instance, and it will, in some sense, pass
through both of them. Where the bits in an ordinary computer can represent
either zero or one, a bit made from a qubit could thus represent both zero and
one at the same time.
For
this reason, a string of only 16 qubits could represent 64,000 different
numbers simultaneously. It’s because a quantum computer could, in principle,
evaluate possible solutions to the same problem in parallel that quantum
computing promises major increases in computational speed.
But
one of the difficulties in building quantum computers is that superpositions of
states can be very fragile: Any interaction with its environment can cause a
subatomic particle to snap into just one of its possible states. Photons are
much more resistant to outside influences than subatomic particles, but that
also makes them harder to control; over the course of a computation, a quantum
computer needs to repeatedly alter the states of qubits.
The
MIT and Harvard researchers’ new paper points toward a quantum computer that
offers the best of both worlds: stability and control. Moreover, photons in
superposition could carry information stored as qubits rather than as ordinary
bits, opening the possibility of a quantum Internet.
Slowing light
Vladan Vuletic, the Lester Wolfe Professor of Physics at MIT; his student,
Haruka Tanji-Suzuki, a member of the MIT-Harvard Center for Ultracold Atoms
(CUA); Wenlan Chen, an MIT graduate student, and Renate Landig, a visiting
student, both at CUA; and Jonathan Simon, a postdoc at Harvard, developed an
optical switch that consists of a small cluster of cesium atoms suspended
between two tiny mirrors in a vacuum cavity. “The only way to make two photons
interact with one another is to use atoms as a mediator,” Vuletic says. “The
[first] photon changes the state of the atom, and therefore it modifies the
atom’s interaction with the other photon.”
When
a photon enters the cavity, it begins bouncing back and forth between the
mirrors, delaying its emission on the other side. If another photon has already
struck the cesium atoms, then each pass through them delays this second photon
even more. The delay induced by a single pass through the atoms would be
imperceptible, but the mirror-lined cavity, Vuletic explains, “allows us to
pass the photon many, many times through the atoms. In our case, it’s like
passing the photon 40,000 times through the atoms.”
When
it emerges from the cavity, the second photon thus has two possible states—delayed
or extra-delayed—depending on whether another photon has preceded it. With
these two states, it could, in principle, represent a bit of information. And
if the first photon was in some weird quantum state, where it can’t be said to
have struck the atoms or not, the second photon will be both extra-delayed and not
extra-delayed at the same time. The cavity would thus serve as a quantum
switch, the fundamental building block of a quantum computer.
Counting photons
Currently, the extra delay is not quite long enough that delayed and
extra-delayed photons can be entirely distinguished, but if the researchers can
increase its duration, the switch could have other uses as well. Many potential
applications of quantum optics, such as quantum cryptography, quantum
communication, and quantum-enhanced imaging, require photons that are emitted
in definite numbers—usually one or two. But the most practical method of emitting
small numbers of photons—a very weak laser—can promise only an average of one
photon at a time: There might sometimes be two, or three, or none. The CUA
researchers’ switch could be tailored to separate photons into groups of one,
two, or three and route them onto different paths.
Because
the switch allows the state of one photon to determine that of another, it
could also serve as an amplifier in a quantum Internet, increasing the strength
of an optical signal without knocking the individual photons out of
superposition. By the same token, it could serve as a probe that detects
photons without knocking them out of superposition, improving the efficiency of
quantum computation.
