Quantum computing: Vibrating atoms make robust qubits, physicists find


Date:
January 26, 2022
Source:
Massachusetts Institute of Technology
Summary:
Physicists have discovered a new quantum bit, or 'qubit,' in the
form of vibrating pairs of atoms known as fermions. The new qubit
appears to be extremely robust, able to maintain superposition
between two vibrational states, even in the midst of environmental
noise, for up to 10 seconds, offering a possible foundation for
future quantum computers.



FULL STORY ==========================================================================
MIT physicists have discovered a new quantum bit, or "qubit," in the form
of vibrating pairs of atoms known as fermions. They found that when pairs
of fermions are chilled and trapped in an optical lattice, the particles
can exist simultaneously in two states -- a weird quantum phenomenon
known as superposition. In this case, the atoms held a superposition
of two vibrational states, in which the pair wobbled against each other
while also swinging in sync, at the same time.


==========================================================================
The team was able to maintain this state of superposition among
hundreds of vibrating pairs of fermions. In so doing, they achieved a
new "quantum register," or system of qubits, that appears to be robust
over relatively long periods of time. The discovery, published today
in the journal Nature, demonstrates that such wobbly qubits could be a promising foundation for future quantum computers.

A qubit represents a basic unit of quantum computing. Where a classical
bit in today's computers carries out a series of logical operations
starting from one of either two states, 0 or 1, a qubit can exist in a superposition of both states. While in this delicate in-between state,
a qubit should be able to simultaneously communicate with many other
qubits and process multiple streams of information at a time, to quickly
solve problems that would take classical computers years to process.

There are many types of qubits, some of which are engineered and others
that exist naturally. Most qubits are notoriously fickle, either unable
to maintain their superposition or unwilling to communicate with other
qubits.

By comparison, the MIT team's new qubit appears to be extremely robust,
able to maintain a superposition between two vibrational states, even
in the midst of environmental noise, for up to 10 seconds. The team
believes the new vibrating qubits could be made to briefly interact,
and potentially carry out tens of thousands of operations in the blink
of an eye.

"We estimate it should take only a millisecond for these qubits to
interact, so we can hope for 10,000 operations during that coherence time, which could be competitive with other platforms," says Martin Zwierlein,
the Thomas A. Frank Professor of Physics at MIT. "So, there is concrete
hope toward making these qubits compute." Zwierlein is a co-author on
the paper, along with lead author Thomas Hartke, Botond Oreg, and Ningyuan
Jia, who are all members of MIT's Research Laboratory of Electronics.



========================================================================== Happy accidents The team's discovery initially happened by
chance. Zwierlein's group studies the behavior of atoms at ultracold,
super-low densities. When atoms are chilled to temperatures a millionth
that of interstellar space, and isolated at densities a millionth that
of air, quantum phenomena and novel states of matter can emerge.

Under these extreme conditions, Zwierlein and his colleagues were
studying the behavior of fermions. A fermion is technically defined as
any particle that has an odd half-integer spin, like neutrons, protons,
and electrons. In practical terms, this means that fermions are prickly
by nature. No two identical fermions can occupy the same quantum state --
a property known as the Pauli exclusion principle. For instance, if one
fermion spins up, the other must spin down.

Electrons are classic examples of fermions, and their mutual Pauli
exclusion is responsible for the structure of atoms and the diversity
of the periodic table of elements, along with the stability of all the
matter in the universe.

Fermions are also any type of atom with an odd number of elementary
particles, as these atoms would also naturally repel each other.

Zwierlein's team happened to be studying fermionic atoms of
potassium-40. They cooled a cloud of fermions down to 100 nanokelvins
and used a system of lasers to generate an optical lattice in which
to trap the atoms. They tuned the conditions so that each well in the
lattice trapped a pair of fermions.

Initially, they observed that under certain conditions, each pair of
fermions appeared to move in sync, like a single molecule.



==========================================================================
To probe this vibrational state further, they gave each fermion pair a
kick, then took fluorescence images of the atoms in the lattice, and saw
that every so often, most squares in the lattice went dark, reflecting
pairs bound in a molecule. But as they continued imaging the system,
the atoms seemed to reappear, in periodic fashion, indicating that the
pairs were oscillating between two quantum vibrational states.

"It's often in experimental physics that you have some bright signal,
and the next moment it goes to hell, to never come back," Zwierlein
says. "Here, it went dark, but then bright again, and repeating. That oscillation shows there is a coherent superposition evolving over
time. That was a happy moment." "Alowhum" After further imaging and calculations, the physicists confirmed that the fermion pairs were holding
a superposition of two vibrational states, simultaneously moving together,
like two pendula swinging in sync, and also relative to, or against
each other.

"They oscillate between these two states at about 144 hertz," Hartke
notes.

"That's a frequency you could hear, like a low hum." The team was able
to tune this frequency, and control the vibrational states of the fermion pairs, by three orders of magnitude, by applying and varying a magnetic
field, through an effect known as Feshbach resonance.

"It's like starting with two noninteracting pendula, and by applying a
magnetic field, we create a spring between them, and can vary the strength
of that spring, slowly pushing the pendula apart," Zwierlein says.

In this way, they were able to simultaneously manipulate about 400 fermion pairs. They observed that as a group, the qubits maintained a state of superposition for up to 10 seconds, before individual pairs collapsed
into one or the other vibrational state.

"We show we have full control over the states of these qubits,"
Zwierlein says.

To make a functional quantum computer using vibrating qubits, the
team will have to find ways to also control individual fermion pairs
-- a problem the physicists are already close to solving. The bigger
challenge will be finding a way for individual qubits to communicate
with each other. For this, Zwierlein has some ideas.

"This is a system where we know we can make two qubits interact," he says.

"There are ways to lower the barrier between pairs, so that they come
together, interact, then split again, for about one millisecond. So,
there is a clear path toward a two-qubit gate, which is what you would
need to make a quantum computer." This research was supported, in
part, by the National Science Foundation, the Gordon and Betty Moore Foundation, the Vannevar Bush Faculty Fellowship, and the Alexander von Humboldt Foundation.

========================================================================== Story Source: Materials provided by
Massachusetts_Institute_of_Technology. Original written by Jennifer
Chu. Note: Content may be edited for style and length.


========================================================================== Related Multimedia:
* Qubits_graphic ========================================================================== Journal Reference:
1. Thomas Hartke, Botond Oreg, Ningyuan Jia, Martin Zwierlein. Quantum
register of fermion pairs. Nature, 2022; 601 (7894): 537 DOI:
10.1038/ s41586-021-04205-8 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/01/220126122405.htm

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