UNSW engineers have demonstrated a well-known quantum thought experiment in
the real world. Their findings deliver a new and more robust way to perform
quantum computations – and they have important implications for error
correction, one of the biggest obstacles standing between them and a working
quantum computer.
Quantum mechanics has puzzled scientists and philosophers for more than a
century. One of the most famous quantum thought experiments is that of the
“Schrödinger’s cat” – a cat whose life or death depends on the decay of a
radioactive atom.
According to quantum mechanics, unless the atom is directly observed, it
must be considered to be in a superposition – that is, being in multiple states
at the same time – of decayed and not decayed. This leads to the troubling
conclusion that the cat is in a superposition of dead and alive.
“No one has ever seen an actual cat in a state of being both dead and alive at the same time, but people use the Schrödinger’s cat metaphor to describe a superposition of quantum states that differ by a large amount,” says UNSW Professor Andrea Morello, leader of the team that conducted the research, published recently in the journal Nature Physics, opens in a new window.
Atomic cat
For this research paper, Prof. Morello’s team used an atom of antimony,
which is much more complex than standard ‘qubits’, or quantum building blocks.
“In our work, the ‘cat’ is an atom of antimony,” says Xi Yu, lead author of
the paper.
“Antimony is a heavy atom, which possesses a large nuclear spin, meaning a
large magnetic dipole. The spin of antimony can take eight different
directions, instead of just two. This may not seem much, but in fact it
completely changes the behaviour of the system. A superposition of the antimony
spin pointing in opposite directions is not just a superposition of ‘up’ and
‘down’, because there are multiple quantum states separating the two branches
of the superposition.”
This has profound consequences for scientists working on building a quantum
computer using the nuclear spin of an atom as the basic building block.
“Normally, people use a quantum bit, or ‘qubit’ – an object described by
only two quantum states – as the basic unit of quantum information,” says
co-author Benjamin Wilhelm.
“If the qubit is a spin, we can call ‘spin down’ the ‘0’ state, and ‘spin
up’ the ‘1’ state. But if the direction of the spin suddenly changes, we have
immediately a logical error: 0 turns to 1 or vice versa, in just one go. This
is why quantum information is so fragile.”
But in the antimony atom that has eight different spin directions, if the
‘0’ is encoded as a ‘dead cat’, and the ‘1’ as an ‘alive cat’, a single error
is not enough to scramble the quantum code.
“As the proverb goes, a cat has nine lives. One little scratch is not enough to kill it. Our metaphorical ‘cat’ has seven lives: it would take seven consecutive errors to turn the ‘0’ into a ‘1’! This is the sense in which the superposition of antimony spin states in opposite directions is ‘macroscopic’ – because it’s happening on a larger scale, and realises a Schrödinger cat,” explains Yu.
Scalable technology
The antimony cat is embedded inside a silicon quantum chip, similar to the
ones we have in our computers and mobile phones, but adapted to give access to
the quantum state of a single atom. The chip was fabricated by UNSW’s Dr
Danielle Holmes, while the atom of antimony was inserted in the chip by
colleagues at the University of Melbourne.
“By hosting the atomic ‘Schrödinger cat’ inside a silicon chip, we gain an
exquisite control over its quantum state – or, if you wish, over its life and
death,” says Dr Holmes.
“Moreover, hosting the ‘cat’ in silicon means that, in the long term, this
technology can be scaled up using similar methods as those we already adopt to
build the computer chips we have today.”
The significance of this breakthrough is that it opens the door to a new
way to perform quantum computations. The information is still encoded in binary
code, ‘0’ or ‘1’, but there is more ‘room for error’ between the logical codes.
“A single, or even a few errors, do not immediately scramble the
information,” Prof. Morello says.
“If an error occurs, we detect it straight away, and we can correct it
before further errors accumulate. To continue the ‘Schrödinger cat’ metaphor,
it’s as if we saw our cat coming home with a big scratch on his face. He’s far
from dead, but we know that he got into a fight; we can go and find who caused
the fight, before it happens again and our cat gets further injuries.”
The demonstration of quantum error detection and correction – a ‘Holy
Grail’ in quantum computing – is the next milestone that the team will address.
The work was the result of a vast international collaboration. Several
authors from UNSW Sydney, plus colleagues at the University of Melbourne,
fabricated and operated the quantum devices. Theory collaborators in the USA,
at Sandia National Laboratories and NASA Ames, and Canada, at the University of
Calgary, provided precious ideas on how to create the cat, and how to assess
its complicated quantum state.
“This work is a wonderful example of open-borders collaboration between
world-leading teams with complementary expertise,” says Prof. Morello.
Image: Visual depiction of the atomic ‘Schrödinger cat’ state. The ‘dead’
state corresponds to the antimony nuclear spin pointing completely downwards;
the ‘alive’ state is the spin completely upwards. A superposition of the two
results in a striking quantum state that displays seven quantum interference
fringes. The number of fringes corresponds to the number of ‘spin flips’
necessary to go from one extreme to the other. In quantum computing, this
corresponds to the number of consecutive errors required to turn a ‘0’ into a
‘1’ or vice versa.
Image credit: Tony Melov
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