Physicists at the University of New South Wales (UNSW) are the first to succeed in creating and manipulating quantum superpositions of a single, large nuclear spin. The superposition involves spin states that are very far apart and are therefore the superposition is considered a Schrödinger’s cat state. The work could be important for applications in quantum information processing and quantum error correction.
It was Erwin Schrödinger who, in 1935, devised his famous thought experiment involving a cat that could, worryingly, be both dead and alive at the same time. In his gedanken experiment, the decay of a radioactive atom triggers a mechanism (the breaking of a vial containing a poisonous gas) that kills the cat. However, since the decay of the radioactive atom is a quantum phenomenon, the atom is in a superposition of being decayed and not decayed. If the cat and poison are hidden in a box, we do not know if the cat is alive or dead. Instead, the state of the feline is a superposition of dead and alive – known as a Schrödinger’s cat state – until we open the box.
Schrödinger’s cat state (or just cat state) is now used to refer a superposition of two very different states of a quantum system. Creating cat states in the lab is no easy task, but researchers have managed to do this in recent years using the quantum superposition of coherent states of a laser field with different amplitudes, or phases, of the field. They have also created cat states using a trapped ion (with the vibrational state of the ion in the trap playing the role of the cat) and coherent microwave fields confined to superconducting boxes combined with Rydberg atoms and superconducting quantum bits (qubits).
Antimony atom cat
The cat state in the UNSW study is an atom of antimony, which is a heavy atom with a large nuclear spin. The high spin value implies that, instead of just pointing up and down (that is, in one of two directions), the nuclear spin of antimony can be in spin states corresponding to eight different directions. This makes it a high-dimensional quantum system that is valuable for quantum information processing and for encoding error-correctable logical qubits. The atom was embedded in a silicon quantum chip that allows for readout and control of the nuclear spin state.
Normally, a qubit, is described by just two quantum states, explains Xi Yu, who is lead author of a paper describing the study. For example, an atom with its spin pointing down can be labelled as the “0” state and the spin pointing up, the “1” state. The problem with such a system is that information contained in these states is fragile and can be easily lost when a 0 switches to a 1, or vice versa. The probability of this logical error occurring is reduced by creating a qubit using a system like the antinomy atom. With its eight different spin directions, a single error is not enough to erase the quantum information – there are still seven quantum states left, and it would take seven consecutive errors to turn the 0 into a 1.
More room for error
The information is still encoded in binary code (0 and 1), but there is more room for error between the logical codes, says team leader Andrea Morello. “If an error occurs, we detect it straight away, and we can correct it before further errors accumulate.”
The researchers say they were not initially looking to make and manipulate cat states but started with a project on high-spin nuclei for reasons unrelated to quantum information. They were in fact interested in observing quantum chaos in a single nuclear spin, which had been an experimental “holy grail” for a very long time, says Morello. “Once we began working with this system, we first got derailed by the serendipitous discovery of nuclear electric resonance, he remembers “We then became aware of some new theoretical ideas for the use of high-spin systems in quantum information and quantum error correcting codes.
“We therefore veered towards that research direction, and this is our first big result in that context,” he tells Physics World.
Scalable technology
The main challenge the team had to overcome in their study was to set up seven “clocks” that had to be precisely synchronized, so they could keep track of the quantum state of the eight-level system. Until quite recently, this would have involved cumbersome programming of waveform generators, explains Morello. “The advent of FPGA [field-programmable gate array] generators, tailored for quantum applications, has made this research much easier to conduct now.”
While there have already been a few examples of such physical platforms in which quantum information can be encoded in a (Hilbert) space of dimension larger than two – for example, microwave cavities or trapped ions – these were relatively large in size: bulk microwave cavities are typically the size of matchbox, he says. “Here, we have reconstructed many of the properties of other high-dimensional systems, but within an atomic-scale object – a nuclear spin. It is very exciting, and quite plausible, to imagine a quantum processor in silicon, containing millions of such Schrödinger cat states.”
The fact that the cat is hosted in a silicon chip means that this technology could be scaled up in the long-term using methods similar to those already employed in the computer chip industry today, he adds.
Looking ahead, the UNSW team now plans to demonstrate quantum error correction in its antimony system. “Beyond that, we are working to integrate the antimony atoms with lithographic quantum dots, to facilitate the scalability of the system and perform quantum logic operations between cat-encoded qubits,” reveals Morello.
The present study is detailed in Nature Physics.
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