One of the fundamental principles of quantum mechanics is that objects can be in two states at the same time. This means that an electron can, for instance, be in two places at once. However, these "superposition" states are never seen in classical, macroscopic objects – one example being Schrödinger's famous cat, who clearly could not be both dead and alive at the same time.

Indeed, the very act of trying to find out whether the cat is alive or dead actually changes its state. This act of measuring, through the so-called quantum mechanical backaction, disturbs the state of a quantum object so that it collapses and behaves like a classical one.

Now, researchers at the FOM Foundation and Delft University say they have succeeded in “opening” the box in which Schrodinger’s cat finds itself by just a small amount. In this way, it is possible to take a “peek” at the cat without destroying its fragile quantum state.

The team, led by Ronald Hanson, replaced the cat with a nitrogen atom in diamond, associated with the nitrogen-vacancy (NV) centre. This particle carries a nuclear spin that can point up (equivalent to the cat being alive) or down (cat dead). In previous work, the same group showed that it could measure a spin’s orientation by coupling the state of the nucleus to a nearby electron. By varying the strength of the coupling between the nucleus and electron, the researchers could actually control the strength of their measurements.

Less backaction

They found that weaker measurements revealed less information but also had less backaction. Analysing a nuclear spin after such a measurement showed that the spin remained in a superposition of two states (albeit slight altered).

Now, Hanson and colleagues have discovered that they can actually “steer” the nuclear spin by applying a series of measurements that vary in strength. Since the outcome of a measurement is not known beforehand, the researchers apply a feedback loop in their experiment. They adapt the strength of a second measurement depending on the outcome of a first and in this way, manoeuvre the nucleus towards a desired superposition state.

Helper qubit

The team used an ancilla qubit, which in this case was the spin of the electron associated with the vacancy of the NV-centre. “This ‘helper’ qubit helps to partially measure the spin of the nitrogen atom of the NV-centre,” team member Machiel Blok told “We measured the electron ancilla qubit’s spin orientation by applying a laser that excites the electron only if its spin is in the up state. The resulting fluorescence signal (bright or dark) thus tells us the state of the electron: bright means that the electron is in an up state and dark means that it is in a down state.

The measurements and feedback loops described in this work could be useful for so-called measurement-based quantum computing in the future, say the researchers. “In this particular scheme, a large entangled state is first created between many qubits,“ explained Blok. “Actual computation is then performed by sequentially measuring the individual qubits while adjusting the measurement settings depending on the results of previous ones.”

Techniques based on measuring the NV centre’s electron spin, which is very sensitive to magnetic fields over tiny volumes, would also be better than conventional methods such as SQUIDs or MRI – especially for estimating magnetic fields in biological samples, said Blok.

The work is detailed in Nature Physics doi:10.1038/nphys2881.

Further reading

Nanodiamonds make magnetic field sensors (Sep 2012)
Nanodiamond probe detects individual target atoms (Jul 2013)
Nanodiamond NVs live longer (Dec 2013)
Nanodiamond-based biolabeling is forever (Sep 2010)