Post date: Feb 25, 2014 8:08:48 PM

There have been a number of great experiments recently. Here is one from a Delft group that verifies some of our theoretical predictions for weak value measurements. They also pursue our ideas for "uncontrolled control" of quantum systems.

Manipulating a qubit through the backaction of sequential partial measurements and real-time feedback

• M. S. Blok,
• C. Bonato,
• M. L. Markham,
• D. J. Twitchen,
• V. V. Dobrovitski
• & R. Hanson

Quantum measurements not only extract information from a system but also alter its state. Although the outcome of the measurement is probabilistic, the backaction imparted on the measured system is accurately described by quantum theory^{1, 2, 3}. Therefore, quantum measurements can be exploited for manipulating quantum systems without the need for control fields^{4, 5, 6}. We demonstrate measurement-only state manipulation on a nuclear spin qubit in diamond by adaptive partial measurements. We implement the partial measurement via tunable correlation with an electron ancilla qubit and subsequent ancilla readout^{7, 8}. We vary the measurement strength to observe controlled wavefunction collapse and find post-selected quantum weak values^{8, 9, 10}. By combining a novel quantum non-demolition readout on the ancilla with real-time adaptation of the measurement strength we realize steering of the nuclear spin to a target state by measurements alone. Besides being of fundamental interest, adaptive measurements can improve metrology applications^{11, 12, 13} and are key to measurement-based quantum computing^{14, 15}.