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 theory1, 2, 3. Therefore, quantum measurements can be exploited for manipulating quantum systems without the need for control fields4, 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 readout7, 8. We vary the measurement strength to observe controlled wavefunction collapse and find post-selected quantum weak values8, 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 applications11, 12, 13 and are key to measurement-based quantum computing14, 15.
At a glance
- Progressive field-state collapse and quantum non-demolition photon counting. Nature 448, 889–893 (2007). et al.
- Quantum back-action of an individual variable-strength measurement. Science 339, 178–181 (2013). et al.
- Observing single quantum trajectories of a superconducting quantum bit. Nature 502, 211–214 (2013). et al.
- Qubit feedback and control with kicked quantum nondemolition measurements: A quantum Bayesian approach. Phys. Rev. B 74, 085307 (2006). &
- Control-free control: Manipulating a quantum system using only a limited set of measurements. Phys. Rev. A 82, 062103 (2010). &
- Quantum control: Squinting at quantum systems. Nature 470, 178–179 (2010).
- Test of weak measurement on a two- or three-qubit computer. Phys. Rev. A 77, 032101 (2008). , &
- Partial-measurement backaction and non-classical weak values in a superconducting circuit. Phys. Rev. Lett. 111, 090506 (2013). et al.
- How the result of a measurement of a component of the spin of a spin-1/2 particle can turn out to be 100. Phys. Rev. Lett. 60, 1351 (1988). , &
- Measurement of quantum weak values of photon polarization. Phys. Rev. Lett. 94, 220405 (2005). et al.
- Spin-bath narrowing with adaptive parameter estimation. Phys. Rev. A 85, 030301(R) (2012). et al.
- Entanglement-free Heisenberg-limited phase estimation. Nature 450, 393–396 (2007). et al.
- Interaction-based quantum metrology showing scaling beyond the Heisenberg limit. Nature 471, 486–489 (2011). et al.
- A one-way quantum computer. Phys. Rev. Lett. 86, 5188–5191 (2001). &
- High-speed linear optics quantum computing using active feed-forward. Nature 445, 65–69 (2007). et al.
- High-fidelity projective read-out of a solid-state spin quantum register. Nature 477, 574–578 (2011). et al.
- Demonstration of entanglement-by-measurement of solid-state qubits. Nature Phys. 9, 29–33 (2013). et al.
- Deterministic entanglement of superconducting qubits by parity measurement and feedback. Nature 502, 350–354 (2013). et al.
- Realization of quantum error correction. Nature 432, 602–605 (2004). et al.
- Experimental feedback control of quantum systems using weak measurements. Phys. Rev. Lett. 104, 080503 (2010). et al.
- Real-time quantum feedback prepares and stabilizes photon number states. Nature 477, 73–77 (2011). et al.
- Stabilizing Rabi oscillations in a superconducting qubit using quantum feedback. Nature 490, 77–80 (2012). et al.
- Contextual values of observables in quantum measurements. Phys. Rev. Lett. 104, 240401 (2010). , &
- Weak values and the Leggett–Garg inequality in solid-state qubits. Phys. Rev. Lett. 100, 026804 (2008). &
- Coherence of an optically illuminated single nuclear spin qubit. Phys. Rev. Lett. 100, 073001 (2008). et al.
- Sensing single remote nuclear spins. Nature Nanotech. 7, 657–662 (2012). et al.
- Detection and control of individual nuclear spins using a weakly coupled electron spin. Phys. Rev. Lett. 109, 137602 (2012). et al.
- Sensing distant nuclear spins with a single electron spin. Phys. Rev. Lett. 109, 137601 (2012). et al.
- Heralded entanglement between solid-state qubits separated by three meters. Nature 497, 86 (2013). et al.
- Room-temperature entanglement between single defect spins in diamond. Nature Phys. 9, 139–143 (2013). et al.
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