Quantum machine

Photograph of the quantum machine developed by O'Connell. The mechanical resonator is to the left of the coupling capacitor (small white square), the qubit to the right of it.

The quantum machine is a machine whose movements can be described by quantum mechanics , while its description is not possible in the context of classical mechanics . The first quantum machine was developed in 2009 by Aaron D. O'Connell at the University of California, Santa Barbara as part of his PhD under the direction of Andrew N. Cleland and John M. Martinis .

Andrew Cleland and John Martini's quantum machine consists of an approximately 40 µm mechanical oscillator that is electrically coupled to a qubit . Various quantum phenomena, such as B. the controlled generation of quantized vibrational states ( phonons ) and the quantum entanglement of these vibrational states with the qubit can be observed. The US journal Science named the quantum machine the scientific breakthrough of the year 2010.

background

The idea that not only objects on the scale of molecules , but also macroscopic objects could follow the laws of quantum mechanics, stems from the beginnings of quantum mechanics in the early 20th century. On the scale of molecules and below, experimental measurements have already provided results that contradict classical physics . However, quantum effects are not easily observable on macroscopic objects. The energy quanta of macroscopic objects are so small that energy changes seem to occur continuously. Furthermore, the interference effects typical of microscopic systems are very efficiently suppressed by unavoidable decoherence effects . With the quantum machine developed by O'Connell, these problems could be partially solved through an optimized resonator design and various other constructive measures.

Structure of the O'Connell quantum machine

Electron microscope image of the heart of the quantum machine, the thin-film resonator. The upper electrode visible in the center of the image obscures the view of the piezoelectric layer and the lower electrode.

The oscillator of the quantum machine developed by O'Connell was a piezo element made of aluminum nitride using thin-film technology , with a diameter of 40 µm and a thickness of 330 nm. This diameter corresponds approximately to the diameter of thin human fuzz , the oscillator of the quantum machine was barely visible to the naked eye recognizable. By applying an electrical alternating voltage between the electrodes, expansion vibrations of the piezo element could be excited, the resonance frequency of this vibration mode being around 6 GHz. By cooling the oscillator to a temperature of 25 mK, thermal influences on the experiment, in particular the thermal excitation of oscillation quanta, could be largely suppressed. The piezo element was capacitively coupled to a Josephson junction that was used as a qubit . The coupling between the resonator and the qubit could be influenced by controlling the Josephson frequency of the Josephson contact and by coupling microwave pulses into the coupling capacitor.

Quantum effects

Various quantum effects could be demonstrated on the quantum machine.

First, the researchers determined the average number of thermally excited oscillation quanta (phonons) of the resonator. The qubit was used as a probe, which was prepared in its basic state . From the low probability of excitation events of the qubit it could be deduced that the mean number of phonons was 0.07, i.e. H. the resonator was in its quantum mechanical ground state with a probability of 93% . ${\ displaystyle | g \ rangle}$ ${\ displaystyle | 0 \ rangle}$

In a next step, individual oscillation quanta of the resonator were excited in a controlled manner. In this experiment, the qubit was first brought into its excited state , whereupon a periodic change ( Rabi oscillations ) of the energy quantum from the qubit to the resonator and back was observed. The resonator and the qubit were thus in an entangled state . The lifetime of the Rabi oscillation was mainly limited by the damping of the resonator; the decay time (more precisely: the relaxation time) was 6.1 ps. Because of this short lifespan, it was not possible to carry out a complete determination of the quantum mechanical state using state tomography. ${\ displaystyle | e \ rangle}$ ${\ displaystyle T_ {1}}$

reception

The experiments of the group from Santa Barbara were published in the journal Nature and were named the breakthrough of the year 2010 by the journal Science . According to the science author Cho, the results open up new possibilities for developing ultra-sensitive force sensors and for generating quantum states of light. Cho sees further applications in basic research in the implementation of tests of quantum mechanics when applied to macroscopic objects.

Individual evidence

↑ ^a ^b AD O'Connell, M. Hofheinz, M. Ansmann, RC Bialczak, M. Lenander, E. Lucero, M. Neeley, D. Sank, H. Wang, M. Weides, J. Wenner, JM Martinis, and AN Cleland, Quantum ground state and single-phonon control of a mechanical resonator , Nature, Vol. 464, April 2010, pp. 697-703, doi : 10.1038 / nature08967 .
^ ^A ^b Adrian Cho, Breakthrough of the Year : The First Quantum Machine , Science , Vol. 330, No. 6011, 2010, p. 1604, doi : 10.1126 / science.330.6011.1604
↑ E. Schrödinger, The Current Situation in Quantum Mechanics , The Natural Sciences , Vol. 23, 1935, doi : 10.1007 / BF01491891 (part 1), doi : 10.1007 / BF01491914 (part 2), doi : 10.1007 / BF01491987 (part 3)
↑ For example, an energy quantum of the oscillation of a pendulum with an oscillation frequency of 1 Hertz is approximately . ${\ displaystyle 6 \ cdot 10 ^ {- 34} \, \ mathrm {J}}$
^ WH Zurek, Decoherence, Einselection, and the Quantum Origins of the Classical , Reviews of Modern Physics, Vol. 75, 2003, pp. 715-765.

literature

Marcelo Alonso, Edward J. Finn: Quantum Physics and Statistical Physics . 2005. ISBN 978-3-486-57762-4
Stephen Gasiorowicz: Quantum Physics . 9th edition 2005. ISBN 978-3-486-27489-9

Web links

Detailed article in the online magazine pro-Physik operated by the DPG
Aaron O'Connell: Making sense of a visible quantum object Video of a TED lecture by Aaron O'Connell about his quantum machine

[oConnell_Nature_2010-1] AD O'Connell, M. Hofheinz, M. Ansmann, RC Bialczak, M. Lenander, E. Lucero, M. Neeley, D. Sank, H. Wang, M. Weides, J. Wenner, JM Martinis, and AN Cleland, Quantum ground state and single-phonon control of a mechanical resonator , Nature, Vol. 464, April 2010, pp. 697-703, doi : 10.1038 / nature08967 .

[Cho_Science_2010-2] A ^b Adrian Cho, Breakthrough of the Year : The First Quantum Machine , Science , Vol. 330, No. 6011, 2010, p. 1604, doi : 10.1126 / science.330.6011.1604

[3] E. Schrödinger, The Current Situation in Quantum Mechanics , The Natural Sciences , Vol. 23, 1935, doi : 10.1007 / BF01491891 (part 1), doi : 10.1007 / BF01491914 (part 2), doi : 10.1007 / BF01491987 (part 3)

[GrößenordnungEnergiequanten-4] For example, an energy quantum of the oscillation of a pendulum with an oscillation frequency of 1 Hertz is approximately . ${\ displaystyle 6 \ cdot 10 ^ {- 34} \, \ mathrm {J}}$

[Zurek_Review_2003-5] WH Zurek, Decoherence, Einselection, and the Quantum Origins of the Classical , Reviews of Modern Physics, Vol. 75, 2003, pp. 715-765.