Abstract
Macroscopic realism is the name for a class of modifications to quantum theory that allow macroscopic objects to be described in a measurementindependent manner, while largely preserving a fully quantum mechanical description of the microscopic world. Objective collapse theories are examples which aim to solve the quantum measurement problem through modified dynamical laws. Whether such theories describe nature, however, is not known. Here we describe and implement an experimental protocol capable of constraining theories of this class, that is more noise tolerant and conceptually transparent than the original Leggett–Garg test. We implement the protocol in a superconducting flux qubit, and rule out (by ∼84 s.d.) those theories which would deny coherent superpositions of 170 nA currents over a ∼10 ns timescale. Further, we address the ‘clumsiness loophole’ by determining classical disturbance with control experiments. Our results constitute strong evidence for the superposition of states of nontrivial macroscopic distinctness.
Introduction
In their original paper, Leggett and Garg (LG) asked whether the flux trapped in a superconducting ring was really ‘there’ when nobody looks^{1}. The systems LG had in mind are micrometre scale loops of superconducting material interrupted with one or more nonlinear elements known as Josephson junctions. Such circuits define two possible states of magnetic flux threading the loop, and modern variants^{2} are among the most macroscopic candidates for a quantum bit (or qubit), the basic constituent of various proffered quantumenhanced technologies such as the quantum computer. When a measurement is made, the qubit is found in one of the two possible states g〉 or e〉 with a probability that oscillates in time. Observation of such socalled ‘Rabi oscillations’ is consistent with a textbook quantum mechanical prediction (which generally ascribes a nonzero complex amplitude to each of the states), but not necessarily inconsistent with a classical ‘valuedefinite’ description (which prescribes that the system is in exactly one state at any given moment)^{3}. The decay envelope of the Rabi oscillations is given by an empirical parameter T_{2}. Huge efforts have been invested in extending this characteristic ‘coherence time’ to the current stateoftheart value of 85 μs (ref. 4), with a view to crossing the quantum errorcorrection thresholds and enabling largescale quantum computation^{5}. The guiding question of LG’s approach extends beyond their prototypical system: is there a fundamental mechanism preventing macroscopic superpositions from persisting, or is the problem merely about resources? LG’s name for the former position is ‘macroscopic realism’, or ‘macrorealism’ for short: objective collapse models such as Ghirardi, Rimini, Weber and Pearle (GRWP) theory^{6,7} or Penrose’s gravitationally induced collapse theory^{8} are specific examples which might make the quantumclassical divide well defined.
Motivated by the need for a strict test which could rule out this worldview, LG considered Q_{1}, Q_{2}, Q_{3} as the value taken by a macroscopic observable Q measured at three consecutive times t_{1}, t_{2}, t_{3}, respectively. LG made the assumption of ‘macrorealism per se’ (MRPS): that these variables can each be assigned a value ±1 at all times. Then the constraint^{1}
will hold. An elementary consequence is
where 〈…〉_{G} denotes the average over a ‘grand’ ensemble (or experimental arrangement) where all three observables (Q_{1}, Q_{2}, Q_{3}) are measured. LG conjoined a premise they termed ‘noninvasive measurability’ (NIM) to reach
the LeggettGarg inequality (LGI). Here, (for i=1, 2, 3) denotes an average over a ensemble identical to the grand ensemble, with the exception that the observable Q_{i} is not measured. If NIM is taken to mean that a suitably careful measurement has no effect on the statistics of measurement outcomes before or after it, it is effectively the premise ===: that the three ensembles in which experiments are actually performed (see Fig. 1a) are equivalent to the grand ensemble. Here, we include the impossibility of backwards causation (sometimes called Induction^{9}) in NIM. When ‘shuffling’ operations S_{1} and S_{2} (which induce coherent oscillations between the two states of interest) intervene respectively between t_{1} and t_{2}, and between t_{2} and t_{3}, LGI can be violated by a quantum mechanical system. If the system is sufficiently large (supercritically macroscopic), on the other hand, macrorealism predicts that no such violation is possible.
LGI or variants thereof have been experimentally tested (and violated) in a wide variety of microscopic experimental systems, sometimes with one or more of a variety of additional assumptions. A review of these experiments may be found in ref. 10, but see also more recent experimental tests on a caesium atom^{11}, delocalized photoexcitations^{12} and a twotransmon system^{13}. The demanding nature of LG tests has so far influenced the slow progress of experiments toward larger objects, meaning that experiments performed to date at best place only loose bounds on the critical macroscopicity.
Here, we show that LG’s approach can be significantly streamlined, resulting in a conceptually cleaner and experimentally simplified protocol. We go on to apply our new protocol to a superconducting flux qubit, thereby pushing the envelope of macroscopicity. The experimental results place constraints on all possible macroscopicrealist theories, and should spur progress towards tests at higher levels of macroscopicity.
Results
A simpler test
MRPS and NIM allow one to reach a simpler constraint
with the ensembles as defined previously. We call this equality the ‘nondisturbance condition’ (NDC). This condition has been derived by others and has been termed a quantum witness^{14}, ‘non disturbing measurement’^{15,16} or ‘no signalling in time’ condition^{17}. It follows from the same assumptions as LGI (Supplementary Note 1 and Supplementary Table 1) and demands a zero effect of the choice of measurement at t_{2} on the statistics of a measurement at the later time t_{3}. Here we suggest, however, that the requirements for the measurement at t_{2} are very minimal—it will be clear shortly that it need not even be a measurement at all, but some generalized operation O about which we need not assume anything. All pertinent properties of O are to be obtained through experiment.
Inspection of the NDC exposes a number of advantages over LGI. First, that there is no need to measure at all at t_{1}. Second, that only onepoint averages, rather than twopoint correlations are required. Third, that the condition is an equality rather than an inequality^{18}. The latter two points imply that any nonzero measurement visibility V:=(max〈Q〉_{obs}−min〈Q〉_{obs})/(max〈Q〉_{ideal}−min〈Q〉_{ideal}) (relating observed statistics to ideal ones) is sufficient to find a violation in principle, whereas previously V> was required.
Measurement invasiveness
The everpresent possibility of a clumsy measurement procedure at t_{2}, giving rise to a violation, rather than any inherent quantum effect, is as important a loophole here as ever. The issue has so far only been addressed with a priori arguments—those from the use of null result measurements^{11,19}, weak measurements^{20,21,22} or the use of an additional ‘stationarity’ assumption^{23}. By contrast, Leggett^{24} and later Wilde and Mizel^{25} have proposed that the problem can be attacked experimentally. This is precisely the approach we adopt here: The classical disturbance of a measurement (which we define shortly) may be determined in a control experiment, rather than assumed zero.
Building on these ideas, here we lay out a precise and operational notion of macrorealism that may be tested in the laboratory. Using conditional probabilities, define the disturbance parameter d_{σ}:=[P(Q_{3}=+1σ, O)−P(Q_{3}=−1σ, O)]−[P(Q_{3}=+1σ)−P(Q_{3}=−1σ)] as a measure of how much disturbance is introduced to Q_{3} by applying O at t_{2} (compared with doing nothing) when the preparation of the system immediately before t_{2} is described by σ. In a pair of control experiments, determine d_{g} and d_{e}, where g and e are the states that the measurement reveals reliably (that is, with 100% chance). These are measures of classical disturbance: see Fig. 1b.
Once the control experiments are completed, the main experiment may begin to determine d_{ρ}. ρ describes the net preparation when g is followed by a shuffling operation S_{1} (see Fig. 1c). According to quantum mechanics, the preparation ρ is described by a density operator . Here, ρ_{g,e}=g, e〉〈g, e are the density operators associated with preparations g, e respectively, α and β are complex numbers satisfying α^{2}+β^{2}=1, and are offdiagonal ‘coherence’ terms. In this language the predictions of a macrorealist theory (for supercritically macroscopic systems) are equivalent to those which follow from putting .
Under the assumption that S_{1} merely prepares weighted convex combinations (statistical mixtures) of the preparations associated with g and e, and does not affect the operation of the measurement at t_{2}, it could be thought natural that the disturbance d_{ρ} should not be higher than the similarly weighted linear combination of each individual disturbance:
or even the much weaker
to cover the possibility that the shuffling operation simply deterministically prepares the worst state (that is, the one with the highest susceptibility to disturbance by O). The fact that (theoretically at least) d_{g}=d_{e}=0 but d_{ρ}≠0 could be thought of as an instance of ‘superactivation’^{26}. Our definition of macrorealism (7) is a noisetolerant version of Maroney and Timpson’s ‘operational eigenstate mixture macrorealism’^{16}.
If the quantum disturbance d_{ρ} is significantly greater in magnitude than the greatest classical disturbance, this implies that the shuffling operation prepared something other than a statistical mixture of g and e. On the quantum view, this would be a coherent superposition of the preparations. On a hiddenvariable view, where preparation of a pure quantum state is actually a stochastic selection from a set of underlying states of reality {λ_{i}}, ρ might access a new set of {λ_{i}} not selectable via either e or g; or indeed represent a distribution over the same {λ_{i}} that is further from equilibrium with respect to O (ref. 16). It is worth noting that the leading theories of macrorealism do not rely on such hidden states, and so (along with a whole class of future theories subscribing to (7)) can indeed be ruled out by our approach.
Experimental results
Now, let us test the protocol experimentally using a superconducting flux qubit, where O is a measurement whose result is not inspected: Schild and Emary^{27} call this a ‘blind measurement’ but here we refer to it as a ‘measurement pulse’ due to the way it is implemented in our system. We find d_{e}>d_{g}, and a violation of (7) d_{ρ}−d_{g}=−0.063177 which is ∼84 s.d. away from zero (shown in Fig. 2). Despite our use of a relatively low fidelity qubit, we are able to reach very low uncertainties by performing 7 × 10^{6} trials per data point (see Fig. 3). A more pristine flux qubit with increased visibility and longer coherence times could display an even stronger violation of the macrorealist view. Our strict test of macrorealism provides evidence for a superposition of magnetic moments equivalent to several hundred thousand static electron spins pointing in opposite directions simultaneously. For further discussion on measures of macroscopicity, see Supplementary Note 3.
Discussion
Note that the visibility of our measurement V ≈ 0.28 is far below that required to find a violation of the LGI—this showcases the advantage of our scheme over standard tests of macrorealism. By eschewing LG’s inequality, but upholding their methodology (as we have done here), improved bounds on macrorealist theories may be obtained more easily. This is in contrast with proposals that tend to increase the complexity both of the logical argument and of the experimental setup; note that the requirements of a recent laboratory test of the LGI^{22} extend to high visibility, partialstrength, nondemolition measurements of twotime correlators via entanglement with a coherent ancilla—requirements that are not necessary in our approach. Furthermore, our reasoning does not use any quantum mechanical assumptions, which, if relied on, can otherwise vitiate the refutation of macrorealism. With the experimental protocol duly simplified, the challenge now is to perform strict tests of macrorealism at much higher macroscopicities—a feat which should be possible as long as the classical disturbance of one’s measurement is not too high.
Methods
Qubit design and fabrication
Our threejunction flux qubit, fabricated using angled evaporation, is placed at the centre of a transmission line Josephson Bifurcation Amplifier (JBA) resonator where it is magnetically coupled (see Fig. 2). The Hamiltonian is
where Δ is the tunnelling energy and is the bias energy, and and are states of definite supercurrent. The energy eigenstates are
where 2x=arctan(Δ/). In our experiment, Δ/h=2.75 GHz and /h≈2.90 GHz (h is Planck’s constant), hence approximately
Our flux qubit has a resonance frequency of 4.0 GHz with a persistent current of I_{p}=170 nA. It is chosen to have an average coherence time T_{2}≈10 ns (orders of magnitude below the best recorded time^{4}) to show the advantages of our approach.
Preparation, control and readout
Initialization of our flux qubit (operating below 10 mK) is achieved by thermal relaxation. Microwave lines provide a mechanism for applying resonant qubit control pulses of fixed duration (2 ns), which rotate the qubit state by an angle proportional to the amplitude of the applied field. The second S_{2} control pulse is applied 18 ns after the S_{1} pulse. The effect of the fixedduration pulses is controlled by modulating the power (see Fig. 3). To calibrate microwave power with the intended rotation angle θ we measured a (2θ, 0) sequence, and fitted a sinusoid to the data. The data agree qualitatively with a simple quantum mechanical model (see Supplementary Fig. 1).
Optionally, we apply a measurement pulse O (at a frequency of 6.5 GHz with total length 12 ns with 4 ns rising time) to the JBA, turning it on at t_{2} (between S_{1} and S_{2}). This operation can be thought of as a measurement of the flux qubit, or (since we do not inspect the result) equivalently, as a completely dephasing operation . The qubit state is finally measured at t_{3}, again through coupling to the JBA in the standard manner^{28,29,30}.
Statistics
Variances in measured quantities were propagated according to elementary rules Var(d_{ρ}−d_{g})=Var(d_{ρ})+Var(d_{g}), giving d_{ρ}−d_{g}/≈84. We conclude that our violation of macrorealism is of extremely high significance (See Fig. 2).
Data availability
All relevant data are available from the authors.
Additional information
How to cite this article: Knee, G. C. et al. A strict experimental test of macroscopic realism in a superconducting flux qubit. Nat. Commun. 7, 13253 doi: 10.1038/ncomms13253 (2016).
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Acknowledgements
We thank Erik Gauger for discussions. We acknowledge support from the MEXT GrantinAid for Scientific Research on Innovative Areas ‘Science of hybrid quantum systems’, grant numbers 15H05870 & 15H05867.
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G. C. K. and MC. Y. conceived and designed the experiment. G. C. K. and A. J. L. wrote the paper. K. K., H. T., H. Y. and S. S. performed the experiment. G. C. K., MC. Y., Y. M. and W. J. M. modelled and refined the protocol. All authors contributed to refining the manuscript.
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Supplementary Information
Supplementary Figures 1, Supplementary Table 1, Supplementary Notes 13 and Supplementary References. (PDF 184 kb)
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Knee, G., Kakuyanagi, K., Yeh, MC. et al. A strict experimental test of macroscopic realism in a superconducting flux qubit. Nat Commun 7, 13253 (2016). https://doi.org/10.1038/ncomms13253
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