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Tracking photon jumps with repeated quantum non-demolition parity measurements.

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Title: Tracking photon jumps with repeated quantum non-demolition parity measurements.
Authors: Sun, L., Petrenko, A., Leghtas, Z., Vlastakis, B., Kirchmair, G., Sliwa, K. M., Narla, A., Hatridge, M., Shankar, S., Blumoff, J., Frunzio, L., Mirrahimi, M., Devoret, M. H., Schoelkopf, R. J.
Source: Nature. 7/24/2014, Vol. 511 Issue 7510, p444-448. 5p.
Subjects: Photons, Quantum error correcting codes, Quantum computers, Ion traps, Electrons, Microwaves
Abstract: Quantum error correction is required for a practical quantum computer because of the fragile nature of quantum information. In quantum error correction, information is redundantly stored in a large quantum state space and one or more observables must be monitored to reveal the occurrence of an error, without disturbing the information encoded in an unknown quantum state. Such observables, typically multi-quantum-bit parities, must correspond to a special symmetry property inherent in the encoding scheme. Measurements of these observables, or error syndromes, must also be performed in a quantum non-demolition way (projecting without further perturbing the state) and more quickly than errors occur. Previously, quantum non-demolition measurements of quantum jumps between states of well-defined energy have been performed in systems such as trapped ions, electrons, cavity quantum electrodynamics, nitrogen-vacancy centres and superconducting quantum bits. So far, however, no fast and repeated monitoring of an error syndrome has been achieved. Here we track the quantum jumps of a possible error syndrome, namely the photon number parity of a microwave cavity, by mapping this property onto an ancilla quantum bit, whose only role is to facilitate quantum state manipulation and measurement. This quantity is just the error syndrome required in a recently proposed scheme for a hardware-efficient protected quantum memory using Schrödinger cat states (quantum superpositions of different coherent states of light) in a harmonic oscillator. We demonstrate the projective nature of this measurement onto a region of state space with well-defined parity by observing the collapse of a coherent state onto even or odd cat states. The measurement is fast compared with the cavity lifetime, has a high single-shot fidelity and has a 99.8 per cent probability per single measurement of leaving the parity unchanged. In combination with the deterministic encoding of quantum information in cat states realized earlier, the quantum non-demolition parity tracking that we demonstrate represents an important step towards implementing an active system that extends the lifetime of a quantum bit. [ABSTRACT FROM AUTHOR]
Copyright of Nature is the property of Springer Nature and its content may not be copied or emailed to multiple sites without the copyright holder's express written permission. Additionally, content may not be used with any artificial intelligence tools or machine learning technologies. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)
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  Data: <searchLink fieldCode="AR" term="%22Sun%2C+L%2E%22">Sun, L.</searchLink><br /><searchLink fieldCode="AR" term="%22Petrenko%2C+A%2E%22">Petrenko, A.</searchLink><br /><searchLink fieldCode="AR" term="%22Leghtas%2C+Z%2E%22">Leghtas, Z.</searchLink><br /><searchLink fieldCode="AR" term="%22Vlastakis%2C+B%2E%22">Vlastakis, B.</searchLink><br /><searchLink fieldCode="AR" term="%22Kirchmair%2C+G%2E%22">Kirchmair, G.</searchLink><br /><searchLink fieldCode="AR" term="%22Sliwa%2C+K%2E+M%2E%22">Sliwa, K. M.</searchLink><br /><searchLink fieldCode="AR" term="%22Narla%2C+A%2E%22">Narla, A.</searchLink><br /><searchLink fieldCode="AR" term="%22Hatridge%2C+M%2E%22">Hatridge, M.</searchLink><br /><searchLink fieldCode="AR" term="%22Shankar%2C+S%2E%22">Shankar, S.</searchLink><br /><searchLink fieldCode="AR" term="%22Blumoff%2C+J%2E%22">Blumoff, J.</searchLink><br /><searchLink fieldCode="AR" term="%22Frunzio%2C+L%2E%22">Frunzio, L.</searchLink><br /><searchLink fieldCode="AR" term="%22Mirrahimi%2C+M%2E%22">Mirrahimi, M.</searchLink><br /><searchLink fieldCode="AR" term="%22Devoret%2C+M%2E+H%2E%22">Devoret, M. H.</searchLink><br /><searchLink fieldCode="AR" term="%22Schoelkopf%2C+R%2E+J%2E%22">Schoelkopf, R. J.</searchLink>
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  Data: Quantum error correction is required for a practical quantum computer because of the fragile nature of quantum information. In quantum error correction, information is redundantly stored in a large quantum state space and one or more observables must be monitored to reveal the occurrence of an error, without disturbing the information encoded in an unknown quantum state. Such observables, typically multi-quantum-bit parities, must correspond to a special symmetry property inherent in the encoding scheme. Measurements of these observables, or error syndromes, must also be performed in a quantum non-demolition way (projecting without further perturbing the state) and more quickly than errors occur. Previously, quantum non-demolition measurements of quantum jumps between states of well-defined energy have been performed in systems such as trapped ions, electrons, cavity quantum electrodynamics, nitrogen-vacancy centres and superconducting quantum bits. So far, however, no fast and repeated monitoring of an error syndrome has been achieved. Here we track the quantum jumps of a possible error syndrome, namely the photon number parity of a microwave cavity, by mapping this property onto an ancilla quantum bit, whose only role is to facilitate quantum state manipulation and measurement. This quantity is just the error syndrome required in a recently proposed scheme for a hardware-efficient protected quantum memory using Schrödinger cat states (quantum superpositions of different coherent states of light) in a harmonic oscillator. We demonstrate the projective nature of this measurement onto a region of state space with well-defined parity by observing the collapse of a coherent state onto even or odd cat states. The measurement is fast compared with the cavity lifetime, has a high single-shot fidelity and has a 99.8 per cent probability per single measurement of leaving the parity unchanged. In combination with the deterministic encoding of quantum information in cat states realized earlier, the quantum non-demolition parity tracking that we demonstrate represents an important step towards implementing an active system that extends the lifetime of a quantum bit. [ABSTRACT FROM AUTHOR]
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  Data: <i>Copyright of Nature is the property of Springer Nature and its content may not be copied or emailed to multiple sites without the copyright holder's express written permission. Additionally, content may not be used with any artificial intelligence tools or machine learning technologies. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract.</i> (Copyright applies to all Abstracts.)
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