Periodical modulation of longitudinal coupling strength for quantum non-demolition qubit readout

Patent metadata

Field	Value
Publication number	US-10013657-B2
Application number	US-201715455105-A
Country	US
Kind code	B2
Filing date	Mar 9, 2017
Priority date	Mar 9, 2016
Publication date	Jul 3, 2018
Grant date	Jul 3, 2018

Abstract

Method and circuit for reading a value {circumflex over (σ)} z stored in a quantum information unit (qubit) memory having a qubit frequency ω a , with a resonator defined by a resonator damping rate κ, a resonator frequency ω r , a resonator electromagnetic field characterized by â † and â, a longitudinal coupling strength g z , an output â out and a longitudinal coupling g z {circumflex over (σ)} z (â † +â). At a quantum non-demolition (QND) longitudinal modulator, periodically modulating the longitudinal coupling strength g z with a signal of amplitude {tilde over (g)} z at least three (3) times greater than the resonator damping rate κ and of frequency ω m with ω m +κ resonant with ω r , wherein the longitudinal coupling strength g z varies over time (t) in accordance with g z (t)= g z +{tilde over (g)} z cos(ω m t) with g z representing an average value of g z and at a QND homodyne detector, measuring the value {circumflex over (σ)} z of the qubit memory from a phase reading of the output {circumflex over (σ)} out .

First claim

What is claimed is:

1. A circuit quantum electrodynamics (circuit QED) implementation of a quantum information unit (qubit) memory having a qubit frequency ω a and holding a value {circumflex over (σ)} z , the circuit QED implementation comprising:
a resonator defined by:
a resonator damping rate κ;
a resonator frequency ω r ;
a resonator electromagnetic field characterized by â † and â;
a longitudinal coupling strength g z ;
an output â out ;
a quantum non-demolition (QND) longitudinal coupling g z {circumflex over (σ)} z (â † +â); and

a modulator periodically modulating the longitudinal coupling strength g z with a signal of amplitude {tilde over (g)} z greater than or equal to the resonator damping rate κ and of frequency ω m with ω m ±κ resonant with ω r ±a correction factor, wherein the correction factor is smaller than |ω r /10| and the longitudinal coupling strength g z varies over time (t) in accordance with:
g z ( t )= g z +{tilde over (g)} z cos(ω m t ) with g z representing an average value of g z ; and

a homodyne detector for measuring the value {circumflex over (σ)} z of the qubit memory from a reading of the output â out .

2. The circuit QED implementation of claim 1 , wherein the correction factor is between 0 and |ω r /100|.

3. The circuit QED implementation of claim 1 , the homodyne detector is for measuring the value {circumflex over (σ)} z of the qubit memory from a phase reading of the output â out .

4. The circuit QED implementation of claim 1 , wherein the signal amplitude {tilde over (g)} z is at least three (3) times greater than the resonator damping rate κ or wherein the signal amplitude {tilde over (g)} z is at least ten (10) times greater than the resonator damping rate κ.

5. The circuit QED implementation of claim 1 , further comprising a signal injector providing a single-mode squeezed input on the resonator such that noise on the phase reading from the output â out is reduced while noise is left to augment on one or more interrelated characteristics of the output â out .

6. The circuit QED implementation of claim 3 , wherein the average value of g z , g z =0 and the single-mode squeezed input is QND.

7. The circuit QED implementation of claim 1 , wherein the qubit memory is a transmon comprising two Josephson junctions with substantially equivalent capacitive values and the longitudinal modulator comprises an inductor-capacitor (LC) oscillator with a phase drop δ across a coupling inductance placed between the two Josephson junctions, the longitudinal coupling resulting from mutual inductance between the oscillator and the transmon, the oscillator varying a flux Φ x in the transmon.

8. The circuit QED implementation of claim 7 , wherein the transmon has a flux sweet spot at integer values of a magnetic flux quantum Φ 0 , Josephson energy asymmetry of the transmon is below 0.02 and Φ x varies by ±0.05Φ 0 around Φ x =0.

9. The circuit QED implementation of claim 7 , wherein a 3-Wave mixing Josephson dipole element is used to couple the qubit and the resonator.

10. The circuit QED implementation of claim 6 , wherein the resonator is detuned from the qubit frequency ω a by |Δ|≥{tilde over (g)} z .

11. The circuit QED implementation of claim 6 , wherein the oscillator inductance is provided by an array of Josephson junctions or by one or more Superconducting Quantum Interference Device (SQUID).

12. A method for reading a value {circumflex over (σ)} z stored in a quantum information unit (qubit) memory having a qubit frequency ω a , with a resonator defined by a resonator damping rate κ, a resonator frequency ω r , a resonator electromagnetic field characterized by â † and â, a longitudinal coupling strength g z , an output â out and a quantum non-demolition (QND) longitudinal coupling g z {circumflex over (σ)} z (â † +â), the method comprising:
at a modulator, periodically modulating the longitudinal coupling strength g z with a signal of amplitude {tilde over (g)} z greater than or equal to the resonator damping rate κ and of frequency ω m with ω m ±κ resonant with ω r ±a correction factor, wherein the correction factor is smaller than |ω r /10| and the longitudinal coupling strength g z varies over time (t) in accordance with:
g z ( t )= g z +{tilde over (g)} z cos(ω m t ) with g z representing an average value of g z ; and

at a homodyne detector, measuring the value {circumflex over (σ)} z of the qubit memory from a reading of the output â out .

13. The method of claim 12 , wherein the signal amplitude {tilde over (g)} z is at least three (3) times greater than the resonator damping rate κ or wherein the signal amplitude {tilde over (g)} z is at least ten (10) times greater than the resonator damping rate κ.

14. The method of claim 12 , further comprising, from a signal injector, providing a single-mode squeezed input on the resonator such that noise on the phase reading from the output â out is reduced while noise is left to augment on one or more interrelated characteristics of the output â out .

15. The method of claim 14 , wherein the average value of g z , g z =0 and the single-mode squeezed input is QND.

16. The method of claim 12 , wherein the qubit memory is a transmon comprising two Josephson junctions with substantially equivalent capacitive values and the longitudinal modulator comprises an inductor-capacitor (LC) oscillator with a phase drop δ across a coupling inductance placed between the two Josephson junctions, the longitudinal coupling resulting from mutual inductance between the oscillator and the transmon, the oscillator varying a flux Φ x in the transmon.

17. The method of claim 16 , wherein the transmon has a flux sweet spot at integer values of a magnetic flux quantum Φ 0 , Josephson energy asymmetry of the transmon is below 0.02 and Φ x varies by ±0.050Φ 0 around Φ x =0.

18. The method of claim 16 , further comprising detuning the resonator from the qubit frequency ω a by |Δ|≥{tilde over (g)} z .

19. The method of claim 16 , wherein a 3-Wave mixing Josephson dipole element is used to couple the qubit Q 1 and the resonator R a .

20. The method of claim 16 , wherein the oscillator inductance is provided by an array of Josephson junctions or wherein the oscillator inductance is provided by one or more Superconducting Quantum Interference Device (SQUID).

Assignees

• Socpra Sciences Et Genie Sec

Inventors

• Bourassa Jerome
• Blais Alexandre
• Didier Nicloas

Classifications

• G11C11/44

  using super-conductive elements, e.g. cryotron · CPC title

• G06N99/002 Primary

  Physics · mapped topic

• H01L39/223

  Electricity · mapped topic

• H01L39/025

  Electricity · mapped topic

• G06N10/40 Primary

  Physical realisations or architectures of quantum processors or components for manipulating qubits, e.g. qubit coupling or qubit control · CPC title