Delayed Choice Qubit

Lorentz Transformations

Justin Dressel

Institute for Quantum Studies

Schmid College of Science and Technology


  • Qubits have a 4D "spacetime" representation
    • Pauli spin basis is a 3D Clifford algebra
    • Measurements rotate hyperbolically into 4D
    • Continuous measurements behave like (stochastic) Lorentz transformations
  • Circuit QED implementations are naturally "delayed choice" continuous measurements
    • Long after a probing microwave field interacts with the qubit, the choice of amplified quadrature selects which type of Lorentz transformation was performed

Example: Monitored Rabi Oscillations

A monitored quantum system (here a transmon qubit) has both smooth unitary dynamics and random measurement backaction

UCB, Nature 511, 570 (2014)

The composite dynamics has interesting theoretical structure

Qubits are 4-dimensional

  • Qubits (e.g., spin-1/2) are 2-level quantum systems

  • But, qubit state space is generally 4-dimensional
|\psi\rangle = \alpha |0\rangle + \beta |1\rangle
ψ=α0+β1|\psi\rangle = \alpha |0\rangle + \beta |1\rangle
\hat{\rho} = \frac{1}{2}\left(p \hat{1} + x \hat{\sigma}_1 + y \hat{\sigma}_2 + z \hat{\sigma}_3\right)
ρ^=12(p1^+xσ^1+yσ^2+zσ^3)\hat{\rho} = \frac{1}{2}\left(p \hat{1} + x \hat{\sigma}_1 + y \hat{\sigma}_2 + z \hat{\sigma}_3\right)

State norm
(preparation success probability)
(max radius of Bloch sphere)

(Conditional) Expectation values

(Bloch sphere coordinates)

\hat{\sigma}_1 = |0\rangle\langle 1| + |1\rangle\langle 0|
σ^1=01+10\hat{\sigma}_1 = |0\rangle\langle 1| + |1\rangle\langle 0|
\hat{\sigma}_2 = -i(|0\rangle\langle 1| - |1\rangle\langle 0|)
σ^2=i(0110)\hat{\sigma}_2 = -i(|0\rangle\langle 1| - |1\rangle\langle 0|)
\hat{\sigma}_3 = |1\rangle\langle 1| - |0\rangle\langle 0|
σ^3=1100\hat{\sigma}_3 = |1\rangle\langle 1| - |0\rangle\langle 0|
x^2 + y^2 + z^2 \equiv R^2 \leq p^2
x2+y2+z2R2p2x^2 + y^2 + z^2 \equiv R^2 \leq p^2
\hat{1} = |1\rangle\langle 1| + |0\rangle\langle 0|
1^=11+00\hat{1} = |1\rangle\langle 1| + |0\rangle\langle 0|

Operator basis: Pauli operators

Orthonormal under operator inner product:

\hat{A}\cdot\hat{B} \equiv \frac{1}{2}\text{Tr}\left[\frac{\hat{A}\hat{B}+\hat{B}\hat{A}}{2}\right]
A^B^12Tr[A^B^+B^A^2]\hat{A}\cdot\hat{B} \equiv \frac{1}{2}\text{Tr}\left[\frac{\hat{A}\hat{B}+\hat{B}\hat{A}}{2}\right]

Pauli Operators form a 3D Clifford Algebra (for spin)

  • The Clifford algebra of 3-space has 8 elements
  • The 4 "grades" of the algebra correspond geometrically to oriented subspaces of differing dimensions
\hat{\sigma}_y\hat{\sigma}_z \quad \hat{\sigma}_z\hat{\sigma}_x \quad \hat{\sigma}_x\hat{\sigma}_y
σ^yσ^zσ^zσ^xσ^xσ^y\hat{\sigma}_y\hat{\sigma}_z \quad \hat{\sigma}_z\hat{\sigma}_x \quad \hat{\sigma}_x\hat{\sigma}_y
\hat{\sigma}_x \quad \hat{\sigma}_y \quad \hat{\sigma}_z
σ^xσ^yσ^z\hat{\sigma}_x \quad \hat{\sigma}_y \quad \hat{\sigma}_z

Geometric meaning:







Line Segment

Plane Segment

Volume Segment

Matrix Representation of 3D Clifford Algebra

  • Simplification:
i\hat{\sigma}_x \quad i\hat{\sigma}_y \quad i\hat{\sigma}_z
iσ^xiσ^yiσ^zi\hat{\sigma}_x \quad i\hat{\sigma}_y \quad i\hat{\sigma}_z
\hat{\sigma}_x \quad \hat{\sigma}_y \quad \hat{\sigma}_z
σ^xσ^yσ^z\hat{\sigma}_x \quad \hat{\sigma}_y \quad \hat{\sigma}_z

Geometric meaning:







Line Segment

Plane Segment

Volume Segment

\hat{\sigma}_x\hat{\sigma}_y\hat{\sigma}_z = i\hat{1}
σ^xσ^yσ^z=i1^\hat{\sigma}_x\hat{\sigma}_y\hat{\sigma}_z = i\hat{1}

(Representation-independent definition of "imaginary unit")

Dot and Wedge Products

The usual Pauli operators are a faithful matrix representation, so the matrix product is precisely the Clifford product

\hat{A}\hat{B} = \frac{\hat{A}\hat{B}+\hat{B}\hat{A}}{2} + \frac{\hat{A}\hat{B}-\hat{B}\hat{A}}{2}
A^B^=A^B^+B^A^2+A^B^B^A^2\hat{A}\hat{B} = \frac{\hat{A}\hat{B}+\hat{B}\hat{A}}{2} + \frac{\hat{A}\hat{B}-\hat{B}\hat{A}}{2}

Symmetric part is dot product

Antisymmetric (noncommutative) part is wedge product

\frac{\hat{A}\hat{B}+\hat{B}\hat{A}}{2} = (\hat{A}\cdot\hat{B})\hat{1}
A^B^+B^A^2=(A^B^)1^\frac{\hat{A}\hat{B}+\hat{B}\hat{A}}{2} = (\hat{A}\cdot\hat{B})\hat{1}
\frac{\hat{A}\hat{B}-\hat{B}\hat{A}}{2} = \hat{A}\wedge\hat{B}
A^B^B^A^2=A^B^\frac{\hat{A}\hat{B}-\hat{B}\hat{A}}{2} = \hat{A}\wedge\hat{B}

Unit operator is an artifact of the matrix representation

\langle \hat{A} \rangle_0 \equiv \frac{1}{2}\text{Tr}[\hat{A}]
A^012Tr[A^]\langle \hat{A} \rangle_0 \equiv \frac{1}{2}\text{Tr}[\hat{A}]

Scalar projection removes representation:

\star \hat{A} \equiv -i\hat{A}
A^iA^\star \hat{A} \equiv -i\hat{A}

Hodge Star operation flips grade k to (3-k):

Cross Product is closed for grade 1 (vectors):

\hat{\sigma}_i\times\hat{\sigma}_j \equiv -i(\hat{\sigma}_i\wedge\hat{\sigma}_j) = \epsilon_{ijk}\hat{\sigma}_k
σ^i×σ^ji(σ^iσ^j)=ϵijkσ^k\hat{\sigma}_i\times\hat{\sigma}_j \equiv -i(\hat{\sigma}_i\wedge\hat{\sigma}_j) = \epsilon_{ijk}\hat{\sigma}_k

(Same "noncommutativity" in classical mechanics)

Why do we care?

For a physical spin, we expect rotations in 3D space to be relevant.

Identifying the implicit Clifford algebra makes geometry explicit.

\vec{s} \equiv \sum_j s_j\,\hat{\sigma}_j
sjsjσ^j\vec{s} \equiv \sum_j s_j\,\hat{\sigma}_j

A spin vector can be interpreted as a true vector in 3D space with this mapping.

\dot{\vec{s}} = \vec{\Omega}\times\vec{s} = -i(\vec{\Omega}\wedge\vec{s}) = \frac{-i}{2}\left[\vec{\Omega}\vec{s}-\vec{s}\vec{\Omega}\right] \equiv \frac{-i}{2}[\vec{\Omega},\vec{s}]
s˙=Ω×s=i(Ωs)=i2[ΩssΩ]i2[Ω,s]\dot{\vec{s}} = \vec{\Omega}\times\vec{s} = -i(\vec{\Omega}\wedge\vec{s}) = \frac{-i}{2}\left[\vec{\Omega}\vec{s}-\vec{s}\vec{\Omega}\right] \equiv \frac{-i}{2}[\vec{\Omega},\vec{s}]

Classical spin precession becomes obviously the same as the commutator evolution generated by a Hamiltonian operator:

\hat{\rho} = \frac{1}{2}[p\hat{1} + \vec{s}]
ρ^=12[p1^+s]\hat{\rho} = \frac{1}{2}[p\hat{1} + \vec{s}]
\text{Tr}[\hat{\rho}\vec{\ell}] = \langle(p\hat{1} + \vec{s})\vec{\ell}\rangle_0 = \vec{s}\cdot\vec{\ell}
Tr[ρ^]=(p1^+s)0=s\text{Tr}[\hat{\rho}\vec{\ell}] = \langle(p\hat{1} + \vec{s})\vec{\ell}\rangle_0 = \vec{s}\cdot\vec{\ell}

The physical correspondence of the state to spin orientations becomes transparent:

What about Measurement?

  • The Clifford algebra of 3D space works for Hamiltonian evolution, since the norm of the state never changes
  • Measurement changes the state norm
  • We either need nonlinear evolution (renormalization), or we need to consider the 4th state component.
\hat{\rho} \mapsto |0\rangle\langle 0|\hat{\rho}|0\rangle\langle 0| = \frac{p-z}{2}|0\rangle\langle 0| = \frac{p-z}{4}(\hat{1}-\hat{\sigma}_z)
ρ^00ρ^00=pz200=pz4(1^σ^z)\hat{\rho} \mapsto |0\rangle\langle 0|\hat{\rho}|0\rangle\langle 0| = \frac{p-z}{2}|0\rangle\langle 0| = \frac{p-z}{4}(\hat{1}-\hat{\sigma}_z)

Example:  ground-state projection

Equivalent to:

p \mapsto (p-z)/2
p(pz)/2p \mapsto (p-z)/2
z \mapsto (z-p)/2
z(zp)/2z \mapsto (z-p)/2

The state norm and spin components become

intertwined by measurement (before renormalization)

Gaussian Measurements

  • Weak measurements provide much more intuition about how to handle the change of the state norm


\hat{\rho} \mapsto \hat{M}_r\hat{\rho}\hat{M}_r^\dagger = C^2(r)[\frac{p-z}{2}e^{-r/V}|0\rangle\langle 0| + \frac{p+z}{2}e^{r/V}|1\rangle\langle 1| + \frac{x}{2}\hat{\sigma}_x + \frac{y}{2}\hat{\sigma}_y]
ρ^M^rρ^M^r=C2(r)[pz2er/V00+p+z2er/V11+x2σ^x+y2σ^y]\hat{\rho} \mapsto \hat{M}_r\hat{\rho}\hat{M}_r^\dagger = C^2(r)[\frac{p-z}{2}e^{-r/V}|0\rangle\langle 0| + \frac{p+z}{2}e^{r/V}|1\rangle\langle 1| + \frac{x}{2}\hat{\sigma}_x + \frac{y}{2}\hat{\sigma}_y]

Hyperbolic rotation of state components in p-z plane by "rapidity" angle r/V

\hat{M}_r = \frac{\exp[-(r-\hat{\sigma}_z)^2/4V]}{(2\pi V)^{1/4}} = C(r)\,e^{r\hat{\sigma}_z/2V}
M^r=exp[(rσ^z)2/4V](2πV)1/4=C(r)erσ^z/2V\hat{M}_r = \frac{\exp[-(r-\hat{\sigma}_z)^2/4V]}{(2\pi V)^{1/4}} = C(r)\,e^{r\hat{\sigma}_z/2V}
P(r) = \frac{\text{Tr}[\hat{M}^\dagger_r\hat{M}_r\hat{\rho}]}{\text{Tr}[\hat{\rho}]} = P_{z=1}\frac{\exp(-(r-1)^2/2V)}{\sqrt{2\pi V}} + P_{z=-1}\frac{\exp(-(r+1)^2/2V)}{\sqrt{2\pi V}}
P(r)=Tr[M^rM^rρ^]Tr[ρ^]=Pz=1exp((r1)2/2V)2πV+Pz=1exp((r+1)2/2V)2πVP(r) = \frac{\text{Tr}[\hat{M}^\dagger_r\hat{M}_r\hat{\rho}]}{\text{Tr}[\hat{\rho}]} = P_{z=1}\frac{\exp(-(r-1)^2/2V)}{\sqrt{2\pi V}} + P_{z=-1}\frac{\exp(-(r+1)^2/2V)}{\sqrt{2\pi V}}

Gaussian pointer r of variance V centered on z eigenvalues +/- 1

C^2(r) = \frac{\exp(-(r^2+1)/2V)}{\sqrt{2\pi V}}
C2(r)=exp((r2+1)/2V)2πVC^2(r) = \frac{\exp(-(r^2+1)/2V)}{\sqrt{2\pi V}}
p \mapsto p\cosh(r/V)+z\sinh(r/V)\qquad z\mapsto z\cosh(r/V)+p\sinh(r/V)
ppcosh(r/V)+zsinh(r/V)zzcosh(r/V)+psinh(r/V)p \mapsto p\cosh(r/V)+z\sinh(r/V)\qquad z\mapsto z\cosh(r/V)+p\sinh(r/V)

State update:

Measurement (Kraus) operator:

prefactor cancels in state renormalization: neglect (This abandons probabilistic interpretation)

Continuous Measurements

  • Every time step dt is a Gaussian measurement with variance:

Instantaneous "rapidity" angle:

\hat{M}_r = \frac{\exp[-dt(r-\hat{\sigma}_z)^2/4\tau]}{(2\pi \tau/dt)^{1/4}} = C(r)\,e^{dt\,r\hat{\sigma}_z/2\tau}
M^r=exp[dt(rσ^z)2/4τ](2πτ/dt)1/4=C(r)edtrσ^z/2τ\hat{M}_r = \frac{\exp[-dt(r-\hat{\sigma}_z)^2/4\tau]}{(2\pi \tau/dt)^{1/4}} = C(r)\,e^{dt\,r\hat{\sigma}_z/2\tau}
C^2(r) = \frac{\exp(-dt(r^2+1)/2\tau)}{\sqrt{2\pi \tau/dt}}
C2(r)=exp(dt(r2+1)/2τ)2πτ/dtC^2(r) = \frac{\exp(-dt(r^2+1)/2\tau)}{\sqrt{2\pi \tau/dt}}
p \mapsto p\cosh(dt\,r/\tau)+z\sinh(dt\,r/\tau)\qquad z\mapsto z\cosh(dt\,r/\tau)+p\sinh(dt\,r/\tau)
ppcosh(dtr/τ)+zsinh(dtr/τ)zzcosh(dtr/τ)+psinh(dtr/τ)p \mapsto p\cosh(dt\,r/\tau)+z\sinh(dt\,r/\tau)\qquad z\mapsto z\cosh(dt\,r/\tau)+p\sinh(dt\,r/\tau)

State update:

Measurement (Kraus) operator:

V = \tau/dt
V=τ/dtV = \tau/dt
\text{In the limit }dt\to 0\text{ the measurement becomes continuous.}
In the limit dt0 the measurement becomes continuous.\text{In the limit }dt\to 0\text{ the measurement becomes continuous.}
\text{Purity-preserving, Markovian measurements fully characterized by timescale }\tau
Purity-preserving, Markovian measurements fully characterized by timescale τ\text{Purity-preserving, Markovian measurements fully characterized by timescale }\tau
\text{Readout }r\text{ then is white noise around the moving mean }\langle\hat{\sigma}_z\rangle
Readout r then is white noise around the moving mean σ^z\text{Readout }r\text{ then is white noise around the moving mean }\langle\hat{\sigma}_z\rangle

Purity-preserving measurement preserves invariant interval:

p^2 - (x^2 + y^2 + z^2) \geq 0
p2(x2+y2+z2)0p^2 - (x^2 + y^2 + z^2) \geq 0

4D Clifford Algebra

Rotations in planes involving the state norm are hyperbolic.  

Rotations in planes not involving the state norm are elliptic.

This is equivalent to the structure of spacetime.

Note: it is simple to "upgrade" the qubit representation to this 4D "spacetime"

3D Clifford algebra is a subalgebra of the 4D Clifford algebra of spacetime

Removing matrix representation changes no physics, but clarifies correspondence


Euclidean 3D

Minkowski 4D (+,-,-,-)

\sigma_1^2 = \sigma_2^2 = \sigma_3^2 = 1
σ12=σ22=σ32=1\sigma_1^2 = \sigma_2^2 = \sigma_3^2 = 1
\gamma_0^2 = 1, \; \gamma_1^2 = \gamma_2^2 = \gamma_3^2 = -1
γ02=1,γ12=γ22=γ32=1\gamma_0^2 = 1, \; \gamma_1^2 = \gamma_2^2 = \gamma_3^2 = -1
\sigma_1 \equiv \gamma_1\gamma_0 \quad \sigma_2 \equiv \gamma_2\gamma_0 \quad \sigma_3 \equiv \gamma_3\gamma_0
σ1γ1γ0σ2γ2γ0σ3γ3γ0\sigma_1 \equiv \gamma_1\gamma_0 \quad \sigma_2 \equiv \gamma_2\gamma_0 \quad \sigma_3 \equiv \gamma_3\gamma_0

Apparent 3D vectors are timelike planes in 4D

Could represent 4D basis as Dirac "gamma matrices" if desired

(note vague connection to relativistic spin 1/2)

4D Clifford Algebra

  • Simplification:
\sigma_1 \quad \sigma_2 \quad \sigma_3 \quad | \quad i\sigma_1 \quad i\sigma_2 \quad i\sigma_3
σ1σ2σ3iσ1iσ2iσ3\sigma_1 \quad \sigma_2 \quad \sigma_3 \quad | \quad i\sigma_1 \quad i\sigma_2 \quad i\sigma_3
\gamma_0 \quad | \quad \gamma_1 \quad \gamma_2 \quad \gamma_3
γ0γ1γ2γ3\gamma_0 \quad | \quad \gamma_1 \quad \gamma_2 \quad \gamma_3






i \equiv \sigma_1\sigma_2\sigma_3 = \gamma_0\gamma_1\gamma_2\gamma_3
iσ1σ2σ3=γ0γ1γ2γ3i \equiv \sigma_1\sigma_2\sigma_3 = \gamma_0\gamma_1\gamma_2\gamma_3

(Representation-independent definition of "imaginary unit")

(but, commutes with even grade, anti-commutes with odd grade)


i\gamma_0 \quad | \quad i\gamma_1 \quad i\gamma_2 \quad i\gamma_3
iγ0iγ1iγ2iγ3i\gamma_0 \quad | \quad i\gamma_1 \quad i\gamma_2 \quad i\gamma_3
2^4 = 16\;\text{elements}
24=16elements2^4 = 16\;\text{elements}

Relative 3D space embedded as even-graded subspace

Planes (of rotation):

3 hyperbolic | 3 elliptic

\hat{\rho} = \frac{1}{2}(p\hat{1} + x\hat{\sigma}_1 + y\hat{\sigma}_2 + z\hat{\sigma}_3) \mapsto \rho \equiv p + x\sigma_1 + y\sigma_2 + z\sigma_3
ρ^=12(p1^+xσ^1+yσ^2+zσ^3)ρp+xσ1+yσ2+zσ3\hat{\rho} = \frac{1}{2}(p\hat{1} + x\hat{\sigma}_1 + y\hat{\sigma}_2 + z\hat{\sigma}_3) \mapsto \rho \equiv p + x\sigma_1 + y\sigma_2 + z\sigma_3
\rho = (p\gamma_0 + x\gamma_1 + y\gamma_2 + z\gamma_3)\gamma_0 = \rho\!\!\!/ \gamma_0
ρ=(pγ0+xγ1+yγ2+zγ3)γ0=ρ/γ0\rho = (p\gamma_0 + x\gamma_1 + y\gamma_2 + z\gamma_3)\gamma_0 = \rho\!\!\!/ \gamma_0

4D "Minkowski" Qubit

Drop 2D matrix representation, preserving physics in Clifford algebra:

Reinterpret algebra as embedded in 4D "spacetime":

Proper 4-vector

Proper time direction (of agent)

(defines relative space for rotations)

Rotations are then obvious spinor transformations of a proper 4-vector:

e^{dt(r/2\tau)(\sigma_3)}\rho e^{dt(r/2\tau)(\sigma_3)} = e^{dt(r/2\tau)(\sigma_3)}\rho\!\!\!/ e^{-dt(r/2\tau)(\sigma_3)}\gamma_0
edt(r/2τ)(σ3)ρedt(r/2τ)(σ3)=edt(r/2τ)(σ3)ρ/edt(r/2τ)(σ3)γ0e^{dt(r/2\tau)(\sigma_3)}\rho e^{dt(r/2\tau)(\sigma_3)} = e^{dt(r/2\tau)(\sigma_3)}\rho\!\!\!/ e^{-dt(r/2\tau)(\sigma_3)}\gamma_0
e^{dt(\Omega/2)({\star}\sigma_1)}\rho e^{-dt(\Omega/2)({\star}\sigma_1)} = e^{dt(\Omega/2)({\star}\sigma_1)}\rho\!\!\!/ e^{-dt(\Omega/2)({\star}\sigma_1)}\gamma_0
edt(Ω/2)(σ1)ρedt(Ω/2)(σ1)=edt(Ω/2)(σ1)ρ/edt(Ω/2)(σ1)γ0e^{dt(\Omega/2)({\star}\sigma_1)}\rho e^{-dt(\Omega/2)({\star}\sigma_1)} = e^{dt(\Omega/2)({\star}\sigma_1)}\rho\!\!\!/ e^{-dt(\Omega/2)({\star}\sigma_1)}\gamma_0



\text{Elliptic rotation in spatial (spin) plane }{\star}\sigma_1 = -i\sigma_1 = \gamma_2\gamma_3\text{ with spin axis }\sigma_1
Elliptic rotation in spatial (spin) plane σ1=iσ1=γ2γ3 with spin axis σ1\text{Elliptic rotation in spatial (spin) plane }{\star}\sigma_1 = -i\sigma_1 = \gamma_2\gamma_3\text{ with spin axis }\sigma_1
\text{Hyperbolic rotation (boost) in temporal (measurement) plane }\sigma_3 = \gamma_3\gamma_0
Hyperbolic rotation (boost) in temporal (measurement) plane σ3=γ3γ0\text{Hyperbolic rotation (boost) in temporal (measurement) plane }\sigma_3 = \gamma_3\gamma_0

Recall: p not probability after rotation

Classical spin "hidden variable" model

4D Matrix Representation

  • While a spinor representation of 4D rotations is efficient, a 4x4 real matrix representation is also computationally useful
\rho\!\!\!/ \mapsto \begin{bmatrix}p \\ x \\ y \\ z\end{bmatrix}
ρ/[pxyz]\rho\!\!\!/ \mapsto \begin{bmatrix}p \\ x \\ y \\ z\end{bmatrix}
e^{dt\,F/2}\rho\!\!\!/ e^{-dt\,F/2} \mapsto \exp\left(dt\begin{bmatrix}0 & r_x/\tau & r_y/\tau & r_z/\tau \\ r_x/\tau & 0 & -\Omega_z & \Omega_y \\ r_y/\tau & \Omega_z & 0 & -\Omega_x \\ r_z/\tau & -\Omega_y & \Omega_x & 0\end{bmatrix}\right)\begin{bmatrix}p \\ x \\ y \\ z\end{bmatrix}
edtF/2ρ/edtF/2exp(dt[0rx/τry/τrz/τrx/τ0ΩzΩyry/τΩz0Ωxrz/τΩyΩx0])[pxyz]e^{dt\,F/2}\rho\!\!\!/ e^{-dt\,F/2} \mapsto \exp\left(dt\begin{bmatrix}0 & r_x/\tau & r_y/\tau & r_z/\tau \\ r_x/\tau & 0 & -\Omega_z & \Omega_y \\ r_y/\tau & \Omega_z & 0 & -\Omega_x \\ r_z/\tau & -\Omega_y & \Omega_x & 0\end{bmatrix}\right)\begin{bmatrix}p \\ x \\ y \\ z\end{bmatrix}
F = E + i B
F=E+iBF = E + i B
E = (r_x/\tau)\sigma_1 + (r_y/\tau)\sigma_2 + (r_z/\tau)\sigma_3
E=(rx/τ)σ1+(ry/τ)σ2+(rz/τ)σ3E = (r_x/\tau)\sigma_1 + (r_y/\tau)\sigma_2 + (r_z/\tau)\sigma_3
iB = (\Omega_x){\star}\sigma_1 + (\Omega_y){\star}\sigma_2 + (\Omega_z){\star}\sigma_3
iB=(Ωx)σ1+(Ωy)σ2+(Ωz)σ3iB = (\Omega_x){\star}\sigma_1 + (\Omega_y){\star}\sigma_2 + (\Omega_z){\star}\sigma_3

Effective "electric field" polarizes spin

Effective "magnetic field" rotates spin

Lorentz rotation generator equivalent to "electromagnetic field tensor"

Describes continuous monitoring of all three qubit measurement axes:

easily modified to add experimental inefficiencies (T1, T2, eta, etc.)


Superconducting Qubit (3D Transmon)

"Qubit" is lowest two energy levels of a nonlinear oscillator

Not spin-1/2, but formalism still useful.

Yale, PRL 107, 240501 (2011)

Continuous Monitoring with Microwaves

Gaussian measurement per dt

Distinguishable qubit states

UCB, Nature 502, 211 (2013)

Phase-sensitive amplifier (LJPA) squeezes field along information-carrying quadrature

\text{Hamiltonian: }H_I = \chi\,\sigma_z\,a^\dagger a
Hamiltonian: HI=χσzaa\text{Hamiltonian: }H_I = \chi\,\sigma_z\,a^\dagger a

Delayed Choice Rotations!

UCB, Nature 502, 211 (2013)

Note the temporal progression:

  1. Field interacts with qubit in cavity
  2. Field leaks from cavity (unsqueezed)
  3. Field propagates away for a delay
  4. Amplifier phase chooses squeezed quadrature
  5. Homodyne measures squeezed quadrature
F = e^{i\phi}(r/\tau)\sigma_3
F=eiϕ(r/τ)σ3F = e^{i\phi}(r/\tau)\sigma_3

Phase of squeezing axis chosen long after field escapes cavity: type of qubit Lorentz rotation depends on this phase!

Causality still Preserved

Same ensemble-averaged (Lindblad) dynamics must occur regardless of (later) choice of measurement (and collected information)

However, the physical story told by the observed readout will be very different 

Korotkov, arXiv:1111.4016 (2011)

Fluctuations vs. Collapse

UCB, Nature 502, 211 (2013)

Story #1:

The squeezing eliminates distinguishability of qubit states, but amplifies the intrinsic uncertainty of the cavity field photon number.

The fluctuating photon number made the qubit energy fluctuate, creating random phase drifts that dephase the qubit in the ensemble average (purely elliptic rotations).

Story #2:

The squeezing suppresses the intrinsic photon number uncertainty, but amplifies the field separation between distinct qubit states..

The cavity photon number does not fluctuate. Instead, continuous weak monitoring of z creates partial collapses that decohere the qubit in the ensemble average (purely hyperbolic rotations).

The later choice of squeezing axis completely changes the physical picture.


  • Two-level systems undergoing Gaussian continuous measurement have a natural mapping to stochastic Lorentz transformations of a 4-vector 

  • Circuit QED measurements have a naturally delayed choice for which Lorentz transformations have occurred 

Thank you!